## Planetary Rotation

Each planet rotates around an axis that passes through its centre of mass. In the case of the Earth this rotation axis is shown in Figure 1.17. It intersects the Earth's surface at the North and South Poles, and the equator is the line half way between the Poles. You can see that the rotation axis is not perpendicular to the Earth's orbital plane (the ecliptic plane) but has an axial inclination of 23.4° from the perpendicular.

As the Earth moves around its orbit the rotation axis remains (very nearly) fixed with respect to the distant stars. This is shown (from an oblique viewpoint) in Figure 1.18. The axis is not fixed with respect to the Sun, and so the aspect varies around the orbit. At A the North Pole is maximally tilted towards the Sun. This is called the June solstice, and it occurs on or near 21 June each year. Six months later, at C, the North Pole is maximally tilted away from the Sun. This is the December solstice, which occurs around the 21st of the month. At B and D we have the only two moments in the year when the Earth's rotation axis is perpendicular to the line from the Earth to the Sun. Over the whole Earth, day and night are of equal length, which gives us the name for these two configurations - the equinoxes. The direction from the Earth

Figure 1.17 The axial inclination of the Earth.

Equator

Earth orbital plane (edge view)

Figure 1.17 The axial inclination of the Earth.

Earth rotation Earth axis

Earth orbit (oblique view)

Figure 1.18 The Earth's rotation axis as the Earth orbits the Sun. This is an oblique view of the orbit, which is nearly circular.

Earth rotation Earth axis