Here is the problem: I need to calculate longitude of observing place. And I know time (in UTC+1) of star culmination (Sirius) and star altitude. I also know RA (right ascension) and Dec (declination) of the star and I also know RA of the Sun for that day. I already calculated latitude but I can't figure out how to calculate longitude.

Thank you.


1 Answer 1


This is my first post, and I've accidentally deleted almost all I've written!

a later edit with an answer

The RA of a culminating star gives you the local sidereal time and comparison with Greenwich Sidereal Time gives your local longitude by noting the difference between the two.

If local sidereal time is less than Greenwich, your position is west of the prime meridian at Greenwich; if it is more, then your position is east of Greenwich.

Four minutes of Right Ascension = One degree of longtitude. One minute of RA = 15 arc minutes of longitude.

A worked example:

Greenwich Sidereal time 17:42
Local Sidereal time 16:36 hours

      Greenwich is ahead (more) than local, therefore local is West.

The difference = 42 minutes plus 24 minutes

= 1 hour 6 minutes of RA to the west of Greenwich

Thus, 60 minutes = 15° longitude

Add 4 minutes = 1° longitude

Add 2 minutes = 0°30' longitude

= longitude 16°30'W

Which is accurate to 0°05' of the usually given figure for my location.

http://www.abecedarical.com/javascript/script_clock.html will show GST

I use a Sidereal Clock app on my Android tablet, set to show both GST and local sidereal time at my location.

information for background

This is the historical background for the search for a reliable method of calculating longitude when at sea

The Quest for Longitude and the Rise of Greenwich – a Brief History

I've edited this several times because I'm finding my way around how this works and made several editing mistakes! I've also tried to add another link, and got an error message, but no explanation. Is there a limit to the number of hyperlinks permitted?

  • $\begingroup$ Does this actually answer the question, though? $\endgroup$
    – user21
    Feb 3, 2016 at 19:05
  • $\begingroup$ When did you read my post @barrycarter? Was it when I had deleted most of it by mistake? I'm new here and want to learn how the forum works. $\endgroup$ Feb 3, 2016 at 19:10
  • $\begingroup$ Yes, I read it when the last paragraph was the only thing there. I'll reread it now (right now, my comments reads "46 mins ago" and your answer reads "edited 44 mins ago", so it looks like I missed it by 2 minutes!) $\endgroup$
    – user21
    Feb 3, 2016 at 19:51
  • $\begingroup$ Can you include an answer in your answer, rather than just providing links to other websites? $\endgroup$ Feb 3, 2016 at 20:04
  • 1
    $\begingroup$ Thanks for your help and your patience. I've just added a proper answer. $\endgroup$ Feb 4, 2016 at 9:12

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