Movng Across the Sky

This afternoon (July 19th,) the Moon will set at about four o'clock. But tomorrow it won't set until five o'clock. This happens because the Moon is racing towards the east (the same way the Earth is turnning,) so the Earth has to turn a little more each day to catch up to it. It disappears behind the mountains in the west - sets - about an hour later each day.

Last year Jupiter was to the west of the bright star Spica, now is to its east. By next year it will be even further to the east, by Antares in Scorpio.

The Moon and the planets (even the Earth itself) have to be in constant motion, or they would fall: the Moon to the Earth, or the planets to the Sun, pulled by the force of gravity. But if there were no gravity, the Moon and the planets would soon be lost in space. Stable orbits are compromises between the tendency to continue moving in a straight line (inertia) and the pull of gravity.

The stars, too, are in motion, or they would fall to the center of the galaxy. The reason we don't see the them change position from year to year, even century to century, is that they're so far away. But they do change position, ever so slightly. This change in the position of a star is called its proper motion.

That's where a red dwarf called Barnard's star enters the picture. It's a dim star in the constellation of Ophiuchus and it can't even be seen without optical aid. But it's so close (six light years,) and moving so quickly, that it has the highest proper motion of any star: about a half of a degree, or the width of the full moon, every two centuries. That doesn't sound like much, but we've been seeing the same constellations for thousands of years. If most stars were as close as Barnard's star, and had a similar speed, the familiar star patterns would be distorted in a few centuries, and be unrecognizable now.

(07/19/06)