How can I prove the Earth is round?

15 May 2018


How can I easily prove to someone that doesn't believe the Earth is round that the Earth is round?


Chris Smith put Andy's question to Cambridge University astronomer Matt Bothwell...

Matt - Well yeah. So luckily it’s quite easy. We’ve known that the Earth is spherical for thousands and thousands of years. You can do with just observations and common sense really. There are a number of ways to do it; I guess two of the most simple ones. One of them, if you go to a beach - we were talking about looking for the green flash before. Well maybe while you’re looking for the green flash, if you’re sitting on a beach and you watch a ship disappearing over the horizon it doesn’t just fade into nothingness; it will actually dip below the horizon and look like it’s sinking down and that’s because the Earth curves away.

Another one, which is probably my favourite one, is to think about lunar eclipses. A lunar eclipse is when you have the Sun and the Earth and the Moon all in a straight line, and the Moon passes into the Earth’s shadow. The shape of the Earth’s shadow on the Moon is always round; this happens no matter what time the lunar eclipse is occurring. If it was a flat Earth, to get a round shadow, that can only happen at midnight. But you can see lunar eclipses at all different times and the shadow is always round, which means that the Earth has to be spherical.

Chris - So if one looks at the Moon; this is a round shape. If one looks at other planets you see things like moons orbiting other planets; they themselves are round. So there’s lots of evidence that these things tend to be round. The Sun is round so it’s kind of difficult to understand why the Earth wouldn’t be.

Matt - No. I’ve never heard of any flat Mars conspiracy theorists.

Chris - Just flat Earth.


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