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

<p><em>a later edit with an answer</em></p>

<p>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. </p>

<p>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.</p>

<p>Four minutes of Right Ascension = One degree of longtitude. 
One minute of RA = 15 arc minutes of longitude. </p>

<p>A worked example:</p>

<p>Greenwich Sidereal time 17:42<br>
          Local Sidereal time 16:36 hours </p>

<pre><code>      Greenwich is ahead (more) than local, therefore local is West.
</code></pre>

<p>The difference = 42 minutes plus 24 minutes </p>

<p>= 1 hour 6 minutes of RA to the west of Greenwich</p>

<p>Thus, 60 minutes = 15° longitude</p>

<p>Add  4 minutes =   1°   longitude</p>

<p>Add   2 minutes =   0°30' longitude</p>

<p>= longitude 16°30'W </p>

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

<p><a href="http://www.abecedarical.com/javascript/script_clock.html" rel="nofollow">http://www.abecedarical.com/javascript/script_clock.html</a> will show GST </p>

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

<p><em>information for background</em></p>

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

<p><a href="http://www.thegreenwichmeridian.org/tgm/articles.php?article=9" rel="nofollow">The Quest for Longitude and the Rise of Greenwich – a Brief History</a></p>

<p><em>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?</em></p>

CC BY-SA 3.0
2016-02-03T19:10:36.577Z
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.
MargaritaMc
CC BY-SA 3.0
2016-02-03T20:04:14.447Z
Can you include an answer in your answer, rather than just providing links to other websites?
Featherball
CC BY-SA 3.0
2016-02-04T09:12:13.773Z
Thanks for your help and your patience. I've just added a proper answer.
MargaritaMc
CC BY-SA 3.0
2016-02-04T09:20:52.830Z
Welcome to Astronomy Stack Exchange :)
Nico
CC BY-SA 3.0
2016-02-03T17:42:36.013Z
2016-02-04T09:16:43.253Z
2016-02-04T09:16:43.253Z
MargaritaMc
<p>Here is the problem: I need to calculate longitude of observing place. And I know <strong>time</strong> (in UTC+1) of star culmination (Sirius) and star <strong>altitude</strong>. I also know <strong>RA</strong> (right ascension) and <strong>Dec</strong> (declination) of the star and I also know <strong>RA</strong> of the Sun for that day. I already calculated latitude but I can't figure out how to calculate longitude.</p>

<p>Thank you.</p>

<observational-astronomy>
Calculating longitude from star culmination
Brob
# Calculating longitude from star culmination

(Asked by: username_0 on 2016-01-30)

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.

**Tags:** observational-astronomy


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## 1 answers

### Answer by username_1 on 2016-02-03. Score: 1 | Accepted answer

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](http://www.thegreenwichmeridian.org/tgm/articles.php?article=9)

*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?*

> **Comments:**
> **username_1**: 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.  
> **username_2**: Can you include an answer in your answer, rather than just providing links to other websites?  
> **username_1**: Thanks for your help and your patience. I've just added a proper answer.  
> **username_3**: Welcome to Astronomy Stack Exchange :)  

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