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Earth's Moon: FAQ
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It is easy to believe that the moon doesn't rotate (spin) since we always see the same face, the near side. But we would also see what we call the far side (it's not dark; it has a day/night cycle just like the near side) if the moon were truly not rotating with respect to the stars as it orbits Earth.

You can demonstrate for yourself that the moon must be spinning with respect to the stars. For the demonstration you will need help from a friend and a rotating office chair. Place the chair in a room so your friend can comfortably sit in the chair, with her arms fully extended in front of her, and rotate around a full circle without bumping into anything. Have your friend hold a magazine with its front cover facing her. Your friend's head is standing in for Earth and her eyes are pretend people watching the magazine standing in for the moon. The walls and furniture in the rest of the room serve as background, unmoving stars in the sky. Pick a spot out of the way and watch.

As your friend slowly turns herself on the chair ask her, several times during one full turn, if she sees the front cover of the magazine as she goes around a circle and if the view of it is changing. She will answer that she sees the front cover and it is not changing, since she is simply holding the magazine at arm's length. She will see the background room (stars) change as she makes her full turn.

What do you see when she rotates the chair? If she starts facing you, you see the back cover of the magazine. As she rotates around, you see the back cover appear more and more edge on. By the time she has made 1/4 turn you see the magazine edge on and neither cover is visible. As she continues to turn you start to see the front cover and by the time she completes 1/2 turn you see the front cover as she is seeing it (as long as her head isn't blocking your view). As she continues turning, the front cover starts to appear smaller and by 3/4 turn the magazine is edge on again. Continuing the turn, the back cover appears until it is fully facing you at the end of her turn. The magazine has made one rotation as it has made one revolution around your friend: this is called synchronous rotation because the rotation (spin) takes the same amount of time as one complete orbit (turn around your friend).

Suppose the moon (magazine) is not rotating with respect to the stars (room). Now your friend picks one wall to keep the back cover facing. As she slowly turns on the chair she also has to slowly turn the magazine to keep the front cover facing the chosen wall. She sees the magazine's front cover slowly turn to edge on at 1/4 turn. Then she starts seeing the back cover until it is full face on at 1/2 turn and then the magazine shrinks to edge on at 3/4 turn. Finishing the turning the front cover now grows to full face on at the end of the turn. At the same time you are seeing the magazine changing its position as it circles your friend but the same cover of the magazine is always facing you.

The moon is rotating at the same rate that it revolves around Earth so we see only the near side hemisphere. It is in synchronous rotation with its orbit.

My friend said that the moon landing is fake because of the flag. When you put it on the surface, there is no wind so how could the flag move?

The flags left on the moon by the Apollo astronauts were all supported across the top by a rod extending outward from the top of the flag pole. This allowed the flag to hang fully spread and be displayed in its full glory. You can see the astronauts assembling the flag pole and extension and then placing the flag at http://youtu.be/k1B2UPkelNw. The only reason the flag moves is because the astronauts are setting it up. For a few seconds after they let it go, it swings a little bit and then that dies out and there is no more motion.

The only "wind" on the moon came when the Lunar Module, carrying the astronauts, turned on its rocket engine and lifted off the surface to rendezvous with the command module orbiting the moon with the third astronaut. The engine exhaust briefly creates wind as it is deflected sideways by the LM's descent stage (which stayed on the moon). (After exiting downwards from the engine in the ascent stage, the exhaust went sideways until the ascent stage was high enough that the exhaust no longer impinged on the descent stage or surface.) You can see the ascent, as captured by the lunar rover camera on Apollo 15 at http://youtu.be/BMBcLg0DkLA It shows debris scattering everywhere, pushed along by the rocket exhaust. You can see the view from the ascending Lunar Module at http://youtu.be/Db_6eF5Zsg0. It displays debris caught and carried by rocket exhaust sideways.

I don't understand why some people think the landings on the moon were faked. Americans built the hardware, went to the moon, and brought back rocks from there.

Stephen Edberg
Astronomer
NASA Jet Propulsion Laboratory
California Institute of Technology


The book "Astronomical Algorithms" by Jean Meeus (Willmann-Bell) has a chapter on calculating lunar perigees and apogees. I also found a large number of websites with a Google search using "calculate moon's perigee and apogee". I expect you can find what you are looking for and check your adopted approach with these resources. I am also fond of the Royal Astronomical Society of Canada's annual Observer's Handbook, which provides events for each day of the year by month, including perigees and apogees.

Stephen Edberg
Astronomer
NASA Jet Propulsion Laboratory
California Institute of Technology

We think that the moon and Earth formed at about the same time, back when our whole solar system was formed. Earth was forming from many chunks of rock and icy material. Possibly a big chunk hit the new Earth and knocked loose a big piece, which became the moon.

As you may have learned, the moon doesn't have any air around it. The air that surrounds our earth acts as a nice blanket to keep us warm and comfy! But the moon, since it doesn't have this blanket, gets much colder than the earth - and much hotter than the earth. On the side of the moon that the sun is shining on, the temperature reaches 260 degrees Fahrenheit. That is hotter than boiling. On the far side of the moon, it gets very cold, -280 degrees Fahrenheit.

