NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: equal altitudes
From: Bill Murdoch
Date: 1997 Dec 04, 3:57 PM
From: Bill Murdoch
Date: 1997 Dec 04, 3:57 PM
We have been back and forth over the last couple of weeks over using different methods of reducing sun sights. It seems that no one objects to using a noon sun sight to determine latitude. It is not hard to find the altitude where the sun stops rising, and the math is simple with the navigational triangle collapsed to a line. You can reduct the sight with simple addition and subtraction. The rub comes with using the noon sight to find longitude. It seems to me (and I have never done it anywhere but in my backyard) that if at noon the sun is passing somewhere near overhead it would work and if the sun is near the horizon it would be difficult at best. With the sun passing overhead it will be rising at 15 deg/hr before noon and setting at 15 deg/hr after noon. For only the shortest of instants would it not be moving. That should be an easy spot to find. Just take a sight while the azimuth is still east and another at the time the azimuth is west and the altitude is the same as the first sight. Split the difference and you have the time of noon; a little arithmetic and you have the lngitude. However, if the sun were near the horizon at noon, it would be rising very slowly in altitude before noon and setting in altitude very slowly after noon. It would be hard to tell the time when it stopped rising and started setting. In fact for someone near the pole for whom the sun neither rises nor sets it would be impossible to tell the time of noon and thus impossible to tell the longitude by this method. The use of sight reduction tables or a calculator to solve the navigation triangle as was pointed out gets around all of this, but books and trig scare most people to death. To bad. They miss a lot. That little calculator can do more. It can not only replace the sight reduction tables; it can also replace the almanac. Try the attached form. It can be used to calculate the GHA of Aries. Any trig calcualtor with one memory should work so long as it keeps 12 digits or more internally. It is not hard to do the almanac calculations. 100 years ago they were all calculated by hand. (I hope the form is without mistakes. I have not had time to exhaustively check it. Let me know if you find something wrong.)