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- Thread starter Wheelwalker
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- #3

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I wouldn't want to do it in my head, but here's a pretty simple formula to get the sidereal time at Greenwich for any date and time. From there, you just adjust for your longitude, as you did above.

https://sites.google.com/site/physi...ts/universaltimetolocalsiderealtimeconversion

If you don't understand it, just google the various terms such as "Julian date." You might also consider googling for "practical astronomy" or "ephemeris calculations" to get a more comprehensive tutorial on time conversion.

If you just want the answer without doing the calculation yourself, there are lots of sites that will do it for you, e.g.

http://www.jgiesen.de/astro/astroJS/siderealClock/

- #4

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Yes you would need time but you would also need position. Azimuth and Altitude must be calculated from the Hour Angle, Latitude and Declination with Altitude to be 90 degrees.

HA = f(gst, time and Longitude minus the Right Ascension)

AZI = f(HA, Latitude, Declination)

ALT = f(HA, Latitude, Declination)

HA = f(gst, time and Longitude minus the Right Ascension)

AZI = f(HA, Latitude, Declination)

ALT = f(HA, Latitude, Declination)

Last edited:

- #5

BobG

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Local sidereal time (which is the same as right ascension) is the Greenwich Mean

You can look up the GMST at midnight for a particular date in an Astronomical Almanac, and then add the current Greenwich Mean Time (solar) to get your current GMST.

Or you can do the easy thing and download a spreadsheet that calculates GMST for any time you want at any given longitude just by setting the azimuth to 180. Or, if your real goal is correlate your location and your telescope's az/el, it will calculate the RA/dec of the object you're looking at directly instead of you having to figure it out manually. (You should do all of this manually at least once, just because, but I don't think I know anyone that would want to do this manually all the time.)

http://www.setileague.org/software/spreadsh.htm

This has several useful spreadsheets. You want the ra_dec.xls spreadsheet to get right ascension & declination.

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