NASA's Cosmicopia -- Ask Us -- Earth and Moon
Earth and Moon
Earth's Birth
How was our planet born?
The Earth was born about 4 and a half billion years ago, at the same time the whole solar system (the Sun, Earth, and other planets) formed.  An enormous cloud of gas started to get smaller and smaller as the gas particles attracted each other with gravity.  Most of the gas went to the center of the solar system and formed the Sun, but several other pieces spinning about the Sun solidified into the planets, including the Earth.
Dr. Eric Christian
How old is the Earth?
Current scientific thinking puts the Earth at about 4.5 billion years old.
Dr. Eric Christian
Distribution of Elements on the Earth
If the Earth formed from elements made in stars, with "dust" forming the Earth via gravity, how did elements settle with like elements?  Why is there gold in one spot, silver in another, copper in another, etc.?
The primary separation of the elements takes place when the rock is molten.  Differences in melting and condensation temperatures and different densities cause the similar molecules to clump.  There are other smaller effects, such as like molecules forming crystals (they fit together better).  There is also some separation (due to differences in condensation temperature) in the proto-solar-system gas cloud.  This is why there are different types of meteoroids.
Dr. Eric Christian
Mass of the Earth
What is the mass of the planet Earth?
The mass of Earth is 5.98 x 1024 kg, or 1.32 x 1025 lb. I was able to look up the mass of Earth at the Welcome to the Planets link and then converted the kilograms to pounds.
Beth Barbier
AOS vs. AU
In a TV show it was noted that something was 1400 "AOS" away.  What does this mean?
There is an AU (Astronomical Unit) , which is defined as the average distance from the Earth to the Sun, but I have never heard of AO or AOS as a unit of measurement.  AOS is used as an abreviation for "Acquisition of Signal" for spacecraft telemetry, however.
Dr. Eric Christian
Why is Earth Spherical?
Why is the Earth spherical?
The Earth (and other planets, and stars) is spherical because the spherical shape is the lowest energy state that a group of matter can be in.  Small asteroids and moons can be non-spherical, but after they reach a certain size (when the force of their gravity can "break" the rock from which they are made), all the bumps are pulled down, and they become more spherical.  There is a maximum size that mountains can attain that gets smaller as the planetoid gets more massive.  So Mars can have larger mountains (Olympus Mons for example) than the Earth can, because it weighs less.  As long as the maximum mountain size is small compared to the radius of the planetoid (true for objects considerably smaller than the Moon), the body will be spherical.
Dr. Eric Christian
How Far to the Center of the Earth?
How many miles is it to the center of the Earth?
The Earth is not a perfect sphere, so the distance to the center of the Earth varies from 6378 km (3963 miles) at the equator to 6357 km (3950 miles) at the poles.
Dr. Eric Christian
Earth's Core
How does Earth's outer core reach the temperature of 5000 degrees and remain liquid, while inner core remains solid?
The inner core may be hotter, but it is at a much higher pressure.  At a fixed temperature (say 5000 degrees), high pressure can make a liquid into a solid.  So, even though the temperature increases as you move into the inner core, the pressure increases faster and wins.
Dr. Eric Christian
Why is Earth's Center So Hot?
Why is the center of the Earth so hot?
It's a combination of radioactivity (the radioactive materials in the Earth generate heat) and the residual heat from the formation of the Earth.  When all of the matter that created the Earth fell together, it picked up kinetic energy falling in.  When it stopped at the proto-Earth, the kinetic energy was turned into heat.  The Earth hasn't cooled yet.  The Moon, being much smaller, has had time to cool and probably has a solid core.
Dr. Eric Christian
What about...?
I'm an astrophysicist, not a geologist. I got my answer from long ago geology courses and research on the web. It's a standard part of geological theory that radioactivity is part of the heating process. See, for example, this Scientific American article .
Why is there not an abundance of fission products observed in materials emitted from either volcanoes or sea-borne vents? Or why are there not more fission products as one penetrates the Earth's crust into deep mines?
There IS increased radioactivity seen in volcanoes (see, for example this news article ).
I don't know about undersea vents, but I wouldn't be surprised if there was increased activity coming from them as well. Your question implies that you think there should be lots and lots of radioactive gas, but you don't need that much radioactivity in the mantle and core.
As for deep mines, local composition is far more important for how much radioactivity there is than depth.
If you want more information, I suggest you contact a real geologist or go to your local college/university library to do some research.
Dr. Eric Christian
Was the Earth a Star?
Why is the Earth's core so hot?  Was the Earth once a star, before it became a planet?
Earth's core temperature is about 6,000° C. By coincidence, this is about the same as the Sun's surface temperature (but much cooler than the Sun's core temperature, which is about 15,600,000° C). The Earth's core is cooling, but at a very slow rate. Over the past three billion years it has probably cooled by a few hundred degrees. Currently, the Earth's core temperature is not changing much because, through radioactive decay (nuclear fission - the breakup of the nuclei of heavy elements, like uranium), it is generating about as much heat as it is losing.
To answer the second part of this question, some definitions are in order.  A star is a self-luminous body that shines by generating energy internally through nuclear fusion (the combining of nuclei of light elements like hydrogen and helium). The Sun is a star. A planet shines by reflected light from the Sun. The solar system has nine "major" planets (of which Earth is one) and innumerable "minor" planets (asteroids and comets of various kinds).
Star masses range from about 0.04 times, to 150 times, the mass of the Sun. The mass of the Earth is 0.000003 times that of the Sun (and the mass of Jupiter, the largest planet in the solar system, is 0.001 times that of the Sun).
Although stars lose mass as they evolve, none lose enough to wind up anywhere near the mass of even the most massive planet.  So, the bottom line is: Stars do not evolve into planets.
For a great animated simulation showing the evolution of stars with masses between 0.1 and 120 times that of the Sun see the stellar evolution simulation created by Terry Herter for his Astronomy 101/103 course at Cornell University. You will need a JAVA enabled browser to view this simulation.
Dr. Ed Tedesco
Weight at Earth's Core
Would your body weigh more or less if standing on the Earth's core?  What about above sea level?
As you go further inside the Earth, the force you feel due to gravity lessens, assuming the Earth is has a uniform density all the way throughout.  Less force means you weigh less.
The reason is that the mass attracting you is inside a sphere, and is given by M = (4/3) * pi * (radius)3 * density
The force you feel is given by F = G * M * (your mass) / (radius)2
This means the net force is F = G * (4/3) * pi * radius * density * (your mass)
(pi=3.14159 and G = Newton's gravitational constant)
So as you go further inside the Earth, the radius is decreasing, so the force you feel is decreasing.  The mass above you oddly enough doesn't contribute at all to any net force on your body.
In reality, of course, the Earth is not of uniform density, and there is a slight increase in force as you go down from the surface, before it begins to decrease again. Still, you weigh less standing on the Earth's core.
As far as what happens above sea level - you must realize that what happens outside the Earth is different from what happens inside the Earth. Inside, as you go deeper and deeper, the mass attracting you is less and less (as stated). Above sea level (the surface of the Earth, specifically) as you go further and further away, the mass remains constant (obviously), but the distance gets larger and larger, which makes the force (given by F = G * M(Earth) * M(you) / r2) smaller.
Notice that the formula that applies inside the Earth is different from the one that applies outside.
Dr. Louis Barbier
Gravity at Earth's Core
Gravity pulls us downward on all sides on the Earth. Does this mean that at the center of the Earth, there is no gravity?
