Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Almanac accuracy, WAS: Re: Hughes Tables.
    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:


    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:


    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 unsubscribe, email NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)