NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Almanac accuracy, WAS: Re: Hughes Tables.
From: Douglas Denny
Date: 2009 Jul 18, 07:40 -0700
From: Douglas Denny
Date: 2009 Jul 18, 07:40 -0700
Agreed entirely. Delta T represents the unpredictable changes in the the Earth's rotation year by year, which is not fully understood. Unpredictable changes in the angular momentum of the Earth do occur though which can now be measured since atomic time was introduced. It is a curious irony that the first Astronomer Royal, Nevil Maskelyne set himself as his first task in the new Greenwich Observatory to determine if the Earth's rotation was constant in rate; which he determined it was. Until this was proven there was no further way forward for the astronomical determination of the longitude which is what the Greenwich Observatory was estabilished for, to assist in solving the problem. 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. Delta -T is uncertain for the past, and is so for the future, so approximations are not possible since it appears to have no regular periodicity. This is why almanacs cannot be used anynore than a century or so either way of today with any accuracy. Delta -T is a problem too in the 'ICE' almanac I use. Although it used official Alamac Office algorithms and data sets, it was produced in the 1980s (I believe) and uses a predicted delta-T thereafter. The predicted delta-T for 2009 for example is 70.7 seconds whereas it is in fact 65.8025 for 2009. So even that programme is not fully 'accurate'. As Humpty Dumpty said in Alice in Wonderland, when the word 'accuracy' is used, "it means just what I choose it to mean -- neither more nor less". Almanacs always need to be used with caution regarding their sources if real accuracy is needed. Douglas Denny. Chichester. England. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
