| 7 | When a comet approaches the Sun
from a great way off it is partly moving sideways and partly falling
towards the Sun. Falling bodies pick up speed, so the comet is moving
faster by the time it gets close to the Sun. Then, on its way back to
the Oort cloud, it gets slower again, just as a cricket ball gets slower
when it is thrown upwards, away from the Earth. Kepler's Second Law (see box 2) explains the variation of the comet's speed in more detail. |