NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Additional error found in H.O. 249
From: Douglas Denny
Date: 2009 Oct 7, 14:13 -0700
From: Douglas Denny
Date: 2009 Oct 7, 14:13 -0700
Quote:- "I do understand that stars have proper motion." They do, but even the fastest moving have completely negligable proper motion and are difficult to measure with the most sophisticated photographic methods over long periods of time. Precession and nutation are the main reasons for apparent star motions, simply speaking measured by and referenced to the changes of the first point of Aries around the equinoctial and the obliquity. The changes are only slight, with a period of 25800 thousand years for full precession around the equinoctial; with nutation having much shorter period linked to the motion of the Moon of 18.6 years but the effects are miniscule: in the order of longitude by 17" and obliquity by 9". Douglas Denny. Chichester. England. ============== Original Message:- Hi Gary What is the magnitude of these errors? Several arc-seconds? An arc-minute? Several arc-minutes? A degree?!!?!?!? My 1853 Bowditch gives a long term almanac, in which the motion or rate for the stars is given for a year, and you just multiply by years and add to the base value. Surprise, it agrees quite readily with current values (not perfectly, but dang close). It didn't seem like a huge delta for a 156 year run (1853 -> 2009). I can't imagine this to be a huge injected error for 1982 -> 2009. Wouldn't one obtain the SHA and declination of a star from the Nautical Almanac for the day in question? Why would it be preferable to obtain this from HO 249 other than convenience? Best Regards Brad --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com -~----------~----~----~----~------~----~------~--~---