The surface of the moon is covered by several feet of broken rock and dust called regolith This material is formed by the impact of meteors. You can see this regolith in some pictures taken by the astronauts of their footprints on the moon.

We call those holes "craters." They are the places where many years ago meteors hit the surface of the moon and put dents into it. There are thousands of big craters, but even more little ones. There are probably millions of little craters on the moon! Some are only an inch or so across.

The moon is made up of various kinds of rocks. These rocks are fairly similar to the rocks on earth. But on earth, we have wind and rain that help wear the rocks down into sand and dirt. There is no air or wind on the moon, so the rocks don't get worn down as they do on the earth.

The bright part of the moon is the part that the sun is shining on. This is like daytime on earth. The dark part is in shadow, like night on earth. Now the moon goes around the earth once every 29 days (approximately).

At new moon, the moon and the sun are on the same side of Earth. We see the part of the moon that is in shadow, so the moon is dark. Then the moon moves around in its orbit. At first quarter, it has gone one-fourth of the way around Earth. Now we can see part of the moon that is sunlit, but part still in shadow. Note that if the sun is setting in the west, the bright part of the moon is on the side toward the sun and the dark part is away.

About a week later, the moon has moved halfway around its orbit. Now it is on the opposite side of Earth, away from the sun. Now we see only the sunlit side - that is the full moon. Note that if the sun is setting in the west, the moon is just rising in the east.

About a week later, the moon has moved now three-fourths of the way around in its circle around Earth. Once again only part of the moon is sunlit and part is dark. Now you can see the moon in the morning, and note that once again the sunlit side is on the side towards the sun, and the shadow side away. Another week and we are back to the new moon.

It's easier to demonstrate if you have a ball to represent the moon and a flashlight for the sun. Have someone stand several feet away, holding the flashlight so it shines on the ball. Hold the "moon" ball and slowly turn around, watching the moon go around you (you are Earth). Do you see the moon's phases?

Anytime there are three bodies (the sun, the moon, or planet) lined up so that one blocks the light from another, we call that an eclipse. During a solar eclipse, our moon moves between us (on Earth) and the sun and blocks the sunlight. During a lunar eclipse, Earth blocks the sun's light that normally lights up the moon. Since we are standing on Earth, what we see is that the moon gets dark. Other kinds of eclipses happen too. For instance if you were standing on the surface of Jupiter (kind of hard, but we can imagine) you might see one of its moons eclipse the sun.

The reason that you don't see the stars during the day is that the sky is too bright. Sunlight scatters around in the air and makes the sky look bright blue. But if you had a telescope and pointed it at a bright star you could still see it during the day! The stars are still there, just hard to see. The moon is bright enough that we can see it during the day or night. It orbits Earth once every 29 days. So during some of that time, it is easiest to see during the day and sometimes during the night.

Yes, the moon has some volcanoes. But as far as I know they are all "dead" volcanoes that have not erupted for millions of years. Most of the craters on the moon are from the surface being hit by asteroids and comets billions of years ago. The moon is a very "quiet" place. There is no air or water to erode the surface, and there are no large earthquakes or active volcanoes to change the surface. Only small rocks may still hit the surface. So it has not changed much in billions of years! Probably the biggest changes recently are the footprints from the astronauts that visited the moon about 30 years ago!

To have rain or snow, we need to have water and an atmosphere of some kind. The moon has no significant atmosphere, so it has no tangible weather at all! Mars has only a very thin atmosphere but it does have weather. Strong winds can blow up big dust storms. Pictures from the Mariner spacecraft show that sometimes thin frost forms on the surface of the planet. Sometimes just after Martian dawn, we see an icy fog rising from the craters! I believe that it is too cold for rain, but frost and icy fogs have definitely been seen. And of course, Mars has polar caps of frozen water and carbon dioxide ("dry ice"). Perhaps it snows at the polar caps. The atmosphere of Venus is very thick and very hot. There is a little water in its clouds, but I don't believe it ever rains. Mercury has no atmosphere. The outer planets - Jupiter, Saturn, Uranus, Neptune, and Pluto - are extremely cold. Their atmospheres are mostly made up of methane, ammonia, nitrogen, and stuff like that. There are probably some ice crystals in their atmospheres too, but they probably just blow around in the strong winds. So there might be a sort of "snow" but not very much like what we are used to on Earth.

Water that would be found on the moon may have existed from the days when our solar system was formed. Comets that may have hit the moon could also be a source of water. Generally we think water, that was part of the moon as it formed, would have probably evaporated away. Water from comets would have evaporated too. However, the area where Lunar Prospector found the possible signature of water is at the very cold south pole of the moon, in a dark, cratered area where the sun never shines. So it seems possible that the water (or ice) has survived there. We are hoping that other observations can be made with other satellites that can confirm whether this is really water on the moon. If so, it would be a great help for human space travel in the solar system!