First, the most important thing to know is that gravity exists absolutely everywhere in the universe.  Every bit of matter exerts a force on every other bit of matter.  This means that you are attracted, and attract, everything in the universe!  The force exerted depends on the distance of the object and the mass.  The Earth exerts the most force on you because it is close (right here!) and very massive.
Forces add like vectors, so their direction is very important.  If you could be at the exact center, the forces that each bit of Earth matter exerted on you would cancel out (up cancelling down, east cancelling west, etc.).  This only occurs for a single point, though, and you would still feel a gravitational force on the rest of your body.
Remember, gravity is universal and exists everywhere.  This is the fundamental law of physics.
Angela Richard
Earth as a Spherical Shell
If I correctly understand Newton's law of gravitation, a mass located at the center of the Earth would tend to be pulled in many opposite directions.  If this mass is a liquid without strong internal coherence forces, it will take the shape of a shell.  Which parameters and values would be comparable with Earth having the shape of a thick shell?
Gravitation is attractive, never repulsive, so, in the absence of other forces there, matter will always tend to move to the center of the sphere.
Dr. Eric Christian
How Do We Know About Earth's Layers?
How do we know that the Earth is made up of three layers?  I can guess that we have not yet explored the center of Earth, or even broken the crust, so how can I be certain that what I read in my textbooks about the standard three-layer theory is, in fact, reality?
The major probe of the Earth's interior is seismic waves from earthquakes.  Scientists can use an array of seismometers to track the speed of the waves as a function of depth, and from this can infer density, temperature, etc.  The sharp discontinuities at the mantle-crust and mantle-core interfaces are especially noticible.
Dr. Eric Christian
Earth's Gravity Variation
The gravity at the poles has been measured to be greater than at the equator. I would like to know which factor contributes more to this phenomenon, whether it is the lesser centrifugal force at the poles compared to the equator, or if it is more due to the proximity to Earth's center? Has any experiment been conducted to find the quantitative role of both of these causes separately and compared?
These values are directly calculable and have been proven by measurements repeatedly.
Radius at poles = 6,356,800 meters
Radius at equator = 6,378,400 meters
omega (angular velocity) = .00007292115 s-1
If you plug these numbers in you find:
gravitational acceleration at poles = 9.8322 m s-2
no centrifugal acceleration at poles
Total acceleration at poles = 9.8332 m s-2
gravitational acceleration at equator = 9.7805 m s-2
centrifugal acceleration at equator = -.0002 m s-2
Total acceleration at equator = 9.7803 m s-2
So it is obvious that the oblateness of the Earth is 250 times more important than the centrifugal acceleration.
Dr. Eric Christian
Why Aren't People on the "Bottom" of the Earth Upside Down?
My six-year-old daughter asked me this question while looking at a globe of the Earth. Can you help me explain this to her?
"Up" and "down" are different depending upon where you are. They come from gravity, which is the force that holds everyone and everything onto the Earth. "Down" points in the direction of gravity, which is toward the center of the Earth, and "up" is in the opposite direction. If you look at a globe, no matter where a person is standing, "up" is the direction away from the center of the Earth and points to the sky.
Dr. Eric Christian
What Keeps Us On the Earth?
What keeps us on the planet?  If the Earth stopped spinning, would we all fall off?  Are there any planets that do not spin?
We do not stay on the Earth because it is spinning, but because of the force of gravity.
I am not aware of any planets that do not spin.
Dr. Louis Barbier
Speed of Earth's Rotation
We have a book that takes us through a project to measure the speed of the Earth's rotation. The project stops prematurely however. It takes us through determining that the Earth rotates 360 degrees in 24 hours, or 15 degrees per hour, then that it goes through 0.5 degrees of rotation in two minutes. That's it. How can I turn this into the speed of the Earth's rotation?
You already have half the answer! The speed of the Earth's rotation can be thought of in two ways - the angular speed (which you've calculated) and the linear speed (of a point on the surface).
Express the angular speed (traditionally referred to by the Greek letter omega) in radians/sec:
omega = 360 degrees / 24 hours = 2 * pi radians / (24 * 60 * 60) second
(pi = 3.14159.....)
= 7.272 x 10-5 radians/sec = 0.00007272 radians/second
Now to convert the angular speed to the linear speed of a point on the Earth's surface, multiply omega by the radius of the Earth, R.
To put it in familiar units, let's express R in miles: R = 3822 miles (roughly). So the linear velocity on the surface V is:
V = omega * R = 0.00007272 * 3822 = 0.278 miles/second
or
V = about 1000 miles/hour  (Surprisingly fast!)
Dr. Louis Barbier
Speed of Earth's Rotation Slowing?
I heard that the rate of Earth's spin is decreasing.  How much is the Earth slowing down?  Has it always been slowing down? Ask an Astrophysicist said that the Earth spins like a figure skater, but they can only spin for so long!
The Earth's spin is slowing down by about 1.5 - 2 milliseconds per century, and that angular momentum is moving into the Moon's orbit, which is getting larger.  The reason for this, and the reason a figure skater can only spin for so long, is friction.  In the case of the skater, it's air resistance and friction with the ice. In the case of the Earth, it's the friction due to tides moving around the Earth.
Dr. Eric Christian
Earth's Rotation Speed via Stellar Measurements?
I am doing a physics science fair project, and I wanted to do something related to astronomy - in particular, the rotation of the Earth in relationship to other stars. I was thinking of setting up a camera and taking a picture of each star I am calculating for, leaving the lens open for approximately 5 seconds. I think if I do this, instead of seeing the star as a specific point of light I will see it as a streak. Do you think this will work? And is there any way that I can calculate the rotational velocity of the Earth by analyzing these streaks?
It takes nearly four minutes for a star at the celestial equator to move one degree (about a finger's width held out at arm's length).  Stars nearer to the poles will move even less (Polaris doesn't appear to move at all in the northern hemisphere).  So in five seconds you won't get much of a streak.  With longer exposures you could maybe do it, but you'll need to accurately measure the length of the track and know how far the star is from the pole (its celestial latitude) to know the total length of the circle it would make if you could photograph it for a full day.
An easier way is to measure transit times.  Measure the time that a star passes the same point (goes behind the roof of a building or crosses an overhead wire) on successive days and divide the time interval by the 360 degrees the star has traveled.  If you miss a day or more because of clouds, it still works if you divide by 360 times the number of days.  You have to keep your eyes in exactly the same spot for this to work, so a mounted telescope (that doesn't move) or just a cardboard tube will help.
Dr. Eric Christian
Earth's Orbit
Since the Earth is constantly moving towards and away from the Sun, are there particular dates - that are constant each year - that the Earth is exactly 1 AU away from the Sun? If so, what are they? If not, what's the approximate cycle for the Earth to reach this point? Every 6 months?
The Earth's orbit varies from 0.983 AU out to 1.067 AU. We are actually closer to the Sun in the winter than in the summer. The orbit crosses the 1 AU point two times a year - in spring and fall.
Dr. Louis Barbier
Foci of Earth's Elliptical Orbit
I know that Earth's orbit is elliptical and that the Sun is at one of its focal points. What is at the other focus? It seems that there should be something that has a graviational effect on the Earth equal to that of the Sun to keep Earth from a circular orbit.
Your question provides the opportunity to elaborate on the connection between Kepler's Laws of planetary motion and Newton's Law of Gravitation.
The simple answer to your question is that there is nothing special at the other focal point. Here's the explanation.
In the late 16th century, Tycho Brahe made the most accurate observations of the positions of the planets at that time, with his state-of-the-art observatory on the Danish island of Hven. In his attempt to model and explain Brahe's observations, Johnannes Kepler devised his laws of planetary motion. The first two are relevant here.
