How we know the Earth's shape

[Music] [Music] [Music] Globes are made to represent the world a globe is a special kind of map it shows the world as we think it might look to someone far out in space a globe shows us the land and water portions of the earth it shows us the oceans and the continents and their general shape but how do we know that the actual shape of the earth may be shown by a globe how do we know what the earth shape really is let's see many thousands of years ago early man looked at his world he thought perhaps that it ended at the horizon he may have asked himself is the world flat and does the horizon mark the falling off place years later people living near the sea noticed an interesting fact they saw that ships heading out to sea in any direction seemed to sink and disappear below the horizon they knew that the ships didn't really sink so they explained what they saw in this way on a flat earth a boat leaving the shore would simply disappear in the distance however if the earth were curved a boat would seem to sink lower and lower as it sailed away so they thought that the earth must be curved ancient Greek scientists thought that the earth was round because of what they sometimes saw when they looked at the moon at certain times the full moon became dark as though a giant shadow were passing over it what happened is shown in this diagram the earth blocked off the sun's light and formed a shadow the Greeks knew that the moon travelled around the earth when it entered the Earth's shadow it seemed to disappear the Greeks called this an eclipse these motion pictures of an eclipse were photographed through a telescope on Mount Wilson in California we can see the edge of the Earth's shadow on the moon the movement is speeded up because this eclipse takes almost two hours it was this same curved shadow crossing the moon which the ancient Greeks saw from the shape of the shadow they were able to tell the general shape of the earth they decided that since the Earth's shadow is round the earth itself must be round Columbus along with many other men knew about the idea of a round earth if the world were round Columbus believed it should be possible to sail around it he failed in his attempt because America was in the way a few years later however a sea captain named Magellan set sail from the country of Spain his ship sailed westward finally after many adventures one ship arrived back at its starting point this voyage proved that the world is round but then about three hundred years ago Isaac Newton a great English scientist said that the earth could not be perfectly round this model made of steel bands shows why he believed this when the model turns slowly it has a round shape but when it turns quickly it bulges out in the middle and flattens on the top and bottom Newton said that since the earth spins rapidly its sides must bulge out and its poles must be flattened later it was proved that Newton was right the distance through the earth from pole to pole is 27 miles less than its distance through the middle this means that the earth is slightly fatter from side to side than from top to bottom the shape of the earth is like a slightly flattened ball in this drawing the flattening and bulging is shown much greater than it really is so we can see it clearly but today satellites traveling around the earth have given us better ways to measure the Earth's shape special receivers on the ground pick up radio signals sent by satellites you are listening to a recording of the signals received from Vanguard one one of the early satellites some information received from a satellite scientists can tell its position they can also tell the path it follows around the earth because the earth bulges slightly in the middle scientists knew that a satellites orbit would slowly change like this however after observing and measuring actual satellite orbits they found other changes they hadn't expected very careful studies of satellite movements proved that the earth not only bulges in the middle and is flattened at the poles but that the earth also dips in a few feet around the northern hemisphere and bows out a few feet in the southern hemisphere remember the differences here and here are very very small only a few feet in many thousands of miles in this drawing the differences are shown much greater than they really are so we can see them clearly the earth may be described as being very slightly pear-shaped but the dips and bulges are so small that if you were far away in space the earth would look perfectly round in recent years scientists have found new ways to look at the earth cameras placed in Rockets have photographed the Earth's from great heights these pictures were taken from a rocket 700 miles above the earth they show clearly the curve of the Earth's surface so man has learned about the shape of the earth in many ways by watching moving ships the Earth's shadow on the moon rapidly turning objects the orbits of satellites and photographs like these as we know now the actual dips and bulges in the earth are very very slight to scientists however even a difference of a few feet in many thousands of miles may be of great importance or normal study a perfectly round globe is still the best model we can conveniently make if the earth itself could be shrunk to the size of a school globe it would appear rounder and smoother than any globe ever made [Music]