Thank you, Jim
You are the second one to send me this explanation. I also found it in
Astrotools. The concept is quite clear and not as complicated as I
thought...and it can get quite complicated.
Thanks again
Otis
----- Original Message -----
From: "Jim Van Nuland" <jvn@No-Spam>
To: "The Astronomy Connection" <sf-bay-tac@No-Spam>
Sent: Tuesday, March 15, 2005 2:02 AM
Subject: Re: [TAC] Fw: Alcyone
> 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
> JVN's web site
>
>
Received on Tue Mar 15 06:29:24 2005