The folks over at the Philly Voice were kind enough to host dueling comments between Flat Earth believers and people who know better.

An illustration of the alternative “Pond Earth Theory.”

From the Flat Earth side:

Rahu and Ketu in Vedic astrology are considered mythology by western astronomy. Rahu is the head of the serpent and Ketu is the body. In mythology, during an eclipse, they thought the head swallowed the sun.

We have to have something rise that causes eclipses in the flat-earth model. We think it’s not necessarily caused by the moon, though it could be.

From a skeptic:

Meanwhile, over a Flat Earth, you can have a small, local sun, overtaking a small, local moon (both moving to the West above the plane of the flat Earth) and the shadow will move West to East. That’s their argument, in a nutshell, and many FE folks are very, very excited about this eclipse because it will prove them right!

The truth is that the case of the eclipse shadow is dependent on linear speed (such as miles per hour) and not angular speed. How fast will the umbra travel? The units of miles per hour require us to do some math with those units.

So if you take the Earth’s rotational surface speed (roughly 1,000 mph at the Equator, more like 800 mph in Philly), and the Moon’s orbital speed (roughly 2,000 mph, eastward), this means the shadow will move faster than the Earth’s surface, to the East.

Personally, I think Phil Plait posted the best explanation back in 1998:

There is an unambiguous effect, though, of the curved Earth, which brings me back to my vacation. My parents live in Sarasota, which is about 1600 kilometers south of where I live near Washington DC. This is equal to about 1/30 of the way around the Earth, or 12 degrees. When I am at home and go out to look at Polaris, the North Star, it is about 40 degrees above the horizon. If I lived at the North Pole, it would be 90 degrees above the horizon, or straight up. However, when I visit my parents, I travel south, and so Polaris appears lower. Much lower, 12 degrees worth! That is very noticeable to the naked eye. On the other hand, stars towards the south appear to be much higher in the sky when I am in Florida. Last year I could clearly see Canopus (the second brightest nighttime star in the sky) to the South, but it never gets high enough to see from my house.

If the Earth were flat, we’d never see this effect. If the Earth were a disk we’d only see it if we traveled along the edge, and not the face. Therefore we must live on a curved Earth, a big ball in space (as a matter of fact, this effect can even be used to determine the circumference of the Earth!).