Prov­ing the Earth is a sphere

Cosmos - - Cosmos Science Club - — AN­DREW MASTER­SON


news re­ports have sug­gested a resur­gence in the the­ory the Earth is ac­tu­ally flat. Some peo­ple un­doubt­edly make the claim as a joke, but ap­par­ently oth­ers are se­ri­ous about this be­lief.

The Earth isn’t flat, of course, but how can we prove that? There are sev­eral ways. Here are a few of our favourites.


A scholar called Eratos­thenes worked out the Earth was a sphere way back in 240 BCE. He did it by whack­ing a stick in the ground in the Egyp­tian city of Syene at mid­day and mea­sur­ing the an­gle be­tween it and its shadow. He re­peated the ex­per­i­ment in the nearby city of Alexan­dria and noted the an­gle was dif­fer­ent by seven de­grees. Ex­trap­o­lat­ing from this, he rea­soned the Earth had to be round – the an­gle would have been iden­ti­cal on a flat plain – and that its cir­cum­fer­ence was 44,100 km. He was out by about 10% – pretty good for a bloke equipped with just two sticks and 10 fin­gers.


Watch a ship sail away from you. Even­tu­ally it will dis­ap­pear over the hori­zon. If the Earth was flat, it wouldn’t do that. It would just ap­pear to grow smaller, and with a pow­er­ful enough te­le­scope you could track it all the way to Africa.


If the Earth was flat, circumnavigation would be im­pos­si­ble. The first per­son to suc­cess­fully sail a ship right around the Earth was a Span­ish sea cap­tain called Juan Se­bas­tian El­cano, who took three years to do it and ar­rived back in his home port in 1522. He was orig­i­nally com­mis­sioned by Fer­di­nand Mag­el­lan, who wanted to have the hon­our of prov­ing the world could be cir­cum­nav­i­gated. The boss, how­ever, got him­self killed in the Philip­pines – leav­ing El­cano and his crew to com­plete the mis­sion. Since then the world has been cir­cum­nav­i­gated mil­lions of times. Pas­sen­ger and cargo planes to do it ev­ery day. In all these mil­lions of jour­neys over hun­dreds of years, no one has ever fallen off the edge of the world and gone hurtling into the abyss of deep space.


If the world was flat and the Sun just spun around in a cir­cle above it, light would hit the en­tire sur­face all at once. Time, there­fore, would be the same everywhere, but it’s not. If it’s 5pm in Australia, ring a flat-earther in Cal­i­for­nia and wake him up. It will be mid­night the day be­fore in Los An­ge­les. Time dif­fer­ences can only hap­pen on a ro­tat­ing sphere.


Since space travel be­came a re­al­ity in the 1960s, as­tro­nauts and satel­lites have taken vast num­bers of pho­to­graphs of the Earth. All of them show a spher­i­cal ob­ject float­ing in space – some­times with an­other spher­i­cal ob­ject, the moon, float­ing nearby. If all these pho­to­graphs had been taken from ex­actly the same spot, it could be ar­gued (as flat-earth­ers some­times do) that all they show is a two-di­men­sional disc. But they haven’t been. They have been taken from mul­ti­ple an­gles at mul­ti­ple dis­tances on mul­ti­ple oc­ca­sions. You just can’t fake all this ev­i­dence. Be­sides, if the Earth was flat, cats would have pushed every­thing off it by now.

Caro­line Jensen / Getty Images

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