88 votes
Accepted

Why does Io cast a hard shadow on Jupiter, but the Moon casts a soft shadow on Earth?

It's due to the larger relative apparent size of the Sun. When the source of light is a point source the shadow is harder, and when it is extended it is softer. Jupiter is approximately 5 times more ...
christopherlovell's user avatar
79 votes
Accepted

The moon has just the right speed not to crash on the Earth or escape into space. What are the odds?

There isn't a "Goldilocks speed" for orbit. If you put two objects in space, and give them a velocity relative to each other, then provided that velocity is less than the escape velocity (...
James K's user avatar
  • 122k
77 votes
Accepted

Is it possible to achieve a stable "selenostationary" orbit around the Moon?

First off, such an orbit wouldn't be a geostationary orbit since geo- refers to the Earth. A more appropriate name would be lunarstationary or selenostationary. I'm not sure if there is an officially ...
zephyr's user avatar
  • 15k
76 votes
Accepted

Is Earth's moon the only one where a total eclipse of the sun covers the entire sun?

As planets get farther from the Sun, the Sun takes up a smaller part of the sky. The Sun is about 31 arc-minutes when viewed from Earth, but just 6-7 from Jupiter and 3-4 from Saturn. Less than 2 ...
userLTK's user avatar
  • 24k
69 votes
Accepted

Where in space would the Earth and Moon appear to be the same size?

When viewing a sphere of radius $r$ at a distance $d$ from the centre of the sphere, you don't see a circle of radius $r$. The extreme lines of sight are tangents to the sphere, as this diagram ...
PM 2Ring's user avatar
  • 14.5k
66 votes

Is it possible to achieve a stable "selenostationary" orbit around the Moon?

As the answer by zephyr describes very well, there are very few stable orbits around the moon, and none of them are stationary. But the moon is tidally locked to Earth. That means that all of the ...
Philipp's user avatar
  • 845
66 votes

Where do we have it from that the Moon is migrating away from Earth?

There are, I think, at least four parts to this argument: the first being the theoretical argument that ties it all together and the remainder being observational evidence for the Moon's orbit ...
Peter Erwin's user avatar
  • 16.7k
65 votes
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Why doesn't the moon twinkle?

The first handful of hits on Google actually return incomplete and even wrong answers (e.g. "Because the Moon is much brighter" which is plain wrong, and "Because the Moon is closer" which is ...
pela's user avatar
  • 38.3k
64 votes

Why is moon light not the same color as sunlight?

The light from the moon is light being reflected from the sun. This is at least one reason you should not expect the Moon to have the same color. Sunlight hitting an e.g. blue object would appear ...
StephenG - Help Ukraine's user avatar
61 votes

Do our sun and moon have names?

What is a name? A name is a word, that is reasonably unique, that is used to identify a person or thing. When a child is born there is no word that identifies it, and so its parents have to choose a &...
James K's user avatar
  • 122k
61 votes

How do we know the moon is smaller than Earth?

Because when the shadow of the moon hits the earth for eclipse, it's only a small shadow that covers a little zone of the earth and lasts a brief moment. When the earth shadow passes on the moon, it ...
bandybabboon's user avatar
  • 4,250
60 votes
Accepted

Is oxygen really the most abundant element on the surface of the Moon?

Yes, that's correct; it's also true for the Earth's crust. The reason is that "rocks" are typically made up of components containing combinations of silicon or one or more metals (e.g., ...
Peter Erwin's user avatar
  • 16.7k
59 votes
Accepted

What is the **actual** average distance of the Moon from Earth?

Mean distance averaged over time for any Keplerian orbit is $a(1+\frac{1}{2}e^2)$, where $a$ is the semi-major axis and $e$ is the eccentricity. Using your NASA fact sheet, I get about 384,979 km for ...
Connor Garcia's user avatar
  • 16.3k
56 votes
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Moon's unusual gravity

As you said, the mass of the Moon is 1.2 percent that of the Earth. Now, if you mean the gravitational acceleration at the surface, it is calculated like this $G\frac{M}{R^2}$, where $M$ is the mass, ...
Tosic's user avatar
  • 1,681
50 votes

If months are based on the moon, then why are the months longer in the Gregorian calendar than lunation?

Ancient Egyptians and Mesopotamians came up with “administrative” calendars of 30 days, that were easier to calculate than “real” lunar months of sometimes 29, sometimes 30 days. At the end of the 12 ...
Pierre Paquette's user avatar
50 votes
Accepted

How does the earth cast straight and reversed shadows on the moon?

