Perplexing Precession

The First Point of Aries really was in Aries when it was named roughly 2,000 years ago. It has crept into the stars of Pisces because of precession, a slow shift in the orientation of the Earth's axis with respect to the rest of the universe.

You've probably seen an example of precession firsthand, even if you didn't realize it. Put a spinning top at an angle on a table and it will precess — its spin axis will slowly circle around the upward direction of the force that the table applies to the point of the top. In exactly the same manner, the spinning Earth slowly precesses because of the force that the tidal gravitational tugs of the Moon and Sun apply to the Earth's slight equatorial bulge.

Because of precession, we see the north celestial pole, which is currently located close to Polaris, swing across the stars in a wide loop around the north ecliptic pole every 26,000 years. The south celestial pole similarly loops around in the far-southern sky. The moving celestial poles drag the whole celestial-coordinate system — the whole grid of declination and right ascension — along with them.

Contrary to popular belief, precession does not shift the Earth's axis with respect to the Earth's own geography. The terrestrial North Pole doesn't move to a new location (at least not much on the time scales we're talking about). Precession won't give walruses a tropical suntan. The only noticeable changes are those that result from the grid of celestial coordinates moving against the stars. In 10,000 years, for instance, Vega will be the north star, and Orion will be a constellation of summer, not winter (for Northern Hemisphere residents).

Because the coordinate grid insists on sliding around this way, a star's right ascension and declination are continually changing. To fix a star's position you need to specify the date for which a right ascension and declination apply. The current standard is "equinox 2000.0," shorthand for "right ascension and declination at the moment the year 2000 began." The previous standard, still encountered on some star charts, was 1950.0.

For moving objects such as the Sun, Moon, and planets, right ascension and declination are often given for the "equinox of date": the R.A. and Dec. values that are correct for the actual date listed. In Sky & Telescope's monthly table of Sun and planet positions, the positions are given in the coordinate system for each date listed.

Rarely, however, do backyard astronomers need to worry about precession. From 1950 to 2000 the coordinate grid crept along the ecliptic by only 0.7°, less than the width of the lowest-power view in many telescopes. And that amount applies only at the ecliptic itself. The total shift is less elsewhere, declining to essentially zero at the ecliptic poles.

Which way does precession go? In most of the sky, it makes a star's right ascension increase each year. That is, an old right-ascension value precedes (is less than) the newer value in amount as well as in date.

As for right ascension itself, just remember that it increases to the east. If you get confused about which way is east on a star map that shows right ascension, this little mnemonic will get you squared away.