[AstroPy] computing latitude/longitude from RA/Dec

[Michael Brewer]

Oh, and please make sure that your RA/Dec coordinates are with respect 
to the true equator and equinox of date (TETE).

On 8/25/22 8:46 PM, Michael Brewer wrote:
> Michael,
>
>   This is fairly easy to do. First convert AltAz to Cartesian 
> coordinates. This is a left handed coordinate system. The X  axis 
> points North, and the Y axis points East. Then rotate clockwise around 
> the Y axis by 90 - latitude and reverse the X and Y coordinates so 
> that the X axis points South and the Y axis points West. You are now 
> in the HADec coordinate system and the LHA ls the angle between the X 
> and Y axes measured clockwise from the X axis.
>
>
> On 8/25/22 7:06 PM, Michael Hoenig wrote:
>> Hi,
>>
>> I currently have an image of the night sky and matching RA/Dec 
>> coordinates of the center.  I also have the azimuth and elevation the 
>> camera was pointed at, and the time.  I am trying to use this info to 
>> calculate the latitude and longitude on Earth where the picture was 
>> taken from, using AstroPy methods...
>>
>> latitude = Dec +/- zenith distance, this is easy.
>>
>> longitude = LST - GMST, where LST = LHA + RA.  This one I'm having 
>> some more trouble with...
>>
>> I can create a Time() object, and then use .sidereal_time('mean') to 
>> get the LST, but of course that requires the location to begin with.
>>
>> I can create a SkyCoord object from my RA and Dec, but is seems a 
>> simple transform_to() isn't sufficient either.
>>
>> What am I missing here?  Is this possible in AstroPy, or do I need to 
>> resort to something like PyEphem or Skyfield?
>>
>> Thanks for any help!
>> Michael
>>
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