Solar Radiation Reaching The Earth

The amount of radiation received by the Earth from the Sun is governed by the geometry of the Earth's motions (Figure 2.3). Its orbit around the Sun is not quite circular and the Sun is offset from the centre. As a result, the Earth is only 147.1 million kilometres distant from the Sun at the nearest point (the perihelion), presently on 3 January, and 152.1 million at the aphelion on 4 July, so 6.5 per cent more radiation reaches the Earth in January (Note 2.E). This would tend to make global temperatures higher in January than in July, i.e. summer in the southern hemisphere hotter than in the northern. This is not observed in practice, as the tendency is masked by the effects of different amounts of ocean in the two hemispheres (Chapter 11).

The Earth moves around the Sun in the plane of the ecliptic and spins on its own axis, which is tilted from a line perpendicular to the plane (Figure 2.3). All of these features are subject to slow, regular changes, described by Milutin Milankovic in 1930, following the ideas of James Croll in 1867. First, the Earth's path around the Sun varies from almost circular to more elliptical and back, each 97 millenia.

Figure 2.3 The geometry of the Earth's movement about the Sun, in an orbit which forms an ellipse on a flat plane (the ecliptic). The Earth spins about an axis which currently points to the North Star, Polaris. Both rotations are clockwise if viewed from the South Pole.

Figure 2.3 The geometry of the Earth's movement about the Sun, in an orbit which forms an ellipse on a flat plane (the ecliptic). The Earth spins about an axis which currently points to the North Star, Polaris. Both rotations are clockwise if viewed from the South Pole.

Second, there is a variation of the tilt (or obliquity) of the Earth's axis, between 21° 59' and 24° 36' degrees and back, each 40,400 years. The tilt (presently 23° 27') is now becoming less, tending imperceptibly to reduce the annual range of surface temperatures. Third, the Earth's tilted axis wobbles (or 'precesses'), describing a cone each 21 millenia. Each of these variations alters the difference between summer and winter temperatures (Chapter 3). Occasionally, the three rhythms come briefly into coincidence, working together then to either maximise or minimise the difference between summer and winter. Rare coincidences of minimum difference probably triggered past ice ages, as a result of the increased precipitation of relatively warm winters and the reduced snowmelt of cooler summers (Chapter 15).

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