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How does precession change the measured coordinates of stars?
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Astronomical map
Star trails over banksia trees, in Gippsland, Vic., Austl. The south celestial pole, located in the constellation Octans, is at the centre of the trails.
Precession does not cause stars to move; rather, it is a cyclic "wobbling" of Earth’s axis of rotation that changes the orientation of our observing platform in space. Because our coordinate systems for mapping the sky are based on the projection of Earth’s poles and equator onto the celestial sphere, this shift alters the measured coordinates of stars over time.123
Specifically, precession causes the following changes:
- Shifting Reference Points: The celestial poles trace out large circles on the sky over a period of approximately 26,000 years. As the poles move, the stars that serve as "pole stars" change over time.13
- Equinox Drift: The celestial equator, which is the projection of Earth’s equator, shifts along with the axis. Consequently, the equinoxes—the points where the celestial equator intersects the ecliptic—drift westward along the ecliptic at a rate of about 50.4 arc seconds annually.1
- Epoch Dependency: Because the celestial equator and poles are constantly moving, the coordinate systems used on precise astronomical maps are only accurate for a specific point in time, known as an epoch.3