NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars using Bennett
From: Frank Reed
Date: 2008 Jul 04, 04:34 -0400
From: Frank Reed
Date: 2008 Jul 04, 04:34 -0400
Ken, you wrote: "Two altitudes were needed, but there was no need for them to be of the bodies used for the lunar distance." Yes, exactly. From a "blackbox" perspective, this is true --almost any two altitudes will do. At some time before or after, you take one sight, of any object preferably near the prime vertical, to get local apparent time (the famous "time sight") and you take another sight of any other object for latitude (probably by a meridian altitude, but there were alternatives). These are then carried forward (or regressed) to the time when the lunar distance is measured. The time sight is carried forward by setting a decent pocket watch to the local time and then adding or subtracting at the usual rate of four seconds per minute of longitude for any distance you've travelled in between (by DR). The latitude sight is carried forward simply by adding or subtracting any change in latitude (again, as estimated by DR). This was workaday stuff in the 19th century. Today we would recognize it as the ordinary process of keeping a running fix. Then from the advanced local time and the adjusted latitude, when you observe a lunar distance, you can skip the process of measuring the altitudes of the bodies as long as you're willing to put up with the extra work involved in calculating their current altitudes. The altitudes taken with a lunar distance are really only required to clear the distance of the effects of refraction and parallax. Using them for anything else is optional. Calculating them is optional. Incidentally, it's not hard to find 19th century navigation authorities who will swear that it is "much more accurate" to calculate the altitudes. They were mistaken. I suspect they confused the ability to carry the calculation of the altitudes out to the last arc second with additional accuracy. Of course since calculated altitudes were actually based on data derived from earlier observed altitudes which were then carried forward, the calculated altitudes would necessarily be less accurate, but generally not by much. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---