system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 What is a lunar eclipse?

I'm really confused, looking online it says it's when the earth casts a shadow on the moon.  But how is it any different from the various stages of the moon already?  Wouldn't a full lunar eclipse be the same thing as a new moon?
gpt
There is a cone behind the Earth where light from the sun doesn't get to. Essentially, night-time (the area on the Earth where the Sun's light doesn't reach) extends upwards from the surface of the Earth, going further up the *more* you are exactly opposite the sun.

By sheer cosmic coincidence, the moon *sometimes* passes by the tip of this cone, which is pretty much exactly enough to cover the whole moon.

So why is this a rare occurrence? Well, in addition to the moon going around the Earth left to right (east/west), it also oscillates a little up and down (north/south). The only time you can get an eclipse (either lunar or solar) is when it happens to be passing down along the correct place on the north/south wobble at the same time it passes the right place on the east/west orbit.


In contrast, the phases of the moon are not caused by the Earth's shadow. They are just nighttime on the moon -- the portion of the moon that is facing away from the sun.