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

**Re: Delta-T**

**From:**Omar Reis

**Date:**2004 Dec 6, 12:01 -0200

>George Huxtable wrote: > This has table 10A giving delta-T for various years from 1620, for which he > quotes +121 sec, to 1998. Unfortunately, this table says nothing about the > errors involved. Meeus' Table 10A differs significantly (for dates around > 1620) from the numbers given in my 1984 Astronomical Almanac, which gives > +98 sec for 1621. Meeus states that the Astronomical Almanac values have > been "corrected" according to a new value for tidal acceleration of the > Moon, given in note S055, issued by the Bureau des Longitudes, Paris, in > 1997, by Chapront et al. The tables agree, for more recent dates. > > Meeus (1998) also gives equation 10.2 giving delta-T "from +948 to + 1600, > and after the year +2000". in seconds, as- > 102 + 102 t + 25.3 t-squared, (Meeus eq 10.2, on page 78) > > where t is the time in centuries from year 2000, so for 1600, t = -4.0 > > This is quite different from the expression Omar quoted, as > "DeltaT=50.6+T*(67.5+T*22.5) (T in centuries since J2000) " > se perhaps we have different editions. Has that equation 10.2 (1998) been > subsequently revised in a later edition of Meeus? Otherwise, where did Omar > get it from?. I have the first edition (1991), to which I have applied the "errata" by the author (1995). The formula is to be used between years 948 and 1620 I was calcuting dT for 1580, so I used it. Omar