What is precession? Do Celestron mounts correct for it?

Precession is the movement of the north celestial pole (NCP) among the constellations over long periods of time. It is caused by external gravitational forces from the Sun, Moon and planets acting on the not-perfectly spherical Earth and changing the tilt of Earth’s rotational axis. This is just like a top’s gyrations on a tabletop, as its spin and weight changes its tilt direction.

The NCP defines 90° north in the equatorial coordinate system. Since the direction of the NCP is changed by precession, the celestial coordinates right ascension (RA) and declination (Dec) are also changed by precession.

The year-to-year change is not very large for most purposes because the entire cycle is very long. The direction of the NCP changes by about 47° over a 26,000-year period, so the NCP changes place among the constellations by about 50 arc seconds per year or about 1° every 72 years. This means that Polaris was not the Pole Star centuries ago and will not be the Pole Star in centuries to come. Vega will be the closest bright star to the NCP in 14,000 AD.

Even though the change is small, it can have significant effects on the accuracy of astronomical observations. As one example, the direction of a north-south line on the sky changes with time. Most star atlases will have a date (examples: Epoch 1950.0 or J2000.0) indicating that the coordinates in the atlas are corrected for precession as of that time or epoch.

Computerized telescopes must also take precession’s changes to coordinates into account for greatest accuracy. Celestron hand control firmware versions 4.15 and later use epoch JNow, meaning they are fully correcting for precession’s effects on stellar coordinates up to the present moment.

Polar finderscopes with reticles must also take account of precession because they show the position of the NCP among the stars. The reticles will show the position of the Pole over a span of years as a line with dates on it.

Updated 10/29/13