ok I know a bit about the celestial sphere and right ascension, declination, equinoxes, solistices, etc. What is the absolute origin in the celestial sphere? I know RA measurements use the vernal equinox, and that declination is +/- 90 degrees north/south of the celestial equator. I thought Polaris might be used as a fixed marker, but I also know precession means that Polaris is close to the NCP, but not exactly coincident with the NCP. Epochs and equinoxes are moments in time. So, what is used as an absolute marker of the celestial sphere from which all other markers, grids are derived?