Online Classroom: Moons semi diameter in perp almanac

This is topic Moons semi diameter in perp almanac in forum Public Discussion of Cel Nav at Online Classroom.

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Posted by cheese'n'egg (Member # 1547) on October 17, 2008, 12:14 AM:

Hi
I am a newbie to this forum, but not to navigation. I was doing some moon sight practice reductions, when I noticed that the SD was always zero on the page of the perpectual almanac. This caused me to go back to the published nautical almanac for that year to get the info. Am I missing something here ? I have down loaded the program again to make sure, hovever the SD still registers as zero.

Posted by Instructor • (Member # 1) on October 17, 2008, 08:40 PM:

I am afraid that we do not compute the SD for the moon. As i recall, we could not get this to the accuracy of the other data, so we decided not to include it.

I will add this note to the tech support. In searching the help file of the program there is an implication that it does do the moon, but that is not right.

The SD of the moon does not enter into any part of routing celestial navigation, so i guess that is why we have not heard back on this till now.

Thanks for pointing this out.

If you need the value of the SD of the moon for past years it can be found at the USNO Astronomical Applications website, under Celestial Navigation Data. It is the first link in our cel nav INET resources in the library. Once there, go to cel nav data data section. Just enter time and date. If moon is not there, change the time some till it shows up.

Posted by cheese'n'egg (Member # 1547) on October 18, 2008, 12:59 AM:

Ah ! That would be why then.
I have to disagree with you about the Moons SD not being relevant. It certainly is when working moon sights from first principles. The same as it would be for the Sun.

Posted by Instructor • (Member # 1) on October 18, 2008, 10:45 AM:

Please explain where we use the semidiameter of the sun or moon when doing routine celestial navigation sight reductions.

In other words, if that column in the almanac were not there at all, what would be the consequences.... not counting its role in what we call the solar IC method, which is an optional technique useful in some circumstances.

Posted by cheese'n'egg (Member # 1547) on October 19, 2008, 11:42 PM:

Okay. Let me first position myself.
When I first started learning the dark arts of navigation, it was at a British Merchant Navy college. Here we had to work from first principles, later we were allowed to use total correction tables. So Yes, if we do not have SD we can use the total correction tables. However I am working from first principles. So the corrections to be applied to Apparent Alt are as below.

Sun

Refraction
Semi Diameter
Parallax

Moon

Refraction
Semi Diameter (after augmentation)
Parallax (Eqautorial horizontal parralax to be
reduced for Latitude)

Posted by Instructor • (Member # 1) on October 20, 2008, 04:23 PM:

OK, very good. I can see that if you had these separate data you could apply them separately.

But it would take some effort these days to find those data in separate tables and, going back to my original point, if you have an almanac, these are all combined into the correction factors we use, as you point out.

My main goal is to be sure that those new to celestial navigation do not think they are missing any data. Everything we need is in the almanac and we will never do any better celestial navigation having access to any other types of tables.

(Even in fact if we got access to higher accuracy ephemeris data, which are available with not too much effort, we would not do better in the long run, as the correction tables and data accuracy and practical sextant accuracy are all pretty well matched in the present system.)

Posted by cheese'n'egg (Member # 1547) on October 21, 2008, 04:24 AM:

I did not mean to create a schism in the camp.
Navigation is a mixture of art and science.
The art of taking, and knowing what is a good sight.
The science of the number crunching.
Then the art of interpreting the results to find out where the devil you were when you did all that.
Going back to the original problem. I think I have found a method of deriving the S.D. from the H.P. of the moon.
SD (moon) = 0.2724 X H.P.
This works very well with my moon problems, and your perp almanac.
I found this lurking in my old notes, God knows where it comes from.