Figure 1.20 A fictitious planet rotating three times per orbit with respect to the Sun.

of 97.8° means that its rotation axis is almost in its orbital plane. We shall return to these oddities when we discuss the origin of the Solar System in Chapter 2.

As in the case of the orbital elements, the axial inclinations and rotation periods of a body are subject to changes, and for the same basic reason - the forces applied by the other bodies in the Solar System. For example, the sidereal rotation period of the Earth is currently increasing, somewhat erratically, by 1.4 x 10-3 seconds per century, largely because of the torque exerted by the Moon on the Earth's tidal distortion. The Earth has had a similar effect on the Moon, and has slowed down the Moon so that it is now locked into a rotation period that keeps it facing the Earth. When one body rotates so that it keeps one face to the body it orbits, it is said to be in synchronous rotation.


Figure 1.21 is an edge view of the Earth's orbit with the positions A and C in Figure 1.18 marked, and the size of the Earth greatly exaggerated. When the North Pole of the Earth is maximally tilted towards the Sun, as at A, there is summer in the northern hemisphere because the surface there is receiving its greatest solar radiation. This is not only because the Sun reaches high in the sky, but also because of the long duration of daylight. By contrast, the southern hemisphere is maximally tilted away from the Sun. □ What season is this hemisphere experiencing?

It is winter in this hemisphere, because solar radiation is thinly spread over the surface and daylight is short. Six months later, at C, the December solstice, the seasons are reversed. It is thus the axial inclination that is responsible for seasonal changes. The eccentricity of the Earth's orbit has only a secondary effect. The Earth is at perihelion in early January, with the northern hemisphere in the depths of winter, and so, as a result of the orbital eccentricity, the seasonal contrasts are reduced in the northern hemisphere, and increased in the southern hemisphere.

Question 1.9

Discuss whether you would expect seasonal changes on Venus. 1.5.1 Precession of the Rotation Axis

So far, the direction of the Earth's rotation axis has been regarded as fixed with respect to the distant stars. This is not quite the case. In fact, it cones around in the manner of Figure 1.22, a

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