Kepler's first law states that all planets move on ellipses, with the Sun in one of its two foci, as you mentioned, and this still holds. Looking at this statement as a purely mathematical description, it is not obvious whether the Sun should be in one or the other focus, or whether there is any special significance to the second focus. That is where a physical argument and/or an observation would have to come in, which would then define the correct initial condition to determine the correct focus.
Brahe's observations also provided the handle to connect Kepler's laws with Newton's Law of Gravitation. The Sun is the source of the gravitation that pulls radially on the planets in our solar system and keeps them in orbit. It also slows them down on their way out to the furthest part of their orbits, and speeds them up on their way back in. If, in fact, there were another equal source of gravitation located in the other focus of the ellipse, there would be no reason for a planet to move faster near the one focus and slower near the other focus, as was observed.
Let us explore this with an illustrative fictitious experiment, diagrammed in the figure below.
Shown are three cases with different initial speed (arrows)
and the respective elliptical orbits in color. In all cases,
the satellite is closest to the planet and Focus 1 when it is moving
with its fastest speed.
We will launch a satelllite from a high tower on a planet that does not rotate (to make things simpler). This figure is obviously not to scale, but to orient you, in the diagram, the tower comes out of the planet on the top here, and the satellite launch goes straight to the right from its top.
When the satellite is launched from the tower at its slowest speed, its orbit looks like the red (innermost) ellipse. It is so slow that after launch it starts to fall toward the planet - and thus speeds up and falls into this type of orbit. As explained, the satellite is fastest where it is closest to the planet: on the side of the orbit opposite the launch tower. We'll call the planet's center Focus 1 (black "plus sign"). Focus 2, for this orbit with the slowest speed, is at the red plus sign (which would not actually be on the tower if the diagram were to scale).
Launching the satellite at a higher speed pushes the far side of the orbit further and further out. In the black (middle) orbit  example, the speed is such that the ellipse turns into a circle. The planet is now in the center of the circle, and a circle is a special case of an ellipse. Because it's a circle, the locations of both foci are the same, at the black plus sign.
Finally, in the green (outermost) orbit case, the launch speed is fast enough for the satellite to continue to increase its distance from the planet. It slows down as it reaches the maximum distance possible with this speed - on the opposite side from the launch point. Again Focus 1 (in the planet center) is close to the point in the orbit where the satellite is at its fastest speed, and Focus 2 for the green orbit (green plus sign) is on the side where the speed is slowest.
Kepler's second law, derived solely from observations at the time of its discovery, states that on a planet's orbit around the Sun, the planet-Sun line sweeps out equal areas during equal time periods (fractions of a planet's orbital period). This quantifies the speed of an orbiting planet based on its distance from the Sun, and is in accordance with actual observations. A planet reaches its fastest speed at perihelion, its closest location to the Sun. This observational fact also provides a basis for the location of the Sun as one of the two foci. Again, it has to be the one on the side where the planet moves fastest.
This web page at the University of Kentucky may be helpful.
Dr. Eberhard Moebius
What Happens if Earth Stops Rotating?
What happens to all the earthlings, if Earth stops spinning? Will we be thrown off the Earth's surface, since Earth is spinning at ~1000 mph?
Of course, this can't happen, but if it did, everything not attached would go flying off to the east, parallel to the surface of the Earth.  The speed would depend upon your latitude.  Only the people at the poles would be safe.  You wouldn't go flying off into space because the 1000 mph maximum (at the equator) isn't enough to overcome gravity, which would still be present.  If you survived, the resulting six month day and six month night would probably take care of you pretty quick.
Dr. Eric Christian
Weight Changes if Earth Stops Rotating
I think that because Earth is rotating, objects on its surface are exposed to outward centrifugal force.  If the Earth stopped rotating, wouldn't everything on Earth appear to weigh more due to the absence of competing centrifugal force?  Does this make sense?
Yes, your logic is correct. If the Earth stopped rotating on its axis everything on Earth, away from the poles, would appear to weigh more due to the absence of centrifugal force.
Mathematically, F = mg, where g is the acceleration due to gravity at the Earth's surface (9.80 kg/m2), m is your mass, and F is the force (in this case, weight) resulting from mass m in gravitational field g.  If you are very far from any massive body then g ~ 0 and so you are weightless (F = m x 0 = 0).
However, for most places on a rotating Earth, we feel a centrifugal force that slightly decreases our weight. This is because for circular motion, the force (Fc) due to acceleration (a) is given by Fc = ma = m(v2/R) Cos(Theta), where v is the speed of the object (you, in this case), R is the radius of the circle it is moving in, and Theta is the angle between the rotation axis and your position on the Earth's surface. For the case of the Earth, R is its radius and Theta = 0 deg at the equator and 90 deg at the poles. So, since Cos (0) = 1 and Cos (90) = 0, the force on you, in a direction away from the rotation axis, is m(v2/R) at the equator and zero at either pole.
Numerically, this means that if your mass is 70 kg, the force of gravity acting on you is F = mg = 70 x 9.8 = 686 (kg/m)2 (a unit called a "Newton"), or 154 pounds.
If you are standing on the equator, the centrifugal force acting on you is Fc = m(v2/R) Cos(Theta) = 70(4632/ 6371000) Cos (0) = 2.4 Newtons = 0.54 pounds.
So, you would weigh about a half-pound less at the equator than you would at the North or South Pole.  (For the purists, the explanation above assumes the Earth is a sphere of uniform density and neglects relativistic forces.)
Dr. Ed Tedesco
Jump!
What would happen if all of the people in the world decided to jump all at once, or all run in the same direction?  Could this change the orbit or rotation of the Earth?
The mass of all the people on Earth is miniscule compared to the mass of the Earth, so neither of these actions would have any effect on Earth's motion.
Dr. Louis Barbier
Night and Day, Seasons
Why do we have night and day? Why are summer days longer than winter days? Why is summer weather hot and winter weather cold?
Check out the Windows to the Universe , the Starchild Web site , the Science@NASA site , or the Bad Astronomy site , which talk about the Earth, seasons, etc. Check Imagine the Universe! for information on the "equation of time" -- the asymmetrical change in the amount of daylight between sunrise and sunset through the year.
Dr. Louis Barbier and Beth Barbier
I know that as we approach the summer solstice, we gain hours of daylight.  Do we gain the same amount of daylight each day as we approach the longest day of the year?
No, the day/night dividing line moves in a curve, not in a linear fashion.  It's due to the interaction of several three-dimensional vectors, and so there are sines and cosines involved (and actually the product of sines and cosines).  For a simple proof, near the poles, the gain of daylight goes to zero before the summer solstice is reached, because the day is already 24 hours long.
Dr. Eric Christian
Balance an Egg at the Equinox?
On September 22, 2000, at exactly 1:27 p.m. EDT, I was able to balance two eggs on a perfectly flat table top.  How long does the autumnal equinox last? I find it very hard to believe that the two eggs are still standing after three days.  Why are they not tipping over?
You can balance an egg any day of the year.  The equinox is not special.  See The Straight Dope web site, for example.  This is a common misconception.
Dr. Eric Christian
Seasons and Sidereal Day
I understand that the Earth's tilt and orbit around the Sun causes the seasons and the seasonal changes in the hours of daylight, but it appears to me when I look at a model of the Earth's orbit around the Sun, night and day should reverse from summer to winter.  If the Earth rotates at a constant speed, and the tilt always faces the same direction in space, shouldn't the position of the Earth in the middle of the night in the winter be the same position of the Earth in the middle of the day in summer?  Obviously, I am wrong, but I do not understand why?
