NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar longitudes, not by lunar distance.
From: Geoffrey Kolbe
Date: 2009 Aug 09, 11:08 +0100
From: Geoffrey Kolbe
Date: 2009 Aug 09, 11:08 +0100
George wrote [NavList 9430] >All he needs to do, once it's been placed firmly on a well-set tripod, is to >set up his instrument so that its vertical axis is truly vertical, for which >the built-in levels should be quite adequate. Then to set it so that its >telescope swings in a near-meridian plane. One way to do this is to >establish North (in the Northern hemisphere), by bisecting the most-Eastern >and most-Western azimuth swings of Polaris. However, that's only possible at >certain times of year; at others, two different stars may have to be used, >but this presents no problem now that stars have been catalogued so >precisely. Then swing by 180� to look Southwards. A couple of practical points are worth noting (again, as Hanno is new to the list) in the use of transit theodolites for measuring azimuth. First is that for altitudes greater than about 50 degrees, it is usually not possible to get your eyeball behind the telescope using a standard transit theodolite. I think Hanno says he lives in San Diego, whose latitude is low enough that he will be able to see Polaris without difficulty using a standard transit theodolite. Here in Northern Europe, it would not be possible to do so. There are so called 'broken' theodolites, where the viewing is done along the axis about which the telescope and vertical scales rotate. There is a 45 degree mirror which redirects the view up the telescope. There is no limit to the altitude one can view using such a theodolite, the most famous example of which is the Wild T4 - but you won't find one of those on ebay! What you do find quite commonly on ebay are theodolites for tracking weather balloons. These are 'broken' theodolites and would be excellent for CN alt/az measurements - except that their scales are quite coarse, usually in degrees only I think, judging by what I can see from the ebay pictures. Second, is that the error in azimuth goes as the tangent of the error in vertical alignment of the theodolite. So, if the theodolite is 1 minute off vertical and you are looking at an object at an altitude of 45 degrees, the error in azimuth could be up to 1 minute. Luckily, standard transit theodolites are self limiting in this regard - see my first point above. Third and final point. As George mentioned, when doing measurements on the sun or moon, it is usual to take a measurement as the edge of the body (top of bottom for altitudes, sides for azimuth) touches the appropriate cross hair and then make a correction to get the centre of the body. For altitudes, the correction (to first order) is simply the semi-diameter of the body. For azimuths, the correction is the semi-diameter divided by the cosine of the altitude. With both these last two points, it can be seen that it is advantageous to observe bodies of low altitude to avoid significant errors in azimuth. Geoffrey Kolbe --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---