Hot answers tagged

110

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 on the opposite side of the earth during that configuration (so that it is night), then you can't see the moon because the earth is blocking your view of it:


84

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 effect GR has on an Earth-Moon transfer orbit. But this takes a little preparation so bear with me while I give a short introduction. I hope everyone who reads ...


79

Human perception is generally logarithmic. For example, the perceived loudness of a sound is measured using decibels, where an decrease of $10 \text{ dB}$ divides the sound intensity by $10$. So if the eclipse were heard instead of seen, "90% coverage" might mean reducing the intensity from $120 \text{ dB}$ to $110 \text{ dB}$, a small change. Perceived ...


67

The moon does not fall to Earth because it is in an orbit. One of the most difficult things to learn about physics is the concept of force. Just because there is a force on something does not mean it will be moving in the direction of the force. Instead, the force influences the motion to be a bit more in the direction of the force than it was before. ...


50

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 visible even when the sun is on the other side. According to NASA, it was Leonardo da Vinci who first explained this. As an example, the brightly lit portion of ...


45

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 every 24 hours, and we'd get a high tide every 24 hours. (There's another high tide when we're exactly on the opposite side of the earth to the moon, but let'...


45

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 1967, incorporated relativistic effects in its modeling of the solar system. This didn't help much. Had they ignored relativistic effects there would have been ...


42

The graph looks exponential because the vertical axis is logarithmic! If you were to re-plot it as linear lumens per square meter, it would be much more v-like, or even u-like. It so happens that a logarithmic plot matches our subjective perception of light intensity better than a linear one would. That's a result of our eyes having evolved to work well in ...


41

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 sunlight and so has a lower "colour temperature". This is a fact, not a perception. EDIT: Just to clear up some confusion - the OP talks about "yellower" because ...


40

Briefly: Because the moon's orbit "wobbles" up and down, so it isn't always in the plane of the earth's orbit around the sun. There's a 2D plane you can form from the ellipse of the earth's orbit and the sun. This plane is known as the ecliptic. The moon's orbit is not exactly in the ecliptic at all times; see this (slightly overcomplicated) picture from ...


38

I stumbled across this question, and while it is an old question and there are some halfway decent answers, I think it deserves a more in-depth response. I completely understand the question, which I think is am excellent question. Trying to understand the phases of the moon is much more difficult than most think. The short answer of it is that you cannot ...


33

Ignoring the "no gravity" part of your question - there is, it's 1/6th that of the Earth - and assuming that you mean "no atmosphere", the flag looks like it's "waving" because the horizontal rod that runs along the top of the flag got stuck part way out. This meant that the flag didn't "unfurl" fully and is hanging like a curtain rather than being stretched ...


30

The relevant "100%" from which you should calculate the percentage isn't the depth of the ocean but the radius of the Earth $$ R\sim 6,378,000\,{\rm m} $$ Multiply this $R$ by $10^{-7}$ and you will get $0.6$ meters, a reasonable estimate for average tides. You must understand that the surface of the ocean always tries to create an "equipotential surface" –...


25

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. Since the moon also orbits around the earth (with a period of a lunar month), this means each side changes, over the course of a lunar month, between facing ...


24

It will never reach such a high velocity. The moon is drifting further from the earth due to tidal acceleration. This process is, at the same time, slowing the rotation of the earth. Once the earth's rotational period matches the moon's orbital period, the earth-moon system will be tidally locked to each other (note: the moon is already tidally locked to the ...


23

It just happens to be a coincidence. The current popular theory for how the Moon formed was a glancing impact on the Earth, late in the planet buiding process, by a Mars sized object. This caused the break up of the impactor and debris from both the impactor and the proto-Earth was flung into orbit to later coallesce into the Moon. So the Moon's size just ...


23

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-relativistic velocities in the Schwarzschild metric is modified by a factor $(1 - r_s/r)(3[1-r_s/r] -2)$, where $r_s = 2GM/c^2 = 0.00885 m$ for the Earth. If we ...


23

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 Earth) given all the "artist's renderings" in the question and the answers, I thought it might be useful to include a diagram that demonstrates the actual scale ...


22

This is a gravitational phenomenon known as tidal lock. It is closely related to the phenomenon of tides on Earth, hence the name. Tidal locking is an effect caused by the gravitational gradient from the near side to the far side of the moon. (That is, the continuous variation of the gravitational field strength across the Moon.) The end result is that the ...


21

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 noticed anywhere except in Mercury's orbit (at least not until they purpose-built experiments to detect minor discrepancies). I know this doesn't give a completely ...


20

Well, I'd like to say that you are almost there. The key point of this question is to know that usually illustrations are just showing the relative positions but not with the real ratio. If the size and distance of the moon is the same as such pictures show, it will much harder to find when it lies at the same side of the sun. Because to see it, the ...


20

I suspect the question may be unanswerable, and possibly even meaningless. As I understand it, the giant impact that resulted in the formation of the Moon would almost certainly have also completely liquefied whatever crust the Earth had at the time, producing a global "magma ocean". Thus, there would've been no traces of the impact left — or rather, ...


19

The two effects are not related. The size appearing larger is a matter of some speculation to this day, but it is purely a psychological effect. If you want to prove this, take a look a the moon while standing up and looking between your legs. It won't look nearly as large. The red/orange color is related to the sunset being red. In fact, it's the same ...


19

(Source, Wikipedia Commons) The moon is generally called a "Harvest Moon" when it appears that way (i.e. large and red) in autumn, amongst a few other names. There are other names that are associated with specific timeframes as well. The colour is due to atmospheric scattering (Also known as Rayleigh scattering): may have noticed that they always ...


19

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 night you compare it to a black sky, it appears white.


18

On the earth-sheltering question, the answer is yes, using material to increase the thermal mass of structures would work just as well on the Moon as on Earth. There might be minor differences due to different materials and lack of water in Moon soils but the general principal would still apply. As for the Moon's core still containing significant heat, ...


18

When we say that the Moon rotates, we don't mean relative to an observer on Earth, because we're also rotating. Maybe best is to think of it from the perspective of the Sun. If you were at the centre of the solar system, looking at the Earth, you'd see the Moon rotates once every 28 days or so. That also happens to be the amount of time it takes for the Moon ...


18

Officially, no -- but there is a weak case to be made that the Moon orbits the Sun rather than the Earth. If you trace the Moon's path in a Sun-centric frame of reference, that path is completely convex. Quoting this Wikipedia article: Unlike most other moons in the Solar System, the trajectory of the Moon is very similar to that of its planet. The ...



Only top voted, non community-wiki answers of a minimum length are eligible