Two-hemisphere global composites of Moderate Resolution Imaging Spectroradiometer (MODIS) data, taken in 2001 and 2002. Observations show that Earth is nearly perfectly round, but must all planets be? Image credit: NASA.

How flat can a planet be?

The Earth is round, Kyrie Irving. But not every world needs to be.

“‘I’ll follow him to the ends of the earth,’ she sobbed. Yes, darling. But the earth doesn’t have any ends.” -Tom Robbins

We know that the Earth isn’t flat, and have known this for hundreds of years. There are many ways to demonstrate this, from ships’ masts disappearing as they sail out over the horizon, to your ability to see farther at higher altitudes, to the longer shadows cast by the Sun at higher latitudes, to measuring the shape of the Moon’s shadow on the Earth during a solar eclipse, to actually going to space and seeing the shape of the Earth for yourself.

But just because the Earth isn’t flat doesn’t necessarily mean a planet couldn’t be. In fact, there are many observations that we make that would be consistent with a flat, circular Earth.

The two ways Earth could cast a circular shadow on the Moon: by being a spherical object (bottom) or a disk-like object (top). Lunar eclipse observations cannot determine the Earth’s sphericity on theirown. Image credit: Windows to the Universe Original (Randy Russell), under a c.c.a.-s.a.-3.0 unported license.