17

A rather stupid question, why can we see the moon when it is between the Earth and the Sun?

An example with excellent drawing.

12123232
  • 313
  • 36
    The premise of this question is wrong. You can't see the moon at night when it is between the earth and the sun as shown in your diagram. – JBentley Jul 06 '16 at 08:53
  • 2
    What you've drawn is a solar eclipse. At that moment, you're not really seeing the moon, only its shadow. When the moon is very close to the sun but not in front of it, it's a "new moon"; only the far side is lit and you can't see the moon very clearly at all. The rest of the time, the moon is at an angle to the sun and you see it partially lit, sometimes at day and sometimes at night. A full moon is when the moon is on the other side of the Earth to the sun. You see it mainly at night. – Ben Hillier Jul 06 '16 at 09:45
  • Wow. People are VERY interested in this question – user122066 Jul 06 '16 at 13:16
  • I think it might be worth noting that during a new moon it will be visible from the night side mostly at dawn or dusk and close to the horizon. It's not usually directly behind the planet but only close to it. – Octopus Jul 06 '16 at 16:21
  • 1
    @JBentley You're spot-on. I've edited the question (and title) to remove "at night" – Monty Harder Jul 06 '16 at 19:31
  • 1
    @MontyHarder I don't know if that is an appropriate edit. The OP drew the night time side of the Earth, so I think he genuinely meant to ask that. Plus the edit changes the nature of the question (and therefore breaks the answers), so should probably be a new question if the OP intends that. – JBentley Jul 06 '16 at 19:35
  • @JBentley I didn't see a stick figure in the drawing indicating where the viewer is, as you've done in your answer below. But if he's really on the night time side, the reason he can't see the moon is, as you've already answered, because the Earth is in the way. Also, the accepted answer does not fit this interpretation. – Monty Harder Jul 06 '16 at 19:54
  • Original poster: what is special about the moon? Suppose you go into a field and hold up an orange so that it is between you and the sun, at any time of day or night you choose. Can you see the orange? How is that situation different than making your arm longer and putting the moon at the other end instead of an orange? – Eric Lippert Jul 07 '16 at 13:04

6 Answers6

112

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:

enter image description here

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:

enter image description here

JBentley
  • 674
  • 2
  • 8
  • 15
48

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.
enter image description here

According to NASA, it was Leonardo da Vinci who first explained this.

As an example, the brightly lit portion of this photo is illuminated directly by the sun. The rest of the moon, though, is still visible and this is due to earthshine:

enter image description here

Image credit: Steve Jurvetson

Jahan Claes
  • 8,080
John1024
  • 3,025
  • 17
  • 14
  • 2
    Please don't use unattributed images like that. The appropriate credit is a google search away. – Emilio Pisanty Jul 06 '16 at 14:18
  • 2
    Note also that the picture doesn't really depict the Moon "between" the Earth and the Sun. The image data puts it at 2007-01-21 03:17 UTC, and the JPL ephemerides give a Sun-Earth-Moon angle of 26° at that time, which means that the Sun-Earth-Moon system looked more or less like this when the picture was taken. Hardly a "between" situation. – Emilio Pisanty Jul 06 '16 at 14:58
  • I don't belive the question is about lighting. It is about occlusion. The OP is asking why isn't it occluded from site during a new moon if it supposedly on the opposite side of the planet. – Octopus Jul 06 '16 at 16:15
  • 1
    @EmilioPisanty it's more 'between' than if it was the far side of the earth – snulty Jul 07 '16 at 11:36
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 of the Earth-Moon system:

Earth-Moon system

This is it. That's how small the Earth (right) and (especially) the Moon (left) are compared to the distance between them.

I did not include the Sun in this diagram. I couldn't. Its diameter would be nearly four times the Earth-Moon distance. And it would be nearly 400 times as distant from the Earth as the Moon:

Sun-Earth system

Not sure how visible the single pixel on the right is, but that single pixel is the Earth-Moon system, shown to scale compared to the little circle on the left representing the Sun, with the distance between the two also at the proper scale.

This scale of things might explain, among other things, why solar eclipses (when the Moon is truly between the Earth and the Sun) are relatively rare.

