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Re: Tinyac almanac program for Windows
From: Paul Hirose
Date: 2010 May 18, 13:37 -0700
From: Paul Hirose
Date: 2010 May 18, 13:37 -0700
Dave Walden wrote: > More playing: Set long&lat = 0 0 0, (lat seems to effect geocentric LHA? > thinking about that one.) coord sys to "apparent geocentric geodetic > (LHA)", gregorian, UT1, de405, and for 9 may 2009 18h, I get back NA > values to NA accuracy. When comparing Tinyac values against an almanac, it's not necessary to set the observer's position. This omission will disable some time and coordinate frame options, but they aren't needed. For example, to read the Greenwich hour angle of a body, select the "terrestrial" button in the Coord Sys dialog. This aligns the coordinate frame to the geodetic latitude / longitude system. (Perhaps this would have been clearer if I'd marked the button "geodetic".) By default the angle goes from -180° to +180° (east positive), but the Coord Sense sub-dialog lets you change the sense of the angle to 0 - 360°, increasing west. That setting is used to compute GHA Aries too. There's no way to read GHA Aries directly, but you can find it by adding the GHA and RA of the same body. You also have the option to select hours as the unit of measure instead of degrees. One use for GHA in time units is the determination of equation of time. For example, what is the equation of time at 1798 Jan 1 12 h GAT? Set time to 12h UT1 (not GAT!), read GHA Sun = +0h04m14.7s (select the east+/west- button to read GHA in that format). That's a first approximation to the equation of time. Change the time to 12h04m14.7s UT1, read GHA Sun = +0h00m00.2s. Add .2s to UT1, and this time the result is perfect. The equation of time is 4m14.9s, UT1 ahead of GAT. The 1798 Nautical Almanac is online, and its value agrees within .2s: http://books.google.com/books?id=1PcNAAAAQAAJ&pg=PR2 That's better than I expected. Of course the precise definition of Greenwich mean time (what we now call UT1) has changed many times over the centuries, and I've read warnings that historic investigators need to take that into account. In the case of Tinyac, for instance, even when you input time in the Greenwich apparent time scale, GAT is converted to UT1 by iteratively computing the equation of time. The algorithm is similar to the one I used above. Then delta T is applied to obtain Terrestrial Time. When UT1 and TT are known, the program has what it needs to compute the position of a body. In my 1798 example, it looks like the modern UT1 implementation has practically perfect continuity with the old mean time. But one data point means little. I'll investigate this further. --