NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Beginner Meridian Passage Question
From: Bill Noyce
Date: 2004 Sep 1, 16:27 -0400
From: Bill Noyce
Date: 2004 Sep 1, 16:27 -0400
> Second, the concept of meridian passage for objects with a declination > greater than the observer's latitude causes me some confusion as they > never dip below the horizon, so under proper viewing conditions Polaris, > Dubhe et al could be observed crossing the meridian twice in a sidereal > or solar day. Half right. The objects that behave this way are those whose declination is larger than (90d - Lat). You are correct that "Meridian Passage" is generally considered to be when the body crosses the half of your meridian that stretches from pole to pole and includes your zenith. > As a side bar, Susan Howell/Practical Celestial Navigation's chapter on > meridian sights states that in a lifeboat situation the position of Polaris > relative to the north celestial pole can be estimated by the position of > Ruchbah, as it is on the same side of the PN and directly in line with > Polaris and the PN. While published in 1976, it has been revised since. > Inspection of the current almanac star pages would indicate the SHA of > Polaris is 320+, while Ruchbah is 338+. Inspecting the rate of change in > one year, it appears that Ruchbah's SHA is changing faster than Polaris's, > so the relationship may be quite different in 2004 than it was in 1976. Am > I missing something? If not, any current rules of thumb (other than finding > a visible star with a like or 180 opposite SHA) for estimating the angle of > Polaris to the North Pole? Not sure this is relevant, but remember that stellar aberration -- the annual movement of stars' apparent positions due to the speed of the earth in orbit as a fraction of the speed of light -- will appear to affect the SHA of Polaris more than other navigational stars, even though its actual position is not affected more greatly. This is simply because you divide the position change by cos(declination) to get change in SHA. I would guess that longer-term changes in SHA for these stars are related much more to the earth's precession than to stars' proper motion, though again I'm not sure this is relevant to your question. Looking at a list of star SHA's, it appears Kochab's SHA is pretty close to 180d away from Polaris's. I suppose the ways to use this info are: a) when you think Polaris is east or west of the pole, use its altitude directly as your latitude; b) when you think Polaris is directly below or above the pole, add or subtract its difference (which you have memorized as about 45'); c) at other angles, estimate the sine or cosine (depending on how you name the angle) and multiply by 45'. > Thanks, > > Bill You're welcome, -- Bill