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Re: Almanac accuracy, WAS: Re: Hughes Tables.
From: Peter Hakel
Date: 2009 Jul 18, 10:51 -0700
From: "douglas.denny@btopenworld.com" <douglas.denny@btopenworld.com>
To: NavList@fer3.com
Sent: Saturday, July 18, 2009 7:40:15 AM
Subject: [NavList 9145] Re: Almanac accuracy, WAS: [NavList 9122] Re: Hughes Tables.
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.
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From: Peter Hakel
Date: 2009 Jul 18, 10:51 -0700
Thank you, Frank and Douglas, for important explanations regarding DeltaT. Indeed, my favorite reference on the subject:
http://eclipse.gsfc.nasa.gov/SEhelp/deltat2004.html
talks about "extrapolations" and "reasonable estimates" for the future values of DeltaT:
"Future changes and trends in ΔT can not be predicted with certainty since theoretical models of the physical causes are not of high enough precision. Extrapolations from the table weighted by the long period trend from tidal braking of the Moon offer reasonable estimates of +67 seconds in 2010, +93 seconds in 2050, +203 seconds in 2100, and +442 seconds in the year 2200."
The same source also discussed uncertainties in DeltaT:
http://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html
Peter Hakel
http://eclipse.gsfc.nasa.gov/SEhelp/deltat2004.html
talks about "extrapolations" and "reasonable estimates" for the future values of DeltaT:
"Future changes and trends in ΔT can not be predicted with certainty since theoretical models of the physical causes are not of high enough precision. Extrapolations from the table weighted by the long period trend from tidal braking of the Moon offer reasonable estimates of +67 seconds in 2010, +93 seconds in 2050, +203 seconds in 2100, and +442 seconds in the year 2200."
The same source also discussed uncertainties in DeltaT:
http://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html
Peter Hakel
From: "douglas.denny@btopenworld.com" <douglas.denny@btopenworld.com>
To: NavList@fer3.com
Sent: Saturday, July 18, 2009 7:40:15 AM
Subject: [NavList 9145] Re: Almanac accuracy, WAS: [NavList 9122] Re: Hughes Tables.
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
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