NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Raw data for bubble
From: Bill Noyce
Date: 2007 Mar 22, 08:35 -0400
From: Bill Noyce
Date: 2007 Mar 22, 08:35 -0400
On 3/22/07, Billwrote: > Oh. I wondered how you got GHA, LHA or declination when only one leg and one > angle are given, and no time. Must be a way, but it is above my head. > > Which leaves me with a pocket calculator, watch, sextant and almanac and the > knowledge an HC and azimuth, and my best guess of 90/270d, is better than a > time sight. I think the original question was, how to predict the time at which the sun has an azimuth of 90 or 270 degrees, given a known or assumed latitude. In this case, you have the co-latitude, the co-declination of the sun, and the azimuth (which is a right angle) as three known parts of the triangle. (The sun's declination comes from a calendar, plus perhaps an estimate of longitude to the nearest 15 degrees. You would have to be more careful if using the moon.) The formulas will come out something simple like cos LHA = tan d / tan L and sin h = sin d / sin L I think the intent is to observe until the predicted h occurs, and record the resulting GMT. From that and the matching LHA you compute GHA and Longitude. Right? And you use the graphical method to interpolate to hit the desired h exactly. From the modern perspective, a sight at this special time gives a LOP like any other; its main advantage is that the LOP runs exactly north and south if you're at the assumed latitude, so any error in the latitude has only a tiny effect on the resulting longitude. -- Bill N. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---