Viktor Toth
  • 1,863
  • 6
    To take this to the next level, there is a site that shows the whole solar system to scale if the moon is the size of a single pixel: http://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html – Kevin Jul 06 '16 at 15:09
  • +1 for scale. Maybe add in the orbital inclination of the Moon? – Chieron Jul 07 '16 at 09:16
15

If it is really between the earth and the sun it is called a "solar eclipse" and and the moon's shadow falls on the earth at certain places, because it is not large enough to cover the whole sun except on a shadow path.

solar eclipse

A total solar eclipse occurs when the Moon completely covers the Sun's disk, as seen in this 1999 solar eclipse. Solar prominences can be seen along the limb (in red) as well as extensive coronal filaments.

One must not look at the moon during the eclipse with naked eyes because of the danger even from the corona of destroying the retina. The intensity is strong enough not to allow for shadows on the moon surface, as seen above.

By contrast, a

total lunar eclipse has the direct sunlight completely blocked by the earth's shadow. The only light seen is refracted through the earth's shadow. This light looks red for the same reason that the sunset looks red, due to rayleigh scattering of the more blue light. Because of its reddish color, a total lunar eclipse is sometimes called a blood moon.

That this does not happen every lunar month ( thanks BenHillier) is because the orbit of the moon around the earth is slanted .

Lunar orbit inclination also determines eclipses; shadows cross when nodes coincide with full and new moon, when the Sun, Earth, and Moon align in three dimensions.

anna v
  • 233,453
  • 2
    Doesn't the lunar eclipse actually happens when the moon is behind the Earth? If it is between the Earth and the Sun, then it's a solar eclipse. – 12123232 Jul 06 '16 at 06:25
  • @12123232 you are correct .It is a solar eclipse .I will edit the answer – anna v Jul 06 '16 at 08:16
  • 2
    "That this does not happen every night ". You meant "every month" I think. It doesn't happen every night because the moon is in a completely different part of the sky! – Ben Hillier Jul 06 '16 at 09:50
  • @BenHillier It is because the moons orbit is not in the ecliptic, if it were then once a day an eclipse would happen someplace on earth – anna v Jul 06 '16 at 10:13
  • 2
    @annav Sorry! I am really convinced on this one. The moon's orbit is, as you correctly point out, not in the ecliptic. And that is the reason why every new moon is not a solar eclipse; and every full moon is not a lunar eclipse. But for the most of the month, the Moon, Sun and Earth are at angles to each other. – Ben Hillier Jul 06 '16 at 10:23
  • @BenHillier I guess you are correct, once a lunar month. Only if the moon took 24 hours to go around the earth instead of 28 days I guess. I edited – anna v Jul 06 '16 at 13:05
  • Nice image. Who owns the copyright? – Jeremy Jul 06 '16 at 21:36
  • @JeremyBanks got it from the wiki link given on solar eclipse. original must be there – anna v Jul 07 '16 at 04:09
13

The diagram you drew is flat, but the solar system is not. The Moon's orbit is not in the same plane as the Earth's orbit. Wikipedia has a nice diagram:

Diagram showing how the lunar orbit is not in the ecliptic plane

Because of this, when the Moon is "in between" the Earth and the Sun, it is usually a little "above" or "below" the Sun as well. You can observe this for yourself: one or two days after the new moon, look for the Moon in the sky at sunset. It will be above the Sun, and the bright arc will point toward the Sun. But it will also be a little off to one side or the other. That angle is a combination of the Moon's orbital angle with the ecliptic and the Earth's axial tilt.

(They do sometimes line up perfectly, and then we have a solar eclipse, as mentioned in other answers.)

zwol
  • 1,857
1

The moon circles around the earth, so half the time it is between the earth and the sun and half the time the earth is between the sun and the moon.

Therefore also the moon rises and sets, the same way the sun rises and sets.

If it's midnight (your are on the opposite site of the earth than the sun) and the moon is betweem the sun and the earth you can't see it. You can see a half-moon in the late evening or early morning, tough.

Robin Roth
  • 111
  • I know, but how can we see it when it is between the sun and the moon? – 12123232 Jul 06 '16 at 06:38
  • 2
    If it's midnight (your are on the opposite site of the earth than the sun) and the moon is betweem the sun and the earth you can't see it. – Robin Roth Jul 06 '16 at 08:18
  • You can see a half-moon in the late evening or early morning, tough. – Robin Roth Jul 06 '16 at 08:19
  • @RobinRoth could you edit your answer to include the information you've put into comments? Without those details, your post doesn't really answer the question as stated. Also, welcome to Physics.SE! If you haven't already done so, please take the [tour]! – Dan Henderson Jul 06 '16 at 12:02