Navigation Question - Ex-Meridians

**New3M** · May 16th, 2012, 07:54 PM

Does anybody out there have a method of doing ex-meridians? Examples of the Coast Guard questions would be great, but anything would be helpful. I've been trying to find some stuff online, but keep coming up blank. Thanks.

**Russ** · May 17th, 2012, 01:39 AM

Originally Posted by New3M

Does anybody out there have a method of doing ex-meridians? Examples of the Coast Guard questions would be great, but anything would be helpful. I've been trying to find some stuff online, but keep coming up blank. Thanks.

See below Russ
http://en.wikipedia.org/wiki/Ex-meridian

**New3M** · May 18th, 2012, 09:59 AM

That uses a Haversine formula which I don't think I really want to tackle. Isn't there some method of using tables?

**richard8000milesaway** · May 18th, 2012, 10:43 AM

Bowditch Tables, pretty straight forward. A little double interpolation as I recall but not difficult.

If you don't have Bowditch vol 2 yet, get one: it's what you'll have in the exam room.

**New3M** · May 18th, 2012, 02:50 PM

I've got one. Just don't have any idea where to start!

**richard8000milesaway** · May 18th, 2012, 05:07 PM

Read Bowditch and figure it out. Everything you need is there.

**LauHalaLana** · May 18th, 2012, 06:21 PM

Ex-meridians used to be used regularly before SatNav and GPS, and are useful if the sun is obscured at LAN, but is visible within a few minutes on either side of LAN (usually about 20 minutes). In the old Bowditch, you entered Table 29 with Declination and Latitude to obtain a factor used to enter Table 30 along with the time between predicted LAN and the ex-meridian observation. That yielded a correction to add to the ex-meridian sight worked by normal LAN formula. It might sound complicated, but it is fast and easy, and we often used it to save a cloudy LAN.

**New3M** · May 26th, 2012, 08:15 PM

Thanks for the guidance. I've been doing a bunch of the CG questions, but none of them seem to be working out. I'm using the right tables (in the new version of Bowditch), right almanac, etc. Yet I can't get any closer than 10' in my calculations (it's not my math). Any other advice? I would think that their questions would work out if not exact, pretty close.