NavList:

   

Reply

At 09:54 PM 12/3/04 +0000, George wrote:

>So where does Michael Dorl's value for delta-t in 1580, and where does
>Omar's equation, come from?

Here are the comments at the front of the piece of code that calculates delta T
in the aa code I got from Mr. Mosier.  I think I added to the Delta T table
to bring it up
to date.

Michael



DeltaT = Ephemeris Time - Universal Time
  *
  * The tabulated values of deltaT, in hundredths of a second,
  * were taken from The Astronomical Almanac, page K8.  The program
  * adjusts for a value of secular tidal acceleration ndot. It is -25.8
  * arcsec per century squared for JPL's DE403 ephemeris.
  * ELP2000 and DE200 use the value -23.8946.
  *
  * The tabulated range is 1620.0 through 2003.0.  Bessel's interpolation
  * formula is implemented to obtain fourth order interpolated values at
  * intermediate times.
  *
  * Updated deltaT predictions can be obtained from this network archive:
  *    http://maia.usno.navy.mil
  * Currently (as of 2002) available series are
  *    tai-utc.dat  Changes by 1 whenever there is a leap second
  *    finals.all   EOP including UT1-UTC, always less than 1 second
  * from which deltaT = 32.184 + (tai-utc) - (UT1-UTC)
  *
  * For dates earlier than the tabulated range, the program
  * calculates approximate formulae of Stephenson and Morrison
  * or K. M. Borkowski.  These approximations have an estimated
  * error of 15 minutes at 1500 B.C.  They are not adjusted for small
  * improvements in the current estimate of ndot because the formulas
  * were derived from studies of ancient eclipses and other historical
  * information, whose interpretation depends only partly on ndot.
  *
  * A quadratic extrapolation formula, that agrees in value and slope with
  * current data, predicts future values of deltaT.
  *
  * Input Y is the Julian epoch expressed in Julian years.  Y can be
  * found from the Julian date JD by
  *     Y = 2000.0 + (JD - 2451545.0)/365.25.
  * See AA page B4.
  *
  * Output double deltat(Y) is ET-UT in seconds.
  *
  *
  * References:
  *
  * Stephenson, F. R., and L. V. Morrison, "Long-term changes
  * in the rotation of the Earth: 700 B.C. to A.D. 1980,"
  * Philosophical Transactions of the Royal Society of London
  * Series A 313, 47-70 (1984)
  *
  * Borkowski, K. M., "ELP2000-85 and the Dynamical Time
  * - Universal Time relation," Astronomy and Astrophysics
  * 205, L8-L10 (1988)
  * Borkowski's formula is derived from eclipses going back to 2137 BC
  * and uses lunar position based on tidal coefficient of -23.9 arcsec/cy^2.
  *
  * Chapront-Touze, Michelle, and Jean Chapront, _Lunar Tables
  * and Programs from 4000 B.C. to A.D. 8000_, Willmann-Bell 1991
  * Their table agrees with the one here, but the entries are
  * rounded to the nearest whole second.
  *
  * Stephenson, F. R., and M. A. Houlden, _Atlas of Historical
  * Eclipse Maps_, Cambridge U. Press (1986)
  */

   

Reply