calculating latitude/longitude from RA/Dec/time

[Michael Hoenig]

Has anyone managed to successfully use the RA/Dec from an image to calculate where on Earth it was taken from?

If I use my phone, I can get the time (and convert to UTC), the azimuth (heading) and even an approximate reading of the altitude/zenith distance.  That should be all the information that is required.

There is some interesting discussion of this problem in threads like this one and this one, but only in passing.  Now that I'm trying to perform the calculations, I'm not sure they're giving me sensible results.

Latitude: this is simply the Declination, +/- the zenith distance.

For longitude, we need to compute the local sidereal time (RA +/- azimuth angle) and the

GMST (Greenwich mean sidereal time) at the time the image was taken.
Longitude = LST - GMST.

Does this look right, or am I missing something?

Grateful for any input, thanks.

[Dustin Lang]

For LST, you want LST = HA + RA, where HA = hour angle.  From HA and Alt you should be able to get azimuth.

If I were trying this, I would probably try to use Astropy - they have some pretty good coordinate transform routines.

In detail, you also need to take into account atmospheric refraction and probably coordinate precession.

cheers,

dustin

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[Michael Hoenig]

Thanks, but....

For LST, you want LST = HA + RA, where HA = hour angle.

Don't I need longitude to calculate the HA in the first place?

[Dustin Lang]

Oh yeah :)  I don't know, I guess you do need the azimuth (this is reminding me of historical challenges finding longitude on ships - there the challenge was timekeeping, but I think they used to time the transits of stars?  ie they would observe a star with a known RA at a known HA = 0 ie at the meridian).  You've got some vector math in your future!

--dstn

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[CaptAndy Sir]

You're doing celestial navigation.  Google for online tools or apps.  Two altitudes with times gives two intersecting LOP (lines of position) just like at sea.  One alt with az will do, but precise az at sea  is too hard.

[Michael Hoenig]

On Saturday, August 13, 2022 at 8:07:38 AM UTC-10 walde...@gmail.com wrote:

You're doing celestial navigation.  Google for online tools or apps.  Two altitudes with times gives two intersecting LOP (lines of position) just like at sea.

But I have more information here than just altitudes and times.  Can't I make use of that?