Prov­ing the Earth is a sphere

Cosmos - - Cosmos Science Club - — AN­DREW MASTER­SON

IN RE­CENT MONTHS

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.

THE AN­CIENT GREEK METHOD

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.

THE HORI­ZON METHOD

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.

THE CIRCUMNAVIGATION METHOD

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.

THE TELE­PHONE METHOD

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.

THE PHO­TO­GRAPHIC METHOD

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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