Otis Finley wrote:
>
> Do any of you use the Alcyone Ephemeris software? If so, what is the
> distinction between the geocentric eq. coordinates and the topocentric
> eq. coordinates. They seem very close.
Hi, Otis,
I don't use Alcyone, but I can discuss the co-ordinate systems.
As you have noticed, they are usually very close, and in many cases,
you can ignore the difference.
"Geocentric" treats all calculations as from the center of the Earth;
in effect, it treats Earth as a point. It's a bit hard to think of the
surface of a point as having longitude and latitude, so you might think
of a very small sphere.
Topocentric adds another dimension: elevation.
If you calculate the elevation and azimuth of a star, the results will
be essentially the same in either system. If you calculate the rise or
set time of that star, (thinking of an ordinary flat horizon), you'll
get the same results.
But if you do that for something nearby, such as the Moon, you'll get
as much as a degree of difference in elevation, and a several minutes of
time difference in rise or set. This is because the radius of the
earth, 4000 miles, is not tiny compared to the distance to the moon,
~240,000 miles. For the Moon, that little angle arctan(4000 / 240000)
about 0.95 degrees. For the Sun, about 9 arc-seconds. For Saturn, 1
arc-second. You see why the two systems converge for distant objects.
If you calculate for a position well above the earth (say, from a
plane at 33,000 feet), the topocentric rise or set time will differ
noticeably, because from a height, you're looking down on the horizon.
This is simplified, as the effect of refraction due to the atmosphere
must also be taken into account. and makes as big a difference.
Occultation and Solar eclipse calculations must be done in
topocentric, and must include the specific elevation of the observer's
location. The actual distance to the sun and moon must be included.
Astronomical calculation is a large field of study, and I've scraped
only the top molecular layer.
Clear Skies!
-- Jim Van Nuland, San Jose (California) Astronomical Association <a href="http://www.svpal.org/~jvn/ ">JVN's web site</a>Received on Tue Mar 15 03:05:59 2005