The Science of Discworld Revised Edition

some regions of the oceans, and take it away from others. The total effect is
tiny
– the surface of the sea rises and falls through a distance of 18 inches (half a metre).
    The coast, where land meets sea, is what creates the big tidal movements. Most of the water is moving sideways (not up) and its motion is affected by the shape of the coastline. In some places the water flows into a narrowing funnel, and then it piles up much more than it does elsewhere. This is what happens in the Bay of Fundy. This effect is made even bigger because coastal waters are shallow, so the energy of the moving water gets concentrated into a thinner layer, creating bigger and faster movements.
    Finally, let’s put the sun back. This has the same kind of effect as the Moon, but smaller. When Sun and Moon are aligned – either both on the same side of the Earth, in which case we see a new moon, or both on opposite sides (full moon) – their gravitational pulls reinforce each other, leading to so-called ‘spring tides’ in which high tide is higher than normal and low tide is lower. These have nothing to do with the
season
Spring. When the Sun and Moon are at right angles as seen from Earth, at half moon, the Sun’s pull cancels out part of the Moon’s, leading to ‘neap tides’ with less movement than normal (these presumably have nothing to do with the
season
Neap …).
    By putting all these effects together, and keeping good records of past tides, it is possible to predict the times of high and low tide, and the amount of vertical movement, anywhere on Earth.
    There are similar tidal effects (large) on the Earth’s atmosphere, and (small) on the planet’s land masses. Tidal effects occur on other bodies in the solar system, and beyond. It is thought that Jupiter’s moon Io, whose surface is mostly sulphur and which has numerous active volcanoes, is heated by being ‘squeezed’ repeatedly by tidal effects from Jupiter.
    Another effect of the Moon on the Earth, discovered in the mid-’90s by Jaques Laskar, is to stabilize the Earth’s axis. The Earth spins like a top, and at any given moment there is a line running through the centre of the Earth around which everything else rotates. This is its axis. The Earth’s axis is tilted relative to the plane in which the Earth orbits the Sun, and this tilt is what causes the seasons. Sometimes the north pole is tilted closer to the sun than the south pole is, and six months later it’s the other way round. When the northern end of the axis is tilted towards the Sun, more sunlight falls on the northern half of the planet than on the southern half, so the north gets summer and the south gets winter. Six months later, when the axis points the other way relative to the sun, the reverse applies.
    Over longer periods of time, the axis changes direction. Just as a top wobbles when it spins, so does the Earth, and over 26,000 years its axis completes one full circle of wobble. At every stage, however, the axis is tilted at the same angle (23°) away from the perpendicular to the orbital plane. This motion is called precession, and it has a small effect on the timing of the seasons – they slowly shift by a total of one year in 26,000. Harmless, basically. However, the axes of most other planets do something far more drastic: they change their angle to the orbital plane. Mars, for example, probably changes this angle by 90° over a period of 10–20 million years. This has a dramatic effect on climate.
    Suppose that a planet’s axis is at right angles to the orbital plane. Then there are no seasonal variations at all, but everywhere except the poles there is a day/night cycle, with equal amounts of day and night. Now tilt the axis a little: seasonal variations appear, and the days are longer in summer and shorter in winter. Suppose that the axis tilts 90°, so that at some instant the north pole, say, points directly at the sun. Half a year later, the south pole points at the Sun. At either pole, there is a ‘day’ of half a year followed by a ‘night’ of half a year. The seasons coincide with the day/night cycle. Regions of the planet bake in high heat for half a year, then freeze for the other half. Although life
can
survive in such circumstances, it may be harder for it to get going in the first place, and it may be more vulnerable to extremes of climate, vulcanism, or meterorite impacts.
    The Earth’s axis can change its angle of tilt over very long periods of time, much