The thing that you've missed is that a "day" is not the length of time that the Earth takes to rotate through 360 degrees.  Instead, the day is defined as the time it takes for the Sun to move from zenith to zenith.  Because the Earth has travelled almost a degree through its orbit, it actually has rotated almost 361 degrees in 24 hours.  Those extra degrees add up over a half year to keep the day synchonized.  But the constellations do shift, so that what you see during the summer is overhead during the day in the winter and vice versa.  The astronomical term for the time it takes the Earth to rotate 360 degrees is "sidereal day", which is 23 hours 56 minutes 4.09 seconds long.
Dr. Eric Christian
Why is Sunrise in a Different Place Each Day?
Why does the position of the sunrise change along the eastern horizon during the year?
The reason is that the axis of the Earth's rotation is tilted relative to the plane of the Earth's orbit around the Sun.  So the circle on the Earth where the Sun is directly overhead moves north and south over the year, from the equator to the Tropic of Cancer, back to the equator, then to the Tropic of Capricorn and back to the equator.  This causes sunrise and sunset to move north and south over the year as well.  This effect also causes the seasons and the shortening and lengthening of the day.
Dr. Eric Christian
For more information on Sun and Moon rise and set times, Moon phases, eclipses, seasons, positions of solar system objects, common astronomical phenomena, calendars and time, and related topics, check out the Astronomical Applications Department of the U.S. Naval Observatory .
Beth Barbier
Why is the Sky Blue?
Why is the sky blue?
The Sun gives off all colors of light, but blue light is bounced around the atmosphere a lot more than red light is (it's called scattering).  The sky is blue because of the blue light bouncing around "lights up" other parts of the sky.
Dr. Eric Christian and Beth Barbier
(April 2000)
Why is the Sunset Red?
Why does sunset look red?
The Sun is always a little redder because of the scattering, but at sunrise and sunset the light has to pass through more atmosphere and loses much more blue light, so appears much redder.
Dr. Eric Christian
How Is Snow Formed?
How is snow formed?  Is it formed when it rains and the raindrop goes back up and hits the cold air?
Hail can be formed by rain rising and freezing, but snow is directly formed in its solid, crystalline form.
Dr. Eric Christian
What Causes Wind?
What causes wind?  Is it due to gravity and the rotation of the Earth?
Both the Earth moving (spinning) and gravity affect the wind.  But the primary cause of the wind is temperature differences, not the moving of the Earth or gravity.
Dr. Eric Christian
Sun's Role in Global Warming?
Isn't it possible that global warming and cooling could be caused by our Sun?  Doesn't it ever change temperatures?  Isn't it possible that the cycles of the ice ages may be caused by cycles of our Sun's temperature?
Though not our area of expertise, this is an area of current scientific study.  Check out the European Science Agency's (ESA) article from September 29, 2000, Climate change: New impressions from space .
Beth Barbier
Earth-Sun Distance and Temperature
What would happen if the Sun moved closer to the Earth by 1 meter?  How much would the temperature increase on Earth?
There would be an increase in temperature if the Earth to Sun distance became much smaller, but 1 meter is insignificant.  For example, the Earth is actually closer to the Sun during the northern hemisphere winter.
Angela Richard
Changes in the Earth-Sun Distance
I read in an article that the Sun is currently closer to the Earth than it has been in a long time.  Why should the Earth-Sun distance change over such a long time scale?
The Earth's orbit changes on several time scales, each of which affect the intensity of radiation we observe at Earth.
The first change is the winter and summer solstice, where the Sun-Earth distance varies between 91,400,000 miles and 94,400,000 miles.
Now on longer time scales, the actual shape of the Earth's orbit changes every 100,000 years, vacillating between more circular and more elliptical.  In this case, when the Earth is closest to the Sun it actually receives 20-30% more sunlight.
In addition, the Earth wobbles on its axis every 26,000 years, changing the time at which winter and summer occur.
Finally, the tilt of the Earth varies every 40,000 years by about 2 arc degrees, which affects the temperature difference between winter and summer.
These changes are termed the "Milankovitch theory" after the geophysicist who first proposed it and are believed to operate together to produce dramatic temperature variations on Earth.  You might want to read more about the Milankovitch theory and solar radiation variations at the U.S. Naval Observatory Astronomical Applications Department and the Encyclopedia of the Atmospheric Environment .
Dr. Georgia de Nolfo
Sidereal Day
Could you please explain a sidereal day?
A sidereal day is the length of time it takes the Earth to rotate 360 degrees.  Since the Earth is revolving around the Sun, it actually has to rotate almost one degree (360 degrees/365.25 days) further until the Sun is in the same place in the sky, which is the definition of a day that everyone is used to.  So a sidereal day is a little shorter, but the stars return to the same positions every sidereal day, so sidereal time is used sometimes in astronomy.
Dr. Eric Christian
If a sidereal day is 23 hours and 56 minutes, what happens to the remaining 4 minutes?
There is a good explanation of sidereal time on the Imagine the Universe website .
Beth Barbier
Lines of Demarcation
If our world turned on a vertical axis, would the equator and the Prime Meridian switch places?
I assume you want to know what would happen if the axis of the Earth moved (it is already turning on an axis that is vertical at the poles).  The position of the equator is determined by the spin axis of the Earth, so if the new axis were at the right point (0 degrees latitude, 90 degrees longitude) then the old Prime Meridian would become the new equator (or at least half of it, the equator is a circle, the prime meridan is a semi-circle).
But the Prime Meridan is just a reference line picked by man, and could have been put anywhere.  It goes through Greenwich, England, but the French were pushing hard for a French origin.  If the axis moved, the new Prime Meridan could be put anywhere.  Luckily, conservation of angular momentum insures that this drastic change in the Earth's axis is not going to happen.
Dr. Eric Christian
Tides
Ever since junior high we were taught that it was the Sun's and Moon's gravity which were directly responsible for causing the tides on Earth.  Now, out of blue, I am told, that that is not so. It's Moon's gradient of its gravity that is causing Earth's tides. Please, help.
The statement that it is the Sun's and Moon's gravity that causes tides is not incorrect, just simplified.  More accurately, it is the gradient of the gravity that causes it.  What happens is that the water on the side closest to the Moon is closer to the Moon than the center of gravity of the Earth and so the Moon exerts more pull on that water and it bulges out (high tide).  On the other side of the Earth, the Moon is pulling more on the center of mass of the Earth than on the water and so the Moon pulls the Earth out from under the water a little bit, causing another bulge or high tide.  That is why there are two high tides per day.  Hope this helps.
Dr. Eric Christian
Lunitidal Interval
I'm trying to set my watch to predict the tides using the Lunitidal Interval.  Can you explain what that is and how it works?
Your question is well beyond our area of expertise or interest, and we cannot answer it. I did find a bit of information on a NOAA website that may get you started, at the Center for Operational Oceanographic Products and Services FAQ .
Beth Barbier
(July 2003)
Contributed by Konstantin Parchevsky, HEPL, Stanford University (May 2007):
The Moon's gravity causes the appearance of two "bumps" of water on the Earth -- one on the Moon's side and one on the opposite side. Theoretically, in the ocean, neglecting the viscosity of water, the highest spot of the "bump" should be just below the Moon.
In reality, near the coast there is a time lag between the moment when the Moon passes the meridian (the so-called culmination) and the moment of the highest tide. This time lag is called the "lunitidal interval". It depends on the viscosity of water and the shape of the coastline and the seabed, and it has to be taken from observations. If you want your watch to predict the tides correctly, you have to input the lunitidal interval. For some cities, you can take it from the table at the end of the watch's manual.
