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» Online Classroom   »   » Public Discussion of Cel Nav   » Ex meridian sights...

Author Topic: Ex meridian sights...
David Burch

 - posted February 17, 2004 11:33 PM      Profile for David Burch           Edit/Delete Post 
We have this part of a post from Bill that is copied since it was originally together with an unrelated subject. Separate topics for separate subjects keeps things tidy.

"I'm also working on latitude by ex-meridian transit (any body) and I don't see how to accomplish this. I'm aware it's not in the course material, but I'm hoping you can help with this.
Bill "

This is now the place we can discuss this subject.

From: Starpath, Seattle, WA
David Burch

 - posted February 18, 2004 12:21 AM      Profile for David Burch           Edit/Delete Post 
This is covered in Bowditch in some detail. Let me for now just say that the procedure involves figuring out what your latitude is when you happened to have just missed the peak of the LAN sight.

There are several approaches. The most practical one is to just forget it, and do a sun line sight reduction and plot the LOP. Wait an hour or so, do another one, and get a running fix, then you get latitude and longitude.

I will come back to this and compare our method with that given in Bowditch. We do of course solve this problem very elegantly in the StarPilot software and calculator programs, but you cannot use them on a USCG test, which is the main place this question might occur... you can use them, however, to test that you did solve the problem correctly.

From: Starpath, Seattle, WA
David Burch

 - posted February 20, 2004 10:44 PM      Profile for David Burch           Edit/Delete Post 
I might add that those who have the CD version of our course materials will find on it the complete sections of Bowditch that treat celestial navigation. There are numerous chapters and we have grouped them together and put them on the cel nav CD as reference materials.
From: Starpath, Seattle, WA
David Burch

 - posted March 06, 2004 11:15 PM      Profile for David Burch           Edit/Delete Post 
to follow up on this topic:

an ex meridian sight is one that is used to determine latitude from a sight or series of sights that missed meridian passage, ie the peak height of the body.

the procedure is:

(1) measure the height and note the time.

(2) then compute the time of mer pass of the body from the DR lon of observation.

(3) then do a sight reduction of the body for Hc for both the time of observation and for the time of mer pass using your DR lat and lon.

(4) subtract the two heights to figure how much the body has descended after passing mer pass. Add this to your obsreved Ho and then do a normal LAN type of analysis for the latitude.

Again, though, it is very unlikely that this type of process would be called for, but it is a good exercise in putting together what you know about the process.

(I might also mention that the StarPilot calculator progam has a function that does this automatically for you. It is called Sight Analyzer, and this is just one side application of that function, not its main job.)

See also related note in the topic called Ex meridian of stars.

From: Starpath, Seattle, WA

 - posted June 20, 2008 10:30 AM      Profile for WJohntri           Edit/Delete Post 
I have a problem with Latitude by EX – Meridian,I have follwed the instructions you gave. No luck.
First I get a LHA that is not in the tables.So I can not figure an HC. Here is the problem.
15th March 1981. DR position LAT 21*42.0N LONG 55*26.0W Ex meridian Sun lower limb.
Chronometer time of the sight 04h02m40s, error 02m24s fast.Hs 66*15.6 index error 2.8’ on the arc.
Hieght of eye 56 What is Lat at Meridian transit. The closest I can get to their Answer of 21*32.0N is 21*31.2N. I get that by using the Dec of Chronometer time. Any help would be much apprecated.

From: Mississippi
David Burch

 - posted June 20, 2008 11:24 AM      Profile for David Burch           Edit/Delete Post 
please give us the information that led to the LHA.... the tables generally include all LHAs, 0 to 360. your LHA will be near 0 or near 360. since it is just missing LAN
From: Starpath, Seattle, WA

 - posted June 20, 2008 11:48 AM      Profile for WJohntri           Edit/Delete Post 
When I use Chronometer time of 04h 02m 40s,with 02m 24s fast and add zd of +4 I get a LHA of 242,Dec of 2S and A-lat 22N.When I do not correct for ZD and use 04h02m40sand correct with the 02m24s fast I get an LHA of 182, Dec of 2S and A-lat 22N.When I look up the LHA the tables go from 0-69 and 291-360. Any idea what I an doing wrong?
From: Mississippi
David Burch

 - posted June 20, 2008 12:17 PM      Profile for David Burch           Edit/Delete Post 
you are at lon 55W which has ZD of +4. this is a noon sight so local time is about noon! ie about 12. so the actual GMT of the sight is about 12+4 is 16 something.

thus we confront a famous problem in USCG cel nav questions.... ie they give you CT and do not tell you if it is AM or PM. Try starting with GMT = 16 02 40 rather than 04 02 40.

From: Starpath, Seattle, WA

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