Q:

Saying that the prime meridian passes through the galactic pole does not seem very specific to me. Is it not necessary to specify at least one more direction (point on the celestial sphere) to locate a meridian?

A:

Yes it is necessary to specify another point on the celestial sphere to fix the celestial coordinate system. This point is the spring or vernal equinox, which is defined as the point where the ecliptic - the apparent yearly path of the Sun crosses the celestial equator moving north. This point is also called the "First Point of Aries," but typical of many confusing labels in astronomy, is now actually located in the constellation Pisces.

We say "now" because the location of the equinox is steadily changing due to the wobbling or precession of the Earth's axis of rotation with a 26,000 year period. As a result, the location of the vernal equinox moves slowly westward along the ecliptic, at about 50 arc seconds per year. The effect is small, but it adds up over the years, and must be considered when plotting accurate positions of the stars.

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