After being a lurker on Quora for many years, I decided to start answering some of the astrophotography and astronomy questions there. This post was inspired by one. There are many ways we know that the earth is not flat. Aristotle is known for several, one of which is that the earth's shadow on the Moon is round. The earth could still be flat and circular and cast a round shadow, but we have more evidence than just a single shadow.

Lunar eclipses allow us to see the shape of the earth’s shadow projected on the Moon. Because the earth and it’s shadow are larger than the Moon, we need to observe lunar eclipses with the sun at different angles in the sky. All of the shadows across the Moon show a circular edge. The only shape that always casts a circular shadow no matter what side it is illuminated from is a sphere.

You can see this for yourself:

Observe several lunar eclipses that occur at different times of the day - right after sunset, the middle of the night, right before sunrise.

Take note of the shape of the earth’s shadow during the partial phases of the eclipse.

The edge of the earth’s shadow on the Moon is always a circular section and the shadow approximately four or five times larger than the Moon. You can’t see the whole shadow at one time, but different lunar eclipses will have the Moon travel through different parts of it. The same shape is seen observed no matter where the sun is located and what part of the shadow the Moon passes through.

The only shape for the earth, that explains this, is one that is close to spherical.

Here are a couple of examples:

and a couple of hours later

If the earth were a disk or some other flat shape, then the eclipse when viewed near dawn or dusk would show a shape like this faked image:

I’ve personally seen many lunar eclipses and have never seen the shadow of a flat earth, or any other shape that isn’t consistent with a ball shaped earth. I've seen the Moon move through the top of the earth's shadow, the middle, and the bottom. I've seen eclipses just before sunrise, just after sunset, and in the middle of the night. The earth's shadow looks circular no matter what direction the sun shines from. The only shape that is consistent with this is a sphere.

The earth only seems flat to someone who’s mental horizons extend no further than their own back yard. Proving that it isn't flat is about as useful and hard as proving that ice is frozen water. I've done this here so that I have it and never have to do it again.