After doing some research it seems to be as follows:
"The First Point of Aries" is considered to be the celestial "prime meridian" (or direction where the x axis points) from which right ascensions are calculated. Due to various reasons this point is NOT constant but drifts.
|Vernal and Autumnal Equinoxes|
When the Earth is at the Vernal Equinox (Spring Equinox) you can find this point by drawing a line from the center of the Earth through the center of the Sun and forward. The First Point of Aries (also known as the Cusp of Aries) is so called because, when Hipparchus defined this point in 130 BCE, it was located in the western extreme of the constellation of Aries. Now it is not anymore there but the point is still called so.
Since the sideral year is about 365 and 1/4 days .. the vernal equinox will happen almost the same time every 4th year at any given place on the Earth (or there is 6 h drift every year) .. but since the numbers are not exact and there are drifts the timing varies.
|"The First Point of Aries"|
Notice also that the Vernal Equinox can basically happen any time during a day or night SINCE the rotation of the Earth and its position on the orbit around the Sun are NOT connected to each other.
|Standard Solar System coordinates|
"J2000" (Current Standard Epoch)
Since the right ascension and declination of stars are constantly changing due to precession, (and, for relatively nearby stars due to proper motion), astronomers always specify these with reference to a particular epoch. The earlier epoch that was in standard use was the B1950.0 epoch.
When the mean equator and equinox of J2000 are used to define a celestial reference frame, that frame may also be denoted J2000 coordinates or simply J2000.
|Right ascension and declination|
Here is how to convert coordinates from J1950 to J2000:
- convert B1950.0 RA and DEC to decimal degrees
- work out the following formulas RA2000 = RA + 0.640265 + 0.278369 * sin(RA) * tan(DEC)
- convert RA back to (what ever you need)
DEC2000 = DEC + 0.278369 * cos(RA)
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