A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2017 Mar 14, 13:49 -0700
The validity until 2100 has one small issue worth understanding: there is no provision to enter a correct value for "delta-T". This leads to gradual drift in the relative positions of the planets, Sun, and Moon. For everything but the Moon, this drift is irrelevant to traditional marine navigation for a hundred years. For the Moon, assuming that the developer of the software used one of the usual extrapolations for delta-T available 25 years ago, such as would have been found in Meeus's Astronomical Algorithms, the Moon's calculated position will gradually drift away from the proper position at a rate approximately quadratic in time. For example, by now, in 2017, many software packages written in that era will estimate about 89 seconds for delta-T while the actual observed value is is about 69 seconds. At the usual rate of 12 seconds equal to 0.1' this implies that the Moon's position in 2017 is wrong by nearly 0.2'. Obviously nothing to worry about, but by the year 2080 this could easily grow to 3 or 4 minutes of arc. That's the only concern with software like this -- the Moon some decades in the future. All of the other ephemeris data should be fine, even a lifetime from now...