Unfortunately, there were no data for Half Moon Bay, CA (near me). So I had to calculate the lunitidal interval by myself. This is a good time to remember a saying: "If you are stuck, read the manual!" Fortunately, there was enough information in the watch manual to do this: "When setting the lunitidal interval for this watch, use the time difference between the Moon's transit over the meridian until high tide."
That's it! Just find the time difference between the moments of culmination of the Moon and the next high tide! First you have to calculate culmination of the Moon. Google gave me a lot of links for the phrase "Moon calculator". The second link is exactly what we are looking for. Just enter the date (such as 9 May 2007) and your position (Half Moon Bay, CA) and press 'Enter'. What we need here is the Moon transit (6:59 a.m.). Then we have to find the time of the next high tide in Half Moon Bay.
This time, Google "tides Half Moon Bay". The first link [similar sites can be found -- ed.] gives us a graphic representation of the tides and the moment of the closest high tide (6:14 p.m.). Time difference is 11h 15m. This is the sought-after lunitidal interval. And it works! Now my watch predicts the tides correctly.
For the record, I am an astrophysicist (solar physicist). I work at Stanford University and perform numerical simulations of propagation of the acoustic waves in the Sun.
What's a good Web site on meteoroids and meteorites?
You might want to check out the Views of the Solar System page on meteoroids and meteorites.
Beth Barbier
Impending Impacts?
Are there any asteroids, comets, or other objects that are going to impact Earth?
Your question about an asteroid impacting Earth is well covered by our sister site, Imagine the Universe!, in Is Earth in danger of being hit by an asteroid? .
Beth Barbier
A Close Call?
My mother saw a news flash on Fox News on July 30, 2002, that there was a meteorite 1.5 miles wide on a collision course with Earth.  I know what happens to "flying objects" from space that suddently collide with our atmosphere.  I am trying to find out if there was any truth to this story.
I think this will answer your questions well: Caveat Impactor
(Don't believe everything you hear on Fox News!)
Beth Barbier
Meteorite Hit Increases Earth's Gravity?
When a meteorite hits Earth, adding mass, does Earth's gravity increase, too?
Yes, but unless the meteorite is really enormous (big enough to destroy life on Earth), the Earth's gravity changes by an amount that is too small to measure.  The Earth is picking up several tons per day of dust and other stuff, not including the very uncommon meteorite.
Dr. Eric Christian
Earth's Magnetic Field
I have seen many drawings of the Earth's magnetosphere , and there are two cusps that make an angle of about 154 degrees with each other.  Do the axes of these cusps point to fixed directions in space as the Earth orbits the Sun? If so, what are the celestial coordinates of their radius vectors?
The cusps are due to the interaction of the Earth's magnetic field and the magnetic field imbedded in the solar wind.  They move around as the Earth moves around the Sun and as the tilt of the Earth's axis moves.  They also move in position, as well as distance from the Earth, as the magnetic field of the Sun changes.  So they don't point at any fixed direction.
Dr. Eric Christian
Solar Wind and Earth's Magnetosphere
I am working on a paper on the interaction of the solar wind with the Earth's magnetosphere.  Do you know of a source giving the magnitude of the magnetic field of the magnetosphere?
Your question has a tremendous amount of latitude in it, both literally and figuratively.  The magnitude of the Earth's magnetic field within the magnetosphere varies greatly with location within the magnetosphere.  It is not a simple question to answer.
The magnetosphere is formed when the flow of the solar wind impacts the Earth's magnetic field (a dipole field), compressing it, causing magnetic reconnection, causing complex currents to flow, etc.  The dipole field changes greatly under the influence of the solar wind.
Try to get a copy of "Physics of the Magnetopause" published by the AGU.  That will help.
The magnetic field in the solar wind near Earth is about 5 nT, or 5 x 10(-5) Gauss.  The magnetic field on the surface of the Earth is about 0.5 Gauss - big difference.
If you were to imagine a spacecraft passing from the solar wind and into the magnetosphere along the Earth-Sun line (at the sub- solar point) you would see the following:
The 5 nT field changes by a factor of 4 as you pass through the Earth's bow shock.  This is a compression wave (discontinuity) that results from a supersonic flow striking the Earth's magnetic field and being stopped).
The magnetic field then climbs by another factor of 4 as the spacecraft passes through the magnetosheath, a turbulent region of flow behind the shock and still upstream of the magnetosphere.
The spacecraft then encounters a region where the flow stops moving toward Earth and flows exclusively around the obstacle that is the Earth's magnetosphere.  This is the magnetopause. Behind the magnetopause the magnetic field is about 40 nT.  So the compression of the flow has picked up almost a factor of 10 in intensity of the magnetic field (and the plasma density with it).
From this point on, it depends greatly on where you go.  If you go Earthward, it is to leading order a dipole field (somewhat squashed, but the scaling is like that).  If you go behind the planet, all kinds of things are happening and location is everything.
I really can't go into much more detail than that without a more specific question, and to be honest I mostly work outside the magnetosphere, so my knowledge is limited.  But basically if you go from the magnetopause to the Earth's surface, you see a magnetic field that grows ever stronger as you decend within the dipole.
Dr. Charles Smith
Earth's Magnetic Field Flip
I know that both the Earth and the Sun undergo a regular shift in their magnetic poles. The Sun's cycle is 11 years and I believe the Earth's is around 26,000 years. Is it true that Earth's pole reversal is due and is going to occur at the same time (or near) to the Sun doing the same thing? When this happens would the Earth and the Sun not react in a similar manner to two identically charged magnets coming together and repel? It seems that our own North Pole tracking data shows that it is already beginning to migrate. When it completes it's reversal what consequences will there be for a civilisation, based on electrical devices? Is the closeness of the Sun not a major factor is global warming?
The Sun shifts it's magnetic field every 11 years, and it has already happened for this solar cycle.  The Earth's magnetic field flip is much more erratic and has happened approximately 25 times in the last 5 million years.  It's been about 740,000 years since the last flip, however, so we're long overdue.  There is evidence that we may be heading towards a reversal (the dipole magnetic field is weakening and the higher order terms are increasing), but we can't predict when it would happen.  Depending upon how quickly the field reversal happens, it could cause problems for things like electric power lines and oil pipelines, and if the field goes to near zero, it might cause a higher background radiation at the ground, but there is no evidence that previous reversals have had any major biological effect.  The forces due to the interaction of the solar and terrestrial magnetic fields are only very small perturbations.
(October 2000)
How do the magnetic poles reverse?  Is it for the same reason that the Sun's poles reverse?
Both the Sun and the Earth are electromagnets, not permanent magnets (despite the Earth's core being made up of iron and nickel).  So it is electric currents moving through the plasma of the Sun and the molten rock of the Earth's interior that generate the magnetic fields.  These currents have instabilities that build up until the field reverses to relieve stress.  The fact that the magnetic pole moves is tied in with the same instabilities, but the flip probably happens pretty quickly on Earth (less than ~ 1000 years is all they can tell).
Dr. Eric Christian
Changes in Earth's Gravitation and Magnetic Field?
Where can I find historical daily data on the Earth's gravitation and magnetic field?
As far as I know, there is no significant change of Earth's gravitation with time, because this is only a function of the Earth's size and mass, both of which are not changing significantly over time. There is only one regular change of the gravitational acceleration over time at a fixed location, and that has to do with the reason for the tides. The gravitational forces of the Moon and the Sun at a specific location change with their relative position in the sky. The effect is of the order of a few 100,000ths of the gravity at the Earth's surface. However, I am not the instant expert on precision gravity related questions.
