<p>I think the Wikipedia article's quotation from <em>The Navigator's Pocket-book. pp. 121–22.</em> means to say that a sighting of a celestial body that's due east or west will contribute less error to the longitude part of a fix than the other sightings. That's certainly true, at least intuitively. Some confusion might come from from poor editing in the second sentence of the quotation. It should say &quot;considerable&quot; rather than &quot;consider,&quot; and &quot;appreciably&quot; rather than &quot;appreciable.&quot;</p>
<p>So what? The article does touch on the importance of using the right celestial  objects for a fix. The consideration is not important just for celestial navigation. In the early days of GPS (even now AFIK), when there was only a small constellation of satelites, the term &quot;GDOP&quot; for &quot;Geometrical Dillution of Precision&quot; was bandied about. The optimal position of four satelites for a fix - minimal GDOP - was said to be three satelites near the horizon at 120 degree intervals and a fourth directly overhead, was given as an example.</p>
<p>The Wikipedia quotation mentions in passing the difficulty of getting things right for sensing when the celestial body crosses the observer's prime vertical. There are lots of difficulties. It's hard to select the stars and planets for a day's work. Azimuth should be one of the criteria used, but not very close to the top of the list.</p>

CC BY-SA 4.0
2023-10-25T22:03:06.450Z
2023-10-25T22:03:06.450Z
stretch
<p>Going through multiple astronomical almanacs or tutorials on astronavigation I encountered a statement that for some particular use (like finding our longitude position) its best to use celestial bodies that are at the moment crossing prime vertical.</p>
<p>Why is it?</p>
<p>Thanks</p>
<p>EDIT:</p>
<ul>
<li><p>Here is a one link to wikipedia article claiming it: <a href="https://en.wikipedia.org/wiki/Prime_vertical" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Prime_vertical</a></p>
</li>
<li><p>Here is link to quora where Bill Hazelton (a PhD seeming to know what he talks about - is claiming the same I posted): <a href="https://www.quora.com/How-do-we-calculate-longitude" rel="nofollow noreferrer">https://www.quora.com/How-do-we-calculate-longitude</a></p>
</li>
<li><p>Here is a link to astronavigation guide that I learnt astronavigation from. I believe it won't help you much, as its in my native language. But maybe with help of Google Translator: <a href="https://www.navipedia.pl/" rel="nofollow noreferrer">https://www.navipedia.pl/</a></p>
</li>
</ul>

CC BY-SA 4.0
2023-10-24T16:51:39.807Z
It is a matter of practicallity.  The body will either be directly due North or due South at that point.  This removes the need to make measurements of other stars, or rely on assumptions about your location which are used in other methods.
Greg Miller
CC BY-SA 4.0
2023-10-24T17:03:10.253Z
It is about Prime Vertical (East-Zenith-West great circle), not Prime Meridian (North-Zenith-South great circle)
3igen3ggy
CC BY-SA 4.0
2023-10-24T22:36:37.207Z
It's hard to imagine the circumstances and setup where that would be so for practical use.  If you can't recall whhere you saw where you saw the statement coulld you give a list of possible sources?
stretch
CC BY-SA 4.0
2023-10-25T01:14:44.113Z
Sorry, I misread your question.  Though that's partly because I agree with stretch, in that I can't imagine that being preferred.  Measuring something on the prime vertical would simplify the problem just as measuring something on the prime meridian (or any known azimuth).  But you have no way of knowing when it crosses the prime vertical without also knowing your location.  So I'd be really interested in a reference too if you have one.
Greg Miller
CC BY-SA 4.0
2023-10-25T04:05:31.493Z
I have added some sources. Will try to search the book-references later.
3igen3ggy
CC BY-SA 4.0
2023-10-27T22:17:31.520Z
@Greg Miller: I'm with you. I had never heard of Prime Verical until seeing the question.
stretch
<navigation>
Why celestial bodies crossing prime vertical are the best to measure for astronavigational purpose?
3igen3ggy
# Why celestial bodies crossing prime vertical are the best to measure for astronavigational purpose?

(Asked by: username_0 on 2023-10-24)

Going through multiple astronomical almanacs or tutorials on astronavigation I encountered a statement that for some particular use (like finding our longitude position) its best to use celestial bodies that are at the moment crossing prime vertical.

Why is it?

Thanks

EDIT:

* Here is a one link to wikipedia article claiming it: <https://en.wikipedia.org/wiki/Prime_vertical>
* Here is link to quora where Bill Hazelton (a PhD seeming to know what he talks about - is claiming the same I posted): <https://www.quora.com/How-do-we-calculate-longitude>
* Here is a link to astronavigation guide that I learnt astronavigation from. I believe it won't help you much, as its in my native language. But maybe with help of Google Translator: <https://www.navipedia.pl/>

**Tags:** navigation

> **Comments:**
> **username_1**: It is a matter of practicallity.  The body will either be directly due North or due South at that point.  This removes the need to make measurements of other stars, or rely on assumptions about your location which are used in other methods.  
> **username_0**: It is about Prime Vertical (East-Zenith-West great circle), not Prime Meridian (North-Zenith-South great circle)  
> **username_2**: It's hard to imagine the circumstances and setup where that would be so for practical use.  If you can't recall whhere you saw where you saw the statement coulld you give a list of possible sources?  
> **username_1**: Sorry, I misread your question.  Though that's partly because I agree with username_2, in that I can't imagine that being preferred.  Measuring something on the prime vertical would simplify the problem just as measuring something on the prime meridian (or any known azimuth).  But you have no way of knowing when it crosses the prime vertical without also knowing your location.  So I'd be really interested in a reference too if you have one.  
> **username_0**: I have added some sources. Will try to search the book-references later.  
> **username_2**: @username_1: I'm with you. I had never heard of Prime Verical until seeing the question.  

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## 1 answers

### Answer by username_2 on 2023-10-25. Score: 2

I think the Wikipedia article's quotation from *The Navigator's Pocket-book. pp. 121–22.* means to say that a sighting of a celestial body that's due east or west will contribute less error to the longitude part of a fix than the other sightings. That's certainly true, at least intuitively. Some confusion might come from from poor editing in the second sentence of the quotation. It should say "considerable" rather than "consider," and "appreciably" rather than "appreciable."

So what? The article does touch on the importance of using the right celestial objects for a fix. The consideration is not important just for celestial navigation. In the early days of GPS (even now AFIK), when there was only a small constellation of satelites, the term "GDOP" for "Geometrical Dillution of Precision" was bandied about. The optimal position of four satelites for a fix - minimal GDOP - was said to be three satelites near the horizon at 120 degree intervals and a fourth directly overhead, was given as an example.

The Wikipedia quotation mentions in passing the difficulty of getting things right for sensing when the celestial body crosses the observer's prime vertical. There are lots of difficulties. It's hard to select the stars and planets for a day's work. Azimuth should be one of the criteria used, but not very close to the top of the list.


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