Your Sky Help: Precession


The north pole of the Earth doesn't stay pointed at a fixed direction in space. Like a wobbling top, the Earth precesses, tracing out a full circle in the sky every 25,800 years. Since the Earth's axis of rotation is inclined 23.5 to the ecliptic (the plane in which it orbits the Sun), the radius of the circle traced out by the north pole in the sky is the same: 23.5. As the Earth's axis slowly turns in space, the position of all the stars as seen from Earth, changes. “Polaris” is only the pole star at the moment; it reaches its closest point to the true north pole in the year 2105. By A.D. 13000 brilliant Vega will point the way north, at which time our Polaris will be an obscure star almost 45 degrees from the pole. Precession is often called “the precession of the equinoxes”, since one of its most obvious consequences is the slow progression of the position of the Sun at the time of the equinoxes from constellation to constellation over the centuries.

Precession is both the bane and the guaranteed meal ticket of the positional astronomer. Astronomers record the positions of celestial bodies in an equatorial coordinate system—based on the Earth's equator and poles. But since the poles move with respect to the stars, any position must be qualified by the date for which the position is valid (its “epoch”). Converting positions from one epoch to another is tedious yet necessary, and has provided gainful employment for generations of unimaginative astronomers. Your Sky takes into account effects of precession over distant spans of time. All catalogues used by Your Sky are referenced to the current standard epoch of J2000.0 and are valid for most non-critical purposes for the 50 year period from 1975 through 2025. Since correcting the positions of tens or hundreds of thousands of stars for precession is time consuming, Your Sky normally neglects the effects of precession as long as the date is within that interval: outside the half century centred on A.D. 2000, precession is calculated for all objects.

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