As far as variations in the Earth's magnetic field at the surface are concerned, two different causes must be distinguished:
The natural magnetic field of the Earth changes over time due to changes in the strength and configuration of its core, which are driven by the hot liquid interior of the Earth.
The magnetic field changes because of gigantic currents that flow around the Earth in the magnetosphere .  Depending on the space weather conditions, the currents in the magnetosphere change over time, and so does the Earth's magnetic field.
Neither type of change is just a change in the strength of the field. The direction of the magnetic field also changes. Therefore, the knowledge of magnetic field strength over time at only one location would not be sufficient to track changes in the Earth's magnetic field.
A detailed introduction into the Earth's magnetic field and related changes, with links to databases, may be found at the National Geophysical Data Center maintained by NOAA.  On this page, General Information provides a very good introduction. Magnetic Declination On-Line allows computation of the magnetic declination (the direction a compass needle will point) for any location on Earth and for any date from 1900 to today (and even into the near future). The World Magnetic Model provides maps with magnetic parameters for specified locations or the entire globe.
Dr. Eberhard Moebius
Distance From Earth's Surface to Space
What's the distance from sea level to the beginning of space?
Earth's atmosphere reaches over 560 kilometers (348 miles) from the surface of the Earth. You can read more about it at the NASA homepage .
Beth Barbier
Earth's Atmosphere and Radiation
A question arose in my astronomy class yesterday when we were discussing the electromagnetic spectrum and the telescopes that study different wavelengths of light.  We put telescopes in orbit to study gamma rays, X-rays, and ultraviolet because those wavelengths cannot penetrate the atmosphere.  But if these have so much energy, why don't they get through the atmosphere?  Also, does the atmosphere absorb or reflect them?
The question of whether light/photons reach the Earth's surface depends on several factors. Recall that light can get both absorbed and scattered by molecules in the atmosphere. The fate of a given photon as it enters the atmosphere depends on the chemical nature of our atmosphere. The Earth's atmosphere is primarily nitrogen and oxygen.
The basic idea to keep in mind is that the atoms and molecules have discrete energy levels which are known to be quantized (from quantum mechanics). Photons also have quantized energy. Because atoms or molecules require discrete energy boosts in order to be excited (i.e. cause an electron to transition from a low energy level to a higher one), the arriving photon, in order to get absorbed, must carry at least the difference in energy between the low and high energy states of the atom.  If the photon is of even greater energy, it may also get absorbed by kicking out the electron altogether (with the electron departing with some extra kinetic energy).
So, photons will get absorbed depending on the chemical/atomic structure of the molecules in the atmosphere and on the energy of the incident photons. For instance, there are two compounds responsible for absorption in our atmosphere in particular: oxygen (O2) and water vapor (H2O).  Molecular oxygen and ozone are the strongest absorbers in the ultraviolet and water vapor, while methane, nitrous oxide, ozone, and carbon dioxide absorb in the infrared.  X-rays can be absorbed in the atmosphere by individual nitrogen or oxygen atoms since they generally have enough energy to kick out an electron (and the chances of seeing an individual atom in a thick atmosphere such as Earth's are pretty high!).
For more on how radiation interacts with matter, see the Interaction of Radiation with Matter at HyperPhysics.
Dr. Georgia de Nolfo
Earth's Atmosphere Over Time
I have read that the Earth is shrinking at 5 feet per year. Working backwards, it would expand 5 feet per year at it current size.  Considering the temperature of the Sun and the effect on the Earth, as we go back in time, how much effect would the Sun exert on the Earth's atmosphere each million years past (1 million, 10 million, 100 million, etc.)?
I'm sure that this number (5 ft/year) is WAY too large.  That would put the Earth 1000 miles larger a million years ago.  5 feet per million years is probably more like it.  The effect on the atmosphere would be small, because it is the total amount of mass of the Earth that provides the gravity that holds the atmosphere in place, and that is only increasing (about 1 ton per hour from micrometeorites).  Over the billions of years that the Earth has been around, it has lost its atmospheric hydrogen and some of its helium just by random diffusion.
Dr. Eric Christian
From the Earth to the Moon
What is the distance from center of the Earth to center of the Moon ?
The National Space Science Data Center (NSSDC) at NASA GSFC has a page on the Moon with the distance from Earth listed as 384,467 km.  It would be standard practice to give center-to-center distances.
What is the distance between the surface of the Earth and the surface of the Moon?
With the center-to-center distance above and the diameters of the Earth and the Moon, you could figure out the distance from surface to surface. From The Nine Planets , the diameter of the Earth is 12,756 km, and the diameter of the Moon is 3,476 km. Just subtract half of the diameters (= the radius), from the center-to-center distance:
Radius of Earth: 6,378 km
Radius of the Moon: 1,738 km
Surface-to-surface distance: 376,351 km
Mountains on Earth and Moon
If the Earth and Moon formed at about the same time, why are mountains on the Moon so smooth compared to the mountain peaks on Earth?
The mountains on the Moon and on Earth, for the most part, formed in completely different ways.  Because the Moon is so much smaller, it cooled most of the way through, and so is mostly rocky now, although it was molten at one time.  Most lunar mountains are caused by impact craters.  On the Earth, the upper rocky crust floats on the molten mantle, and mountains are caused by plate tectonics, when enormous plates of the crust bump up against each other.  The deepest craters on the Moon are about the same size as Mt. Everest, however.
Dr. Eric Christian
Asteroids and the Atmosphere
In photos, you can see that the Moon is battered from asteroids and meteors, but the Earth has been hit but isn't battered.  Is this because of the Earth's atmosphere?  Why does the Moon have no atmosphere?
You are asking two related questions, so let me tackle them in sequence. First, you asked why the Moon is covered with impact craters from meteors and asteroids, while the Earth seems to be much luckier in that respect. You answered part of that question yourself: the Earth has an atmosphere, which protects its surface from meteors up to a moderate size. These meteors evaporate upon entry into the atmosphere and can be seen as "shooting stars."  Larger meteors, though, reach the ground. There are quite a few famous meteor craters on Earth, for example, the Meteor Crater near Flagstaff, Arizona, or the Noerdlinger Ries, an almost perfectly circular valley in Germany. Admittedly, these are fewer craters than are found on the Moon, and most of the Earth's craters are much younger. In fact, the Earth was pelted with many impacts at the same time when the Moon was hit so badly, up to about 3.5 billion years ago.
Why don't we find these craters anymore on Earth? The answer to this part of the question is that the Earth is geologically active and has a severe climate, which leads to hefty erosion by water, ice and wind over the long time scale of billions of years. In other words, the Earth's surface has been completely reshaped several times over by the ongoing plate tectonics, which moves the continents around and builds up mountain ranges. On the other hand, all these formations, including impact craters, are subject to severe weathering already on the scale of hundreds of millions of years.
Let me come back to your second question: Why does the Earth have an atmosphere and not the Moon? The answer lies in the fact that the Moon features so much less mass than the Earth; it has only about 1/80 the mass of the Earth. As a consequence the strength of gravity on the Moon's surface is about 1/6 that on Earth, and the escape speed, i.e. the minimum speed that an object must have to escape the gravitational grip of the Moon, is much less (about 1/5) than that for Earth. (For the return trip from the Moon the Apollo lander had to achieve only a much lower speed than the Saturn rocket when leaving the Earth.) Therefore, atoms and molecules of a gas are much more likely to escape the Moon than the Earth. Being at the same distance from the Sun, the average temperatures on Earth and the Moon are very similar (not the extremes because of the differences in the atmosphere). Therefore, the average energy of the atoms and molecules in a gas, or their average speed, is the same on Earth and on the Moon. As a consequence the Moon has lost all of its gases in the distant past. To play with the idea of keeping or losing an atmosphere you may want to try out an in-class activity from my general astronomy class.  You will find it at
Why Does the Moon Stay?
