56 / Basics of Environmental Science
(March to September) is five days longer than the winter (September to March), a situation that
changes slowly. The gravitational attraction of the Sun, Moon and, to a much smaller extent, the
planets on the slight bulge around the Earth’s equator cause the position of the Earth at the equinoxes
to move westward by 50.27" (“=seconds of arc) a year, so it takes 25800 years for them to complete
a full cycle and return to their starting position. This is called the precession of the equinoxes and it
alters the dates at which the Earth is at perihelion and aphelion. About 13000 years from now we will
be at aphelion (furthest from the Sun) in January.
It is within the tropics that the surface receives the most intense insolation and, therefore, is heated
most strongly. The movement of air and oceans then carries heat from the tropics into higher latitudes.
This transport of heat produces the main ‘climates’ of the world and our day-to-day weather.
To either side of the equator, the prevailing winds are from an easterly direction and they are so
reliable that sailing ships made extensive use of them. The name ‘Trade Wind’ has nothing to do
with commerce. ‘Trade’ used to mean ‘course’ and ‘to blow trade’ meant to blow in a constant
direction. Such was their importance that eminent scientists theorized about their cause and it was
from their calculations that the first understanding arose of the way the atmosphere transfers heat.
In 1686, the astronomer Edmond Halley (1656–1742) suggested that hot equatorial air rises and is
replaced by cooler air from higher latitudes. He was almost correct, but he could not explain why
the returning air arrived from the north-east and south-east, rather than from due north and south.
This was explained in 1735 by George Hadley (1685–1768). He realized that the Earth rotates
beneath the air, changing its apparent direction of flow, but a century passed before the French
physicist Gaspard Gustave de Coriolis (1792–1843) discovered what really happens. In 1835 he
proposed what is now known as the ‘Coriolis effect’ (or ‘force’, although no force, in a mechanical
sense, is involved).
Figure 2.17 Seasons and the Earth’s orbit