NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Polaris isn't so easy!
From: Frank Reed CT
Date: 2004 Apr 6, 15:11 EDT
From: Frank Reed CT
Date: 2004 Apr 6, 15:11 EDT
Pardon a brief list mechanics question:
There's a slightly annoying "quirk" with this mailing list, and I'm wondering if anyone knows how to fix it. Most messsages to the list arrive with the "reply-to" address set to the list address, which is the way it should be. That way when you compose a reply, it becomes part of the conversation rather than just advice for one individual. But for a small number of list members, such as Jim Thompson, the "reply-to" goes directly to the individual. Obviously, if you catch it before you send, you can simply change the address to the list address, but I know that I don't always catch it. The two messages below, for example, were replies that I expected to go to the list but didn't. Is this a setting that the list member has control over? Or is it some issue with the listserv?
And now, back to the navigation:
REPLY 1:
Jim T wrote:
"Does Polaris show up in an artifical horizon (pan of oil)?"
I've seen it very clearly reflected in the waters of the Mystic River estuary on nights when the water is perfectly still. I'm sure I could have taken a sight between the star and its reflection on those nights, but it isn't something I've yet bothered to do. I would say (just as a guess based on my own experience) that reflection off of water takes out about a magnitude and a half to two magnitudes in brightness, so the reflection of Polaris is pretty faint, but well within the range of normally visible stars at dark sites. Reflection off a liquid mercury horizon takes out only a fraction of a magnitude so the reflected image would be nearly as bright as Polaris in the sky if you can manage to scrounge up a sufficient supply of mercury.
One thought that might help with your sights: get dark adapted.
You mentioned that navigators had used Polaris for centuries for latitude. That's true, of course, but it's interesting to note that it had its ups and downs. Shooting the North Star was apparently relatively unpopular in the 19th century.
REPLY 2:
Jim T wrote:
"Preset the sextant for predicted altitude"
By the way, this is a recommended practice for almost any sight but especially for sights where the angle will change very slowly for an extended period of time: Noon Sun, Polaris, any other meridian sight, and yes, lunars. Sometimes students of navigation get the idea that this is somehow cheating; how can we know the thing we're trying to measure? It isn't because a navigator is never lost. You always have an approximate idea where you are and so pre-setting the sextant to within a degree or so of the expected angle should be considered normal practice.
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
There's a slightly annoying "quirk" with this mailing list, and I'm wondering if anyone knows how to fix it. Most messsages to the list arrive with the "reply-to" address set to the list address, which is the way it should be. That way when you compose a reply, it becomes part of the conversation rather than just advice for one individual. But for a small number of list members, such as Jim Thompson, the "reply-to" goes directly to the individual. Obviously, if you catch it before you send, you can simply change the address to the list address, but I know that I don't always catch it. The two messages below, for example, were replies that I expected to go to the list but didn't. Is this a setting that the list member has control over? Or is it some issue with the listserv?
And now, back to the navigation:
REPLY 1:
Jim T wrote:
"Does Polaris show up in an artifical horizon (pan of oil)?"
I've seen it very clearly reflected in the waters of the Mystic River estuary on nights when the water is perfectly still. I'm sure I could have taken a sight between the star and its reflection on those nights, but it isn't something I've yet bothered to do. I would say (just as a guess based on my own experience) that reflection off of water takes out about a magnitude and a half to two magnitudes in brightness, so the reflection of Polaris is pretty faint, but well within the range of normally visible stars at dark sites. Reflection off a liquid mercury horizon takes out only a fraction of a magnitude so the reflected image would be nearly as bright as Polaris in the sky if you can manage to scrounge up a sufficient supply of mercury.
One thought that might help with your sights: get dark adapted.
You mentioned that navigators had used Polaris for centuries for latitude. That's true, of course, but it's interesting to note that it had its ups and downs. Shooting the North Star was apparently relatively unpopular in the 19th century.
REPLY 2:
Jim T wrote:
"Preset the sextant for predicted altitude"
By the way, this is a recommended practice for almost any sight but especially for sights where the angle will change very slowly for an extended period of time: Noon Sun, Polaris, any other meridian sight, and yes, lunars. Sometimes students of navigation get the idea that this is somehow cheating; how can we know the thing we're trying to measure? It isn't because a navigator is never lost. You always have an approximate idea where you are and so pre-setting the sextant to within a degree or so of the expected angle should be considered normal practice.
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
