Geeky astronomical coordinate question:
Yesterday was the Vernal Equinox. The RASC handbook says it happened
at 23:21 UT, but I wanted to check that in XEphem. I couldn't figure
out how to get it to tell me directly, so I went to the Sky View
and right-clicked on the Sun at different times to find out when
it crossed 0 declination. That happened four hours after the date
the RASC lists, at about 03:15. Most tables list the 23:21 time
as the time of the equinox.
Subsequent research, and running it on the JPL Horizons simulator
seems to indicate this is a difference between astrographic
coordinates (used by xephem) and apparent coordinates (RASC and
other tables). But none of my references are very clear what the
difference is and why they differ so much. The best I've been able
to come up with is that apparent coordinates are corrected for the
motion of the Earth during the light travel (approx 8 minutes)
from the body being observed, while astrographic coordinates are
only corrected for the other body's motion during that time.
But Horizons says both sets of coordinates are corrected for
light travel time.
Anybody know what the difference is, or know a good reference that
explains it?
Also, is there a better way to ask XEphem for the time of the equinox?
...Akkana
