NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Almanac accuracy
From: Paul Hirose
Date: 2009 Jul 19, 15:46 -0700
From: Paul Hirose
Date: 2009 Jul 19, 15:46 -0700
douglas.denny@btopenworld.com wrote: > For a full detailed explanation see the "Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac" published jointly be the American and UK Nautical Almamac Offcies. That's the title of the 1961 book, now out of print. The latest version was published in 1992 with a different title: http://aa.usno.navy.mil/publications/docs/exp_supp.php A paperback version is available with a 21st century copyright (I forget the exact year), but its contents are identical to the 1992 book. It's a good idea to check the publisher's online errata sheets. I once wasted days trying to debug a complicated set of formulae in the Supplement. They never worked right, so I gave up. Years later I discovered the online errata. There was a typo! > They now have 'MICA' which is the same thing but with updated interfacing for modern computer - and you _pay_ for it Although MICA and the almanacs must be purchased, the main resources needed to replicate them are free on the Web. These are the IAU SOFA source code, the USNO NOVAS source code, the JPL DE ephemerides (data files and the Fortran 77 source code to utilize them), and the Hipparcos star catalog. > Terrestrial Dynamical Time (TDT) represents the uniform precision time base by definition (based on atomic time) for astronomical events and predictions, whereas Universal Time is the 'apparent' (real) time as measured in practical terms on Earth. The difference between the two is represented by delta-T and is adjusted by leap seconds. One leap second was added in Dec.2008 and another leap second will be added in Dec 2009. "Terrestrial Dynamical Time" is the old name for something now called "Terrestrial Time". The difference TT-UT1 equals delta T. That value is not "adjusted by leap seconds", since leap seconds are applied to UTC, not TT or UT1. You're mistaken about a leap second at the end of 2009. The official bulletin from the IERS says there will be no leap second: http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat At the end of 2009, UT1 is predicted to be about .1 second ahead of UTC. If a leap second were inserted, that difference would increase to 1.1 seconds, an excessive amount. -- --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---