A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Mike Mayer
Date: 2014 Mar 22, 20:08 -0500
Thanks! The sun data is for 0h Terrestrial Time. I hadn’t really thought about it.
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Paul Hirose
Sent: Saturday, March 22, 2014 5:51 PM
Subject: [NavList] Re: GHA = GAST - RA Astronomical Almanac vs Nautical Almanac
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.
I filter out messages with attachments or HTML.
View and reply to this message: http://fer3.com/arc/m2.aspx?i=127345