You could plant something, but it would die. There is no atmosphere (it needs carbon dioxide) and no water. The sunlight would burn it during the lunar day, and in the nighttime it would freeze. I don't know if the soil would provide the nutrients that it would need, because it is just rock dust; there are no organic components that earth plants need to fix nitrogen, and so on. Life on earth is very special and very precious!

We think that the ice on the moon came from comets! Comets are made up of mostly ice with some rocks and dirt mixed in. We think that most of the water on the earth probably came from comets that crashed into the earth when the earth was very, very young. The ice on the moon may have come the same way. Anoither source of water might be the hydrogen that comes to the moon in the solar wind. Most of the water on the moon evaporated away a long time ago. But the ice at the South Pole stayed there because it is very, very cold and is in a dark area where the sun never shines.

Yes, it is! But it is moving only about an inch farther away each year.

No, I don't think so. The moon is way too small in mass (too little material) to have ever been a star.

Galileo was responsible for naming the major features on the moon. You may know that he was the first person to study the night sky using a telescope. He thought the dark, smooth areas were seas, and called them "maria" (Latin for seas; "mare" is the singular). For instance, the first Apollo landing occurred in Mare Tranquilitatis (the Sea of Tranquility). Of course we know now that there are no seas. The "seas" look flat from ancient lava flows. But the names stayed.

The gravity on the surface of the moon is one-sixth of Earth's, so the astronaut could certainly throw that rock a lot farther. Did you know that one of the Apollo astronauts took a golf club to the moon and hit a golf ball a really long way? Even so, the gravity is strong enough that the ball or rock would not go into orbit or leave the moon. But it would go six times as far.

Of course we know that this can't happen, because there is no air and a plane couldn't fly fast enough to escape the earth's gravity. But we can pretend. A 747 airplane normally flies at about 400 miles per hour. The moon is about 250,000 miles away. So if we divide 250,000 by 400, we find that the plane would take 625 hours - or 26 days - to fly to the moon! Boy that would be a looong trip! Twenty-six days of eating airline food - yuck!

It depends on how fast the spaceship can travel. When the Apollo astronauts went to the moon, it took about two days.

Have you looked at the moon and noticed the dark patches? Some people think that they make the moon look like it has two eyes and a big smile. The next time the moon is nearly full, it would be a good time to look in the early evening at the moon and see if you can see the "face." In other cultures people see different things on the moon. The Japanese people talk about the rabbit on the moon. I have looked at the moon and seen the "rabbit" too - it looks like a rabbit is walking up the left side of the moon. You might want to look for the rabbit too.

The moon is something that even the cavemen must have seen and given a name to. Maybe something like "big light in the sky at night when the sun isn't around." According to my dictionary, the Old English word for the moon was mona. In Latin it was mensis. In Greek it was mene (mee-nee). The words moon and month come from the same roots. That is probably because a month was originally measured by the phases of the moon. It takes 29.5 days for the moon to go from full moon to full moon. But there have been many changes to the calendar since that was true, so now months are a little longer and people don't pay too much attention to the phases of the moon anymore.

Right now NASA has no immediate plans to send a human back to the moon. NASA scientists and engineers have been studying how to live on the moon, so it is probably possible.

Some people talk about the Dark Side of the moon as if it is a specific place, but this isn't correct. As the moon orbits Earth, different parts are in sunlight or the dark at different times. It takes roughly 29 days for the moon to circle Earth. Since it keeps the same side toward Earth, this means that the moon turns once every 29 days. This is hard to visualize, but you can try it with a ball (for the moon) and a flashlight (for the sun, and you as Earth), perhaps with some help from your teacher. This is also why the moon has phases.

When the astronauts went to the moon, they wanted to be on the side facing Earth so they could communicate with us, and also they wanted to be in the sunlight so they could see. So they went around the full moon. They stayed only a few days. If they had stayed for two weeks, they would have ended up in the dark during the new moon!

If we sent a space probe, we would have to decide where to put it based on what kind of studies it would be doing. For instance, if you wanted to study radio waves from the stars, you might want to be on the far side of the moon so you wouldn't get any interference from Earth's TV and radio waves. But you would also have to set up a communications relay station so you could communicate with the probe.

The moon actually CAUSES the tides. If there were no moon, we would have no tides. The tides arise due to the pull of the moon's gravity. On the side of Earth nearest the moon, the moon's gravity is the strongest and it pulls up the water slightly (high tide). On the side of Earth furthest from the moon, the moon's gravity is the weakest and the water can move a little away from the moon (which is also high tide). This also affects Earth itself. During high tide Earth rises by an inch or two, not enough for us to notice.

Actually everything DOES reflect sunlight. If something doesn't reflect light, it looks completely black. There aren't many things like that around. If you stand outside in the sunlight, you are seeing because the sun's light is bouncing off of everything and your eyes see that light. When you are inside, you see things because the light from the lamps or the fluorescent lights bounces off things in the room.

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Last Updated: 1 Jul 2014