When using the Universal Gravitational Constant, it shows that the force between the Sun and our Moon is greater than the force between the Earth and our Moon. Why does the Sun not pull it away from the Earth?
Because the Moon is in orbit around the Sun (as well as in orbit around the Earth).  Another way of looking at it is that the center of gravity of the Earth-Moon system is revolving around the Sun, and the Earth and the Moon are revolving around their common center of gravity.  The Sun is pulling on the Moon, but the Earth is "falling" right along with it, so they stay together.
Dr. Eric Christian
What's the difference between a solar and a lunar eclipse?
You might want to look at the Sky and Telescope eclipse page .  It addresses this question.
Beth Barbier
Moon Dog
The other night I looked up into the sky and saw a large ring around the Moon.  I was told it might be a "Moon Dog".  Is that true?
It's possible that you did see a "moon dog".  Check out the Imagine the Universe! site's answer on this subject.
Beth Barbier
Position of Earth in Space
There is probably a great deal of influential motion beyond our supercluster of galaxies. The local group moves to keep pace with the supercluster.  Our Milky Way, with increased speed and a longer path to follow, races to maintain its position. Our Sun orbits the galactic center at a higher speed and covers a greater distance. Earth must travel a still greater distance and speed to orbit the Sun. Would it be wrong to assume that the Earth's position in space is determined by ALL this galactic motion?
And why can't we assume that the boundaries of atoms are set by heavenly motion? Is it possible that what we call "the strong nuclear force" is nothing more than the result of small entities racing in acute paths, keeping pace with larger bodies?
Although the Earth's position in space IS determined by the sum of various motions (Earth around Sun, Sun around Galaxy, Galaxy in supercluster, etc.), there is NO evidence that this has anything to do with atoms.  Why should the boundaries of atoms be set by heavenly motion?
The theory of General Relativity fits the motions of the Earth extremely well, and Quantum Chromodynamics explains ALL observations of the strong nuclear force.  Your hypothesis would have the strong force be different at different places and velocities.  The evidence (stars, etc. seem to "burn" the same throughout the Universe) is against you.
Dr. Eric Christian
Speed of the Earth
I am curious to find the speed of the Earth around the Sun.  A measurement in either km/h or mph would be most helpful.  I believe the formula is something like distance traveled / time =3D kmh or mph (d/t=3Dkmh mph).  The time is approx 365 days or approx 8760 hours.  How do I find the distance and is my formula correct?
The average radius of the Earth's orbit around the Sun is 93 million miles, so the distance is
radius * 2 * PI = 584 million miles
584,000,000 miles / 8,764 hours = 66,660 mph
Dr. Eric Christian
Why Do We Always See the Same Side of the Moon?
I'm a 5th grade science teacher, and my students have a hard time understanding the concept that the same side of the moon always faces Earth, even though the moon rotates on its axis. Do you have any suggestions that might help them?
One good activity involves having students play the parts of the Earth and the Moon. There's a description on this Astronomical Society of the Pacific site .
Beth Barbier
Rotation of the Moon
According to my science text book, the Moon rotates on its axis in 29 1/2 days. The Moon also revolves around the Earth in 29 1/2 days. Yet in one of the questions, the book said that the Moon spins on its axis in 27 1/3 days and the time from one full moon to the next was 29 1/2 days. I looked in the encyclopedia, and it said the Moon rotates on its axis and revolves around the Earth in 27 1/3 days. Which one is correct?
The Moon rotates on it's axis and revolves around the Earth in 27 1/3 days.  The time from one full moon to the next is 29 1/2 days. The reason they are not the same is that during the 27 1/3 days that it take the Moon to go around the Earth, the Earth has moved relative to the Sun, and it takes another 2 days or so before the Moon is directly on the other side of the Sun (which is what gives a full moon). It is similar to the reason why the Earth rotates on its axis once every 23 hours 56 minutes and 4.1 seconds, but its 24 hours from noon to noon.
Dr. Eric Christian
"Far Side" of the Moon
My students are having trouble understanding that the same side of the Moon always faces Earth, even though the Moon rotates on its axis.  Do you know of any good models or demonstrations that might help them understand this concept?
One good activity involves having students play the parts of the Earth and the Moon. There's a description on this Astronomical Society of the Pacific site: The Moon: It's Just a Phase It's Going Through...
Beth Barbier
Origin of Moonlight
What is the origin of the light on the Moon? Is it a reflection of the sunlight that hits the Earth or is it light that hits the Moon directly from the Sun?
Most of the light from the Moon is direct reflection of sunlight.  A lunar eclipse happens because the Earth's shadow crosses the Moon, preventing the Sun from directly illuminating the Moon.  You can see some "earthlight", reflection of sunlight that hits the Earth, then hits the Moon and bounces back to the Earth.  When there is a crescent moon, you can frequently (if the sky is dark) still see the "unlit" portion of the Moon.  That light (much dimmer than normal moonlight) is from the Earth.
Dr. Eric Christian
Shadows on the Moon?
Does an object on the Moon have a shadow?
Shadows are caused by sunlight, and since the Sun shines on the Moon, there must be shadows. In fact you can determine the height of objects on the Moon by the length of their shadows (just like on Earth).
Dr. Louis Barbier
Always Night on the Moon?
Why are all pictures of the Moon taken at night?
I'm not certain what you're referring to here.  If you mean that pictures on the Moon show a black sky, you should know that there isn't an atmosphere on the Moon to reflect sunlight, so its sky always looks black.  The surface of the Moon, however, does reflect sunlight, so the surface facing the Sun is well-lit, and that side is experiencing daytime. (Think about the definition of day and night on Earth.)
Beth Barbier
Water on the Moon
How much water was in the hole that the crater made that hit the Moon?
Water was found to exist as very small, very spread out ice crystals in craters on the north and south poles of the Moon.  Craters are the result of ancient meteorites that hit the Moon's surface.  The amount of ice is very uncertain, but estimates are 10 - 300 million tons spread over the polar regions.
Dr. Eric Christian
Meteorites and the Moon
What is the name of the force that causes meteorites to impact on the Moon? Is it gravity?
The short answer is "no". Not directly at least. Meteoroids impact the Moon (the Earth, and other planets) because the trajectory of their orbits and that of the Moon intersect. Of course, gravity is responsible for the shape of the orbits themselves, so one might indirectly say gravity is responsible. But not in the way I think you are asking.
Imagine, for example, when you fire a bullet from a gun at a target. While it is true that there is a gravitational attraction between the bullet and the target, the bullet would still hit the target if there were no gravity. Its trajectory will intersect the target location.
Meteoroids are of two distinct classes. Some revolve around the Sun (like the planets do) and have orbits of small eccentricity (nearly circular) and those orbits are near the plane in which the planets orbit. The more common type is associated with debris streaming off comets as they approach the Sun. This debris ends up scattered along the (highly eccentric) orbit of the comet. As the Earth's orbit passes through these debris "clouds" we have meteor showers. Since the Moon is moving along with the Earth, it will often experience many of these meteoroids too, which will strike the Moon's surface.
Dr. Louis Barbier
Dust on the Moon
I recently saw part of a religious television show explaining that when the Apollo mission landed on the Moon, they were expecting about 40 - 50 feet of dust, due to the Moon's gravitational field pulling in the dust and its lacking an atmosphere to burn it up.  They proclaimed that because there was "...only ten thousand years worth of dust...", therefore the Universe is only 10,000 years old, and God must have created it.  What's the scientific explanation?
