02-26-2016, 11:56 PM
(This post was last modified: 02-27-2016, 12:00 AM by jeremyparker.)
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?
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.
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
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?
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.
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