Aha! I think you'll find that the answer is that those are not photos of Earth's shadow on the Moon at all! Look at the photo of the Earth and the Moon seen from the spacecraft Voyager 1 as it was ...
uhoh's user avatar
  • 30.7k
49 votes

Does the moon have days?

The answer depends on your definition of a day. If you define a day as we usually define it in the Earth (time between the Sun is at noon or average time between sunrises, a 24 hours day in Earth), ...
Pere's user avatar
  • 1,760
49 votes

Why doesn't the Moon, like the Sun, turn bright red when it's close to the horizon?

The moon does significantly redden when it's close to the horizon, especially if you can see it over the ocean or from very flat ground, where you have a clear view all the way to the horizon. A clear ...
Darth Pseudonym's user avatar
47 votes
Accepted

If the Moon were impacted by a suitably sized meteor, how long would it take to impact the Earth?

As several people have said, this is incredibly unlikely. Part of the reason why is that the "circling the drain" effect you describe doesn't really happen for solid objects much less dense than black ...
Steve Linton's user avatar
  • 10.3k
46 votes

Can a lunar eclipse happen at night and be followed by a solar eclipse the next day?

No. A lunar eclipse occurs when the moon is on the opposite side of the Earth as the sun. A solar eclipse occurs when the moon is on the same side of the Earth as the sun. And it takes the moon 14 ...
James K's user avatar
  • 122k
45 votes
Accepted

Can the moon appear to be in any position in the sky?

Like the sun, the moon, from our perspective on the surface of the Earth, rises in the East and sets in the West. However, it does not rise exactly due East and set exactly due West. If you were to ...
Unique Worldline's user avatar
45 votes
Accepted

Puzzling quotes from astronauts about earth size

The Earth is 4 times the diameter of the Moon. The Earth viewed from the Moon will therefore appear to have 4 times the angular diameter of the Moon viewed from the Earth. The Moon is easily obscured ...
ProfRob's user avatar
  • 152k
45 votes
Accepted

Apparent Ring of Craters on the Moon

Actually, most of the surface of the Moon is covered with craters like that. The exception are the large dark 'seas', which are in fact basaltic plains. The seas are mostly present on this side of the ...
Glorfindel's user avatar
  • 4,790
45 votes

How does the earth cast straight and reversed shadows on the moon?

There is already a good and correct answer from @uhoh, but I thought I’d add another to show off some of my favorite teaching images, and to comment on what we do learn from lunar eclipses. The ...
Eric Jensen's user avatar
  • 4,864
45 votes
Accepted

Why is there a year 1 B.C., a year 0, and a year 1 A.D. in NASA’s Five Millennium Canon of Solar Eclipses?

This is all explained on the home page All dates are astronomical dates. These match CE (or AD) dates for positive values but differ by 1 for BCE dates. The Gregorian calendar is used for all dates ...
James K's user avatar
  • 122k
44 votes
Accepted

Why does the Moon appear gray when passing between the Sun and the Earth?

That's what it really would look like if you were there with DSCOVR. The albedo of the Moon is only about 0.136, about half of the Earth's average albedo. Of course the part with clouds is higher. I ...
uhoh's user avatar
  • 30.7k
44 votes

What ground-based observational evidence is there that the Moon doesn't have an atmosphere?

The Moon is so close that establishment of an atmosphere would be easy: you take a photo. An atmosphere always is thicker at the surface and gets exponentially thinner outward. This would be visible ...
planetmaker's user avatar
  • 19.4k
44 votes
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Does the Earth constantly lose mass?

You are wrong that "to keep any object in circular motion requires energy" In a circular orbit, the force of gravity is always perpendicular to the motion of the moon, so no work is done by ...
James K's user avatar
  • 122k
43 votes

Why is moon light not the same color as sunlight?

Reflected moonlight is actually slightly reddened compared with the incident solar spectrum (Ciocca & Wang 2013). That same light is then transmitted through out atmosphere in exactly the same way ...
ProfRob's user avatar
  • 152k
42 votes
Accepted

How do we distinguish old craters from new ones on the Moon?

Aside from the excellent points made in James K's answer, there are other ways to date craters. For example, when the rays of one crater overlay those of another, we know that the former is younger ...
Dan Hanson's user avatar
  • 1,161

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