That’s Maths: Earth’s shape and spin won’t make you thin
Newton’s theory that the Earth is an oblate spheroid, flattened by rotation, was proven in the 18th century
Oranges and lemons: variations of gravity have important geophysical consequences – but when it comes to weight control, they can’t substitute for the old reliables
The Earth is approximately spherical, but there is a slight flattening towards the poles. This is a consequence of the rotation when the planet was forming during the early history of the solar system.
The gravitational attraction of a sphere of uniform density is the same as if all the mass were concentrated at the centre. Thus, it is the same everywhere on the surface. But is the Earth a perfect sphere?
Newton believed that rotation would bring about a flattening, with the polar radius smaller than that at the equator. This would make it an oblate spheroid: flattened like an orange. Dominique Cassini and his son Jacques took a contrary view: their measurements in France indicated that the Earth is elongated at the poles, a shape called a prolate spheroid, more like a lemon.
If the Earth were flattened, the length of a meridional arc of one degree of (geographic) latitude would increase from equator to poles. If the Earth were elongated, as held by the Cassinis, the length of such a meridional arc would decrease towards the poles. But which was true? There was a lively controversy between the Orangemen and the Elder Lemons.
Exploring the issue
To resolve the issue, in 1753 the French Academy of Sciences proposed a geodetic expedition to make measurements of the meridian arc in polar and tropical regions. Two groups set out from Paris, one to Peru and one to Lapland. The northbound group was led by Pierre Louis Maupertuis.
It is an amazing fact that, by means of a humble pendulum, we can determine the geometric shape of the Earth. We can calculate the relative strength of gravity at two latitudes by measuring the period of the pendulum at the two locations.
In 1673, the French astronomer Jean Richer found that his pendulum clock, which kept good time in Paris, lost 2.5 minutes per day in Cayenne. This means that the gravitational attraction in Cayenne is about 0.35 per cent smaller than in Paris. From this we can calculate that the effective force of gravity increases by about 0.5 per cent as we go from equator to pole.
Among the scientists accompanying Maupertuis was Alexis Claude Clairaut, a notable French mathematician, geophysicist and astronomer. In his Theory of the Figure of the Earth, Clairaut published a formula that allows the ellipticity of the Earth to be calculated from surface measurements of gravity.
An analysis of the Lapland measurements confirmed that the Earth is an oblate spheroid, as Newton had predicted. On hearing the result, Voltaire congratulated Maupertuis “for flattening the Earth and the Cassinis”.
Variations of gravity have important geophysical consequences for inertial navigation, GPS location and so on. But when it comes to weight control, they are no substitute for the old reliables: good diet and regular exercise. A man weighing 100kg can lose about half a kilogram by moving from Anchorage to Zanzibar.
Of course, his body mass does not vary – only the pull of the Earth upon it.