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If declination is the degree measurement from the celestial equator roughly the Earth equator unless I've misunderstand plane. "The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator." https://en.wikipedia.org/wiki/Celestial_equator
Where is zero degrees for Right Ascension, is it the GMT Line or International Date Line or some other line?
0 RA is the direction from the center of the Earth to the vernal equinox. You've already imagined the plane in space created by projecting the equator into space. Now consider the plane formed by the mean orbit of the Earth; this is the ecliptic. The two planes intersect in a line. The ray from the center of the Earth toward where the Sun is in March is the vernal equinox; the ray from the center of the Earth toward where the Sun is in September is the autumnal equinox.
The big difference is that the location of the vernal equinox among the stars changes very slowly, taking about 26,000 years to sweep all around the ecliptic. The plane that passes through the north and south pole and the Royal Observatory, Greenwich, sweeps through all the stars every day.