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May 22, 2010 5:23 PM   Subscribe

Help me understand the phases of the moon. Why is it that a New Moon is visible at all?

Ok, I have what can only be a sort of dumb question about the phases of the moon, that I'm sure everyone else figured out the answer to sometime around the fourth grade. Basically, I think I've never quite understood how the phases of the moon work--I particularly don't get how it's possible for there to be a "New Moon" in the sky, given the way I understand the phases of the moon to work.

The problem is this: Take a look at any typical diagram of the earth, the moon, and the moon's phases. Here's an example. According to such diagrams, the moon is "New" when it's between the earth and the sun. However, when it in this position, it should not be observable by those on the nighttime side of earth, facing away from the sun. Right?

It seems like those on the night side of earth would need to look through the earth to see the moon in it's "new" phase, even if the moon is orbiting on a pretty steep angle.

So...help me here? Have I managed to discredit the Copernican model of the solar system and proved I'm living in a projected Matrix-type world? Or, is there something something a little off in the way I'm thinking about the moon and its orbit? Open to both possibilities. Thanks for any insight.
posted by doubtless to Science & Nature (16 answers total) 10 users marked this as a favorite
 
This nifty (flash) lunar phase simulator might help you visualize the situation better.
posted by dantekgeek at 5:31 PM on May 22, 2010 [21 favorites]


Sometimes light reflecting from the surface of the Earth illuminates the new moon making it visible during the day.
posted by hortense at 5:34 PM on May 22, 2010


According to such diagrams, the moon is "New" when it's between the earth and the sun. However, when it in this position, it should not be observable by those on the nighttime side of earth, facing away from the sun. Right?

And do you ever see a new moon during the night? (Dawn and dusk, maybe. Night, no.)
posted by DU at 5:34 PM on May 22, 2010


To expand on DU, we only see the full moon at night. So the more full the moon is, the more it is visible at night. If it is half full, it is up during half of the night (approximately ).
posted by Monday at 5:51 PM on May 22, 2010


Awesome, thanks Metafilter!
posted by doubtless at 5:51 PM on May 22, 2010


So...OK...Shameful confession time. I am another person who never 'got' this. Don't judge me.

From the simulator, it looks like the 'new moon' is just the dark side of the moon facing the earch. Is that right? I was always told that it was because the earth was casting a shadow on the moon and that didn't make sense to me.
posted by SLC Mom at 5:52 PM on May 22, 2010


SLC Mom: You are correct. The only time the Earth casts a shadow on the moon is during a Lunar Eclipse, when they line up perfectly. This does happen, but not very often, and always during a "full moon".

List of lunar eclipses
posted by Mwongozi at 5:57 PM on May 22, 2010


I was always told that it was because the earth was casting a shadow on the moon and that didn't make sense to me.

That's the definition of a lunar eclipse, which is about the only way to see a completely dark moon in the middle of the night. Except that it isn't quite completely dark, as some sunlight is refracted around the earth by the atmosphere, and this gives the moon a lovely red glow during an eclipse. It's quite a sight.
posted by FishBike at 5:57 PM on May 22, 2010



From the simulator, it looks like the 'new moon' is just the dark side of the moon facing the earch.


The Moon is tidally-locked to the Earth, which means the same side always faces the Earth. What we call the "dark side" is the side that faces away from Earth, which we cannot see; it's not truly 'dark' in an illumination sense, but 'dark' in a 'darkest Africa' sense. When there's a New Moon from our viewpoint on Earth, the "dark side" (the side facing away from the earth) is at a Full Moon.
posted by AzraelBrown at 6:10 PM on May 22, 2010


Thanks folks!
posted by SLC Mom at 6:35 PM on May 22, 2010


BTW, a great way to really internalize the motions of the moon is to observe it (if possible) at the same time every day. Say, when you are walking out to your car in the morning. Night is the best, obvs, but dusk and dawn are OK too. Daytime is possible but difficult for multiple reasons.

Seeing the patterns in your observations, explanations and diagrams will start to make a lot more sense.
posted by DU at 6:50 PM on May 22, 2010


However, when it in this position, it should not be observable by those on the nighttime side of earth, facing away from the sun. Right?

That's because the diagram is wrong. It's implying that this is a top-down view of three bodies travelling through space in the same plane.

However if you look at the bottom right-hand figure of the flash simulator linked above, you will see that the moon's orbit is inclined at an angle to the plane of the earth's orbit around the sun.

There are therefore two separate intersecting planes of movement. (And it is the intersection of these planes that is the point at which eclipses can occur).

The large figure of the flash animation makes the same mistake as your original linked diagram. It is there also wrong. (I may write to the people who created this figure). If you plotted the two intersecting planes that are shown in the bottom right hand figure onto a flat surface 'viewed from above' (as in the main figure), the moon's orbit would be an ellipse, not a circle.

It is the mistaken representation of the moon's orbit as a circle that lies in the same plane as the orbit of the earth around the sun, that is responsible for a lot of confusion.
posted by carter at 8:19 PM on May 22, 2010


It seems like those on the night side of earth would need to look through the earth to see the moon in it's "new" phase, even if the moon is orbiting on a pretty steep angle.
You're halfway correct; you can't see the new moon at midnight. But you can see it near dawn or dusk.

Another way to think about this is that the closer the moon is to New, the closer to the sun it appears in the sky. So you'll always see a crescent moon fairly near the sun — obviously not too easy to see it during the day, but occasionally you can. And likewise, the full moon is going to be opposite the sun, from your POV, so it'll be at the zenith near midnight. Like the new moon, the full moon will rise and set near the same time the sun does, but it'll be on the opposite side of the sky. At half-full, the moon will rise/set near noon/midnight. All approximate, of course, because of orbital inclination and eccentricity and whatnot, but it's pretty close.

Along the same lines, think about how Venus is the dawn or evening star. Because it's an inner planet (that is, closer to the sun than we are), the apparent angle between it and the sun, from our perspective, is always well under ninety degrees. So it appears close to the sun in the sky, and we can only see it during the brief moments when it's above the horizon but the Sun isn't. The outer planets, on the other hand, might be anywhere on the ecliptic from our POV.
posted by hattifattener at 10:52 PM on May 22, 2010 [1 favorite]


There are a few things that go together to make new moons visible.

Firstly, the path of the moon around the earth is at an angle to the equator. This means that the moon will only occasionally pass between the earth and sun, causing an eclipse. Mostly it passes above or below the sun's apparent position in the sky, which means that a small amount of the illuminated face of the moon will be facing towards us at all times, other than during an eclipse.

Secondly, the illuminated part of the moon is very bright - even after the reflected light passes through our atmosphere, it's as bright as something lit by a sunny day on earth.

Thirdly, the apparent separation of the moon and the sun means that the new moon can be above the horizon just before sunrise, so the glare of the sun doesn't wash it out.

When you put these things together you can see that even a very new moon can be visible if you're in the right place at the right time.
posted by Joe in Australia at 2:38 AM on May 23, 2010


When you put these things together you can see that even a very new moon can be visible if you're in the right place at the right time.

Example.
posted by dhartung at 5:35 PM on May 23, 2010 [1 favorite]


Linked from the article identified by dhartung is this explanation of when the youngest possible moons can be seen. Apparently the new moon simply cannot be seen when it is less than 7.5 degrees from the sun - I never knew that!
posted by Joe in Australia at 8:19 PM on May 23, 2010


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