Earth Sciences / 55spring becomes autumn). Only at the spring and autumn equinoxes is the noonday Sun directly overhead
at the equator. At noon on 21 June it is directly above the tropic of Cancer and at noon on 21 December
it is directly above the tropic of Capricorn. This is how the two tropics are defined.
Our orbit around the Sun is not circular, but slightly elliptical. We are at our closest to the Sun
(perihelion) on 3 January and furthest away (aphelion) on 4 July and, therefore, we receive
about 7 per cent more solar radiation in January than we do in July. This should make southern-
hemisphere summers warmer than those in the northern hemisphere, and the winters cooler, but
in reality the situation is reversed. This is due partly to the masking of so small an effect by the
general circulation of air, and partly to the fact that at present the northern-hemisphere summer
Figure 2.16 Chemical composition of the atmosphere with height