A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2015 Sep 28, 00:02 -0700
Ocean navigatior has an article this month on using Rude star finder to precompute sights for nautical twilight.
I have a couple of comments on the athors technique.
First he gives an example of finding the time of morning nautical twilight, then computing LHA Aires for setting HO 2101 D to locate which stars will be available to shoot when the horizon becomes visible. Well, you don't need to go through all that trouble, its dark outside, just go out and look at the sky and see the stars, they will be there when the horizon becomes visible. And if it isn't the first night at sea then you should remember which stars you shot yesturday morning, at sailboat speeds they will be there this mornign too at altitudes close enough to be found in the index mirror. Using the HO 2101 D only makes sense for evening stars since precomputing the altitude and azimuth will allow you to find the star and take the observation when the stars would not have been visible yet to the naked eye so you can take the sight earlier in the evening while the horizon is as clear as possible.
Second, his method for figuring out the LHA of Aries is unneccisarily complicated. He says find the time of twilight from the Nautical Almanac table, adjust that time for the DR longitude to find the GMT of twilight at your longitude, use that GMT to re-enter the NA and take out GHA Aries, and then subtract out your longitude to determine LHA Aries. See what he has done here, he first adds the longitude and then, at the end subtracts out the longitude, these two operation cancel out. All you have to do is determine the time of twilight at the Greenwich meridian which is what is tabulated in the twilight table, theis GMT is the local time at Greenwich. Then look at the GHA Aries for that time and that is also the LHA Aries at Greenwich. That will be the same LHA aries at the point of twilight at your longitude. This is accurate to about one-half of a degree of LHA so is sufficiently accurate for star finder setting. (GHA Aries changes only 59' per day so making the adjustment for longitude can never exceed 59' even if your longitude is one degree east and can never exceed half a degree in west longitudes.)