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112 votes

Why can we see the moon when it is between the Earth and the Sun?

The premise of this question is wrong. If the moon is in between the earth and the sun (as shown on your diagram), and you can see the moon, then it is day, not night: If on the other hand, you are ...
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100 votes
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Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

The trouble with orbital mechanics is that it rapidly gets exceedingly complicated and hard to make intuitive sense of. However I think there is a reasonably straightforward way to show how little ...
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77 votes

Why does the full Moon appear?

The orbital plane of the Moon around the Earth is at an angle to the orbital plane of the Earth about the Sun. This diagram was drawn to show why we do not have eclipses more often but is also ...
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71 votes
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Why does the full Moon appear?

One of the reasons people often have bad intuitions like yours about the relationship between the Earth and the Moon is because they've never seen an accurate picture. The distance from the Earth to ...
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64 votes
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Is there a maximum distance from a planet that a moon can orbit?

The concept you're looking for is that of a planet's Hill sphere. If a planet of mass $m$ is in a roughly circular orbit of radius $a$ about a star of mass $M$, then the radius of this "sphere" is ...
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63 votes

Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

The Jet Propulsion Laboratory has incorporated general relativistic effects in its numerical integration of the planets since the mid to late 1960s. For example, the JPL DE19 ephemeris, released in ...
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58 votes

If you sliced the moon in half perfectly, would it hold together?

Assuming you don't separate the halves to any great extent, it would immediately fuse back together due to its self-gravity. The tidal forces exerted by the Earth on the Moon are only sufficient to ...
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51 votes

How can a full moon be seen south of an observer's location?

Here is a picture that may help in describing this. First row: Sun (red), Earth (blue) and Moon (black) to scale (axes - in km). You can see just how far away the sun is... and on this scale the ...
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49 votes

Why does moonlight have a lower color temperature?

I refer you to the picture below, taken from Ciocca & Wang (2013). This clearly shows that the spectrum of the moon (normalised to have a similar overall strength as sunlight) is redder than ...
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48 votes
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Why can we see the moon when it is between the Earth and the Sun?

The sun doesn't just illuminate the moon directly. The moon is also illuminated by sunlight reflected from the earth. This is called earthshine. This makes the parts of the moon that face us ...
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46 votes
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I was told that if the Moon had a retrograde orbit, tides would have a faster rhythm. Why is that?

Suppose the Moon didn't orbit the Earth at all, so it just stayed at some fixed point while the Earth rotated underneath it: In this case every point on the equator would pass directly under the Moon ...
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45 votes
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If the moon had a mirror surface, would the earth be equally illuminated as by the sun during full moon, or would it require a different mirror shape?

No, because of the sizes of their surfaces. Let's make these simplified assumptions: The Earth and the Moon are both spheres 1 AU from the Sun. The total amount of sunlight an object receives is ...
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44 votes

If the moon became a black hole, how much would it eat in a day?

It'd suck in very little mass. One thing to understand about black holes is that they have super-strong gravity, but only when you are very close to their event horizon. Otherwise they're just normal ...
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40 votes

If the moon had a mirror surface, would the earth be equally illuminated as by the sun during full moon, or would it require a different mirror shape?

You seem to be asking if the reflection of the sun from a spherical mirror, a convex surface would be the same as the reflection from a flat mirror. A convex mirror is dispersive The image in ...
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39 votes

How can a full moon be seen south of an observer's location?

To answer the question, your friend needs to understand that the sun is not a little thing on the other side of the globe. It is very large, and it only looks small because it is far away. Also the ...
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36 votes

What is a simple argument to prove that the stars in the sky are further away from the Earth than the Moon?

The fact of parallax in the observed position of the moon was known in ancient times. This makes possible a calculation of the distance of the moon in terms of Earth radii. No parallax was then ...
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35 votes
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What does "rarely" mean in NASA's statement: "technically referred to as an exosphere because it’s so thin, its atoms rarely collide."?

Any given gas molecule goes a long way on average without colliding with another gas molecule. The mean free path for a gas molecule is on the order $$ \lambda\sim \frac{k_BT}{d^2p} $$ where $k_B$ is ...
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34 votes

What is a simple argument to prove that the stars in the sky are further away from the Earth than the Moon?

Lunar occultations. Just missed the moon block Mars last month. Not sure when Mars will block a star, thereby proving the stars are further than the moon.
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  • 23k
33 votes

Why is it so dark during a solar eclipse?

The most significant difference is that in a total eclipse the moon obstructs the sun's light outside of earth's atmosphere whereas at sunset, the light is obstructed by the horizon within the ...
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  • 476
28 votes

Why does a full moon seem uniformly bright from earth, shouldn't it be dimmer at the "border"?

If the moon were a uniform sphere, it would indeed appear dimmer at the edges of a full moon. Surface roughness is the main reason the edges are not as dim as the sphere model predicts. When viewing ...
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  • 12.7k
27 votes

Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

A few sanity checks without actually computing anything: First, the error due to neglecting general relativity is so small that it didn't affect prediction of lunar eclipses and wasn't actually ...
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  • 6,319
26 votes

Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

I'll start the ball rolling on this one. My GR knowledge is probably not good enough to make this a truly satisfying answer... The gravitational acceleration for an object moving radially at non-...
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  • 110k
26 votes
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Why is the sky of the moon always dark?

The moon does have a night and a day, but this isn't as fully connected to your question as you might think. The moon is tidally locked with the earth, meaning that the same side always faces earth. ...
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  • 518
23 votes

Why can we see the moon when it is between the Earth and the Sun?

While excellent answers have already been provided (yes, it's Earthshine; yes, when the Moon is between the Sun and the Earth, you don't see the Moon at night, you see it from the daylit side of the ...
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  • 1,813
23 votes

If the moon became a black hole, how much would it eat in a day?

It would suck in basically nothing. If the mass of the moon were concentrated in a black hole, you could draw a sphere around that black hole the size of the moon, and for everything outside that ...
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23 votes

Why didn't the debris collapse back into the Earth at the time of Moon's formation?

A lot of debris has probably fallen back to earth. To stay in orbit you need enough angular momentum to overcome attraction. But if the collision happened at an angle a portion of the debris could ...
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22 votes

Why is the surface of the Moon white?

The moon appears white from the Earth for two reasons. The first is that the reflected spectrum of sunlight is very broad and contains no very significant features. On this basis, the spectrum of the ...
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21 votes

If you sliced the moon in half perfectly, would it hold together?

General relativity is a more accurate theory than Newton's mechanics/gravity for calculating gravitational effects. But here, since we are dealing with a low-speed problem in the weak-field limit, the ...
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  • 4,368
20 votes
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Why is the surface of the Moon white?

The moon is actually grey. You can see this if you look at images taken in space, or, preferably, on the moon itself. For example, this one, of Buzz Aldrin: (Courtesy of NASA) But, seeing as how at ...
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  • 1,297
20 votes

Why does a full moon seem uniformly bright from earth, shouldn't it be dimmer at the "border"?

I've wondered the same thing, why the edges of the full Moon don't look darker, and why the terminator for a quarter-Moon phase doesn't look dimmer than the point at the edge of the Moon that is ...
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