The bakerian lecture, 1974 A view of earth and air

Abstract

My theme is the Earth and its atmosphere as viewed by an interstellar sightseer who arrives in the solar system, able to see to the limits of present human knowledge but no further, and able to turn the clock back but not forward. This celestial snooper sees a bleeding Sun pouring out its substance as a supersonic ‘solar wind’, within which the magnetic Earth carves out a tadpole-like cavity, the magnetosphere, with a tail stretching millions of kilometres downwind. In the magnetosphere ionized atoms predominate from a height of 60 000 km down to 2000 km; below that are layers where neutral hydrogen, helium, atomic oxygen and nitrogen are dominant in turn. Here, particularly at heights between 100 and 1000 km, analysis of the orbits of artificial satellites has revealed strong winds and huge variations in density (by a factor of up to 100) and temperature (by up to 600 K), largely controlled by the Sun. Looking back on past ideas of the atmosphere, the visitor from space notices how often scientists have forsaken correct ideas in favour of novelty. The shape of the Earth itself has excited curiosity since the stone age, and the saga of its discovery is a microcosm of human endeavour: first seen as a sphere, measured by Eratosthenes; then fashioned by the fertile mind of Newton into a flattened sphere; and now revealed as a slightly pear-shaped spheroid with a potato-shaped equatorial section, a surface on which continents may move like rigid plates over a lake of treacle. The celestial visitor sees scientists measuring the Earth’s figure correct to one metre, and soon, possibly, correct to a few centimetres by means of laser ranging to artificial satellites.