(02-26-2016, 11:56 PM)jeremyparker Wrote: Moon shots:
The moon is tricky to shoot and to reduce correctly - why? It's big and bright and easy to identify, but it jiggers about, so hour angle and declination are more complicated to calculate, and hides parts of itself, so it's more difficult to select the correct limb. It's so close that parallax and augmentation become significant issues. All of these introduce more steps in the process and more opportunity for error. On top of all this, if we are shooting it at night, its own reflection often confuses the horizon. But why are we shooting it at night, when the sky is already full of stars and planets?

 My reply- twilight, but I'd better be ready for it.


Consider this: When we take a single sight of the sun we cannot obtain a position. No matter how accurate and precise we are in our observation, time keeping, reduction and plotting, all we obtain is a position line and we cannot determine position until we have a second LOP to cross. With a daytime sun sight all we can do is advance our LOP by dead reckoning until we take the second sight. So, despite all our efforts towards accuracy in the sight, we now introduce a whole raft of unquantifiable errors: our helmsman's ability to hold a course and report it correctly, any unknown error in our compass and log, the magnitude of currents and leeway. How wonderful if we had a second body available at the time of the first sight ... and sometimes we do: the Moon.

My reply- here, here!  The Moon is often most agreeable to sights during the day.  Someone once put a wrench close to our compass (bulkhead mounted) and caused us to doubt our course.  Happily it was resolved.

In my opinion - and experience - the only real value in taking a sight of the moon is by day so that we can cross two LOPs and obtain a position. So, yes, I do shoot the moon when it contributes to the better accuracy of my position.

My question to the forum is: Does anybody bother with Noon Sights - and why?

Jeremy

My reply- I do.  It's more of habit than anything else and I like determining latitude.  Getting an approximate Longtitude from Noon-sight is a benefit, but not one I put too much value in.  To be fair, when I want Longitude I'll do "meridian by equal altitudes".  Sights will be taken about 20 minutes before LAN and then when the Sun drops back to the AM Hs.

As to the Moon, don't you just wish the one wasn't so bright at times?

Welcome aboard Jeremy!

Fred

(02-29-2016, 04:51 AM)jeremyparker Wrote: My follow up question perhaps belongs in a different thread - Who bothers with Noon sights, and why?
I ask because there seems to be an almost religious attachment to the noon sight amongst many navigators and a belief that it is necessary to determine latitude to obtain a fix! Consequently we hang around on deck for 20 minutes awaiting the magical zenith moment while our lunch is going cold in the galley below, only to have the only clouds of the day obscure the sun right about LAN. Why not snap off and reduce a quick series of sights whenever we please and cross the resultant LOP with that of our advanced morning sight. This still renders our position; we can still determine Lat and Long. We only NEED a Latitude if we are performing a Time Sight - an observation by which we determine LHA and thence Longitude - but when using the intercept method precise Lat is not necessary.

(02-29-2016, 04:51 AM)jeremyparker Wrote: Lunars? Great in theory, terribly difficult in practice, particularly from the heaving deck of a small sailing vessel!!

Jeremy