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Comment by jmtrevarton | Hacker News Reader

I still don't understand why there's a new moon when the moon is between the earth and the sun and a full moon when the earth is between the moon and the sun. Maybe if I stare at the model longer it'll magically make sense.

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On the new moon, when the moon is between the earth and the sun, the light reflects back towards the sun, so you don't see it i.e. the moon is dark.

As for the full moon, the earth and sun are on the same side of the moon, so the sun is lighting the moon "from behind you" and you can see that.

I keep thinking there should always be a little bit of a solar eclipse when there's a new moon. I think my model might need a z axis. Or maybe something in my mental model just still isn't right.

That's right.

In these missed eclipse the moon is usually a little too high or a little too down. The moon revolves on a plane that is tilted respect the Earth's plane of revolution. These two planes of revolution intersect at the the two points called nodes.

See this picture

https://en.wikipedia.org/wiki/File:Eclipse_vs_new_or_full_mo...

If the new moon happens to fall on the node then you will get a solar Eclipse.

Thank you! That's actually really helpful!

The model also places the moon way further from the Earth than it really is or else you wouldn't be able to see it as separate from the Earth, so that messes with creating a mental model for how the whole thing works as well.

It might make more sense if you consider that the three objects are not usually aligned, due to the tilt of the moon's orbit around the earth. In the case of the full moon, this allows us to see the fully illuminated moon even when we are between it and the sun.

Yeah that is the part that makes it confusing. Thank you!

The new moon is in the sky when it's daytime. Crescents show at dawn/dusk. Full moon is night only.

hence the eclipse today

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