NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Eqn. of time
From: John Cole
Date: 2007 Jan 10, 18:01 -0800
From: John Cole
Date: 2007 Jan 10, 18:01 -0800
FrankReedCT@aol.com wrote: > And let's not forget > _http://www.analemma.com/_ (http://www.analemma.com/) . > It's a detailed site on all aspects of the analemma and the equation of > time, and also one of the oldest amateur web sites on the Internet with really > good graphic content on a technical subject. It dates back to the late 20th > century (!). > Thanks, Frank, for the reference. A great site, very well done. Now, perhaps the historians and/or old timers can help with the following. My first exposure to celestial navigation was many years ago as an undergraduate in applied mathematics. Navigation at sea was disposed of in perhaps one lecture during a semester course on spherical astronomy taught by dons who generally looked down on anything of practical use and certainly never showed us how to use an almanac. We were taught that right ascension was tabulated and converted to GHA knowing Greenwich Sidereal Time (GHA Aries): GHAMS (Mean Sun) + RAMS = GST (GHA Aries). GHA star + RA star = GST When I became interested in practical celestial navigation forty years later right ascension had disappeared from the scene and GMT had almost disappeared as well. In some old texts I have noted excerpts from almanacs where for the sun two quantities R and E were tabulated (together with declination) as a function of GMT where R was defined as RAMS + 12h and E was defined as 12h - ET (Equation of Time). So GHAS = GMT + E. Thus during the period from the early 1900s sun data in the nautical almanacs seem to have evolved from tabulations of RA, through tabulations of R and E, to today's tabulations of GHA (or SHA in the case of stars). I assume the almanacs have always been subject to fads in the schools including fads in notation, academic astronomers versus practical navigators, competition among publishers of almanacs for sales based on improved ease of use, politics, and so forth. Can anyone share with us the history of this evolution? John Cole --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---