The first attempt to measure the amount of meteoritic dust falling onto Earth was made by Hans Pettersson in the 1950s.  His measurement came up with a maximum infall rate across the entire Earth (what scientists call an upper limit) of about 15 million tons per year.  His sample was contaminated by volcanic dust, etc., and the real number (measured out in space) is only about 20,000 - 40,000 tons per year.
Creationists ignore the new measurements, and the fact that Pettersson's value was an upper limit, and misinform the public.  Interestingly enough, there are plenty of places on the Moon where the dust is more than 100 feet deep, but most of the dust is from meteoric impacts on the Moon itself throwing up debris.  NASA was NOT expecting a deep layer of dust where Apollo landed (in the highlands).  Remember, we had already landed unmanned probes on the Moon before Apollo (Surveyor).
For more information, you can check the Talk.Origins archive .
Dr. Eric Christian
Your Weight on the Moon
If the Moon size is about 1\60 that of the Earth, then why do we only weigh 1\6 on the Moon of what we weigh on Earth?
The mass of the Moon is less than 1/80 (0.0123) that of the Earth, and its diameter is a little more than a quarter (0.273).  Gravitational acceleration is proportional to M / (R * R), so for the Moon (0.0123) / (.273 * .273) = 0.165 or about 1/6 that of the Earth.
Dr. Eric Christian
Moon's Gravity
In my class, I am part of a team role-playing that we on the surface of the Moon and become lost.  We must figure out how to launch a rescue beacon. How strong is the gravitational pull on the surface of the Moon?  Specifically, what kind of force would be necessary to lift a rescue beacon satellite from the surface into the Moon's orbit?  Would a weather ballon be able to lift the rescue beacon off the surface?
The gravity on the Moon is about 1/6th that of Earth, which comes out to about 5.3 feet per second per second acceleration (32 * 0.165 = 5.28 feet/sec/sec).  To get something off the surface of the Moon you'll need energy equal to the mass of the object times this acceleration times the height you want to achieve (M x G x H).  A balloon gets its lift for the same reason a boat floats, it weighs less than what it's displacing.  If there is no air to displace, you get no lift.
Drs. Eric Christian and Louis Barbier
Compass on the Moon
On the Moon, would a compass point at the Earth?  I though it might when the Moon is near enough in the magnetic field of the Earth.
The magnetic field of the Earth only extends about a quarter of the way to the Moon.  So the compass wouldn't point at the Earth, and since the Moon has nearly no magnetic field, it wouldn't be much good at all.
Dr. Eric Christian
Magnetism on the Moon
The Moon doesn't have much of a magnetic field, so would a magnet stick to an iron bar on the Moon?  Could the magnet induce magnetism in the iron bar?
Definitely, a magnet would stick to an iron bar on the Moon. In fact, it will stick to an iron bar any place in the universe, provided the temperature is not excessively high. The effects of magnetism, such as a magnet and iron sticking together, are based on the presence of a magnet and have nothing to do with the question whether the environment is within a larger magnetic field or not. If astronauts bring a permanent magnet to the Moon they will observe that it will attract any piece made of iron.
They only question may be whether the magnet keeps its magnetic quality or not. If a magnet is heated to more than about 700 degrees centigrade, it will lose its magnetism and behave like an ordinary hunk of iron. However, on the Moon it is not hot enough that this would happen. So the magnet will still work.
Now to the second question: will the magnet induce magnetism on the iron? Yes, it will. Again, any permanent magnet will induce magnetism on a piece of iron on the Moon. The only difference between the Earth and the Moon is that the Earth's magnetic field can induce magnetism on a piece of iron by itself, similar to what a refrigerator magnet can do. This can't happen on the Moon, but any magnet still can induce magnetism there.
Dr. Eberhard Moebius
Flashlight on the Moon
Will a flashlight work on the Moon?
Nothing in the flashlight (basically just batteries, wires, switch, and bulb) requires air, so it would work fine.
Dr. Eric Christian
Wind on the Moon?
In a picture the American flag on the Moon appears to be waving in the wind.  How is this explained?
There is no air on the Moon.  NASA embedded stiff wire in the American flag so that it wouldn't just hang straight down, and adjusted the wire so that the flag appeared to be waving.  They thought (probably rightly) that a flag as flat as a board wouldn't look right.
Dr. Eric Christian
Parachute on the Moon
Would it be possible to use a parachute to descend to the Moon from a mile above the Moon's surface?  I believe it wouldn't be as effective as on the Earth, but could you specify how effective, if at all?
Parachutes use air resistance to slow down a descent.  Since there is no air on the Moon, they would be completely ineffective.  The parachute would fall at the same rate as a rock.
Dr. Eric Christian
Gunfire on the Moon
Recently, I was involved in a debate about the usefulness of a gun on the Moon.  Some people argued that it wouldn't fire because oxygen would be required for combustion. Others thought that it would fire, and possibly could be used as a propulsion device.  Which is the correct answer?
Actually, both arguments have some element of truth.  You do need oxygen to ignite the firearm and space is for all intents and purposes a vacuum (with no available oxygen to take part in combustion).  Now engineers have been able to get around this difficulty over the years with rocket fuel by mixing both a fuel and an oxidizer together in the rocket's combustion chamber.  For example in solid rocket boosters (SRB) the typical mixture consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent).  You can read more about SRBs on the Kennedy Space Center site.
Now as it turns out, guns are also self-igniting systems, where the mixture of fuel and oxidizer is usually located inside the bullet itself.  So indeed, guns can be fired in space, although you won't hear the typical accompanying "boom"!  Of course, a firearm could indeed be used as a method for propulsion, although there may be more efficient ways to acquire propulsion in space.  See this site on rocket propulsion for more details on propulsion in space.
Dr. Georgia de Nolfo
Moon Survival Test
When I was in high school, I was given a test about surviving on the Moon with a limited number of items.  The test asked me to rank the items by importance.  Do you know where I might obtain a copy?
The StarChild website has such a test: Problems in Space .
Beth Barbier
Moon Leaves Earth's Gravity?
Will the Moon leave the Earth's gravity?
The Moon will not leave the Earth's gravity, even though the orbit of the Moon is increasing slightly.  The Earth's rotation is slowing down (due to "tidal braking"), and to conserve angular momentum the Moon is accelerating.  The Moon's orbit increases by about 3 cm/year.
The Earth and the Moon eventually will be "locked" together with each only having one side constantly facing the other.  (Right now the same side of the Moon faces the Earth, but all sides of the Earth see the Moon. In the future this will not be true!) Life on Earth will be quite different then, but this won't occur for billions of years yet. When it does occur, the Moon's orbit will be 50% larger than it is now, and a month will be about 50 days.
Dr. Louis Barbier
See Great Wall from the Moon?
Can you see the Great Wall of China from the Moon?
The Great Wall can barely be seen from the Shuttle, so it would not be possible to see it from the Moon with the naked eye.
Beth Barbier
The Name of the Moon
Why does the Earth's moon not have a name (like Saturn's moons:  Titan, Mimas, etc.)?
The Moon was called Selene or Artemis by the Greeks and Luna by the Romans. I'm sure other cultures also had names for the Moon.  But in English, Moon (from Mona and Moone in Old and Middle English)  was used before anyone had any idea that the other planets had moons.  So it was more a case that the specific name for the Moon was extended to mean small bodies revolving around planets elsewhere.  The Moon's name is the Moon.
Dr. Eric Christian