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I work whit Heavensat program to understand the behavior of satellites around the Earth, lately something surprised me. The question is that "Are Right ascension / Declination values of a satellite invariant for different observer on the earth at a fixed time?"

I thought that the answer is "Yes", But in this program when I change the observer location, the values of RA/dec will change!

I used some other algorithm to calculate RA/dec manually and check the program. The output was the same!

These are the output of heavensat for a fixed satellite and time for two different observer site:

enter image description here I changed the observer site a little and you can see the changes in RA/dec enter image description here

Is there anything I should know about Right ascension and Declination? Or it has different concept for satellites?

Thanks

Saeed Naseri
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  • Right ascension and declination refer to the equatorial coordinates system, which does not depend on the observer. However, I see on the images that time is local, and local time depends obviously on the location of the observer. Does the problem disappear if you switch to UTC? – Jean-Claude Arbaut Nov 16 '14 at 16:19
  • Thanks, But I've read this article, beside I learned astronomy course at college. I know it doesn't depend on the observer, but how can you explain the output of program? – Saeed Naseri Nov 16 '14 at 16:24
  • What about the second part of my comment? – Jean-Claude Arbaut Nov 16 '14 at 16:24
  • Unfortunately it has no effect. it can be downloaded here: http://www.heavensat.ru/download.php?file=heavensat246.zip – Saeed Naseri Nov 16 '14 at 16:27
  • After downloading this and a database from space-track, I have no clear answer: elevation and azimuth are correctly related to the observer (for instance, the green oval line is where elevation=0, which is the "horizon", from the satellite), but I can't explain why RA and DEC vary when you change the observer. Maybe it would be better to ask directly to Alexander Lapshin ([email protected]). – Jean-Claude Arbaut Nov 16 '14 at 17:11

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RA and DEC indicate the position of the satellite on the background sky. Star positions can be described with RA/DEC because they are infinitely far and do not move relatively to one another when you change your position. Satellites are much closer, so if you move they will move compared to the star background.

This is why if you change observer site you will see the satellite RA and DEC will change.

They do not define the satellite position but the direction toward the satellite.

gosnold
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