Vernal Equinox -- astrometric vs. apparent?

Akkana Peck

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

#3180

Akkana Peck <yahoo@...> writes:

Geeky astronomical coordinate question ... 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.When I packaged the XEphem computation routines so that they could be
used by Python programmers, I went ahead and attempted to write up an
explanation of how these coordinates differ.  My explanation might not
be very clear - and might even be wrong if there are points of the
science that I misunderstood - but here it is, in case it helps you:

    

-- 
Brandon Craig Rhodes   brandon@...   

#3181

Change the equinox from 2000 to equinox of date and see what happens.

Bud

Show quoted text

On 3/21/2011 11:35 AM, Akkana Peck wrote:
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


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Akkana Peck

#3182

Brandon Craig Rhodes writes:
When I packaged the XEphem computation routines so that they could be
used by Python programmers, I went ahead and attempted to write up an
explanation of how these coordinates differ.  My explanation might not
be very clear - and might even be wrong if there are points of the
science that I misunderstood - but here it is, in case it helps you:

    Thanks! That's by far the best explanation I've found. I wrote
a little PyEphem program to watch the three sets of coordinates
change, and convinced myself that was indeed the issue. Also,
next_equinox() is a nice straightforward way to get the exact
time, next time I want to verify it ... much easier than trying
lots of dates by hand in XEphem.

I'm still amazed the difference relativistic deflection + nutation +
aberration of light makes a difference of four hours in the time
of the equinox. I don't suppose PyEphem offers a way to separate out
these effects, by any chance? I'm guessing nutation is the big one in
this case.

	...Akkana

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Vernal Equinox -- astrometric vs. apparent?