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Re: GHA = GAST - RA Astronomical Almanac vs Nautical Almanac
From: Paul Hirose
Date: 2014 Mar 22, 15:46 -0700
From: Paul Hirose
Date: 2014 Mar 22, 15:46 -0700
Mike Mayer wrote: > Anyone know what I'm missing? In comparing the Sun's GAST - RA from the Astronomical Almanac to the GHA in the Nautical Almanac there are cases that seem to be too far off. For example, for January 1, 2014 at 0 UT I get: > > GAST = 6h42m17.0580s = 6.704738333h (Page B13) > RA = 18h45m35.31s = 18.759808333h (Page C6) It's a matter of time scales, Mike. Sidereal time is a function of Earth rotation, so you enter the table with UT1. On the other hand, the geocentric apparent RA of the Sun is independent of Earth rotation. It's computed from a planetary ephemeris and a precession / nutation model. Nowadays both use the TT (Terrestrial Time) scale. That should be noted at the top of each page in the table. (I don't have the 2014 Almanac.) At Jan 1 0h, TT was 67.28 seconds ahead of UT1, so by taking the entries "at the same time" from both tables, you were acually taking out values applicable more than a minute apart. --
