A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robin Stuart
Date: 2018 Oct 23, 13:24 -0700
You wrote: “When does it say the December solstice will occur (in the UTC time scale)?
Since Skyfield agrees with the JPL Horizons Web interface https://ssd.jpl.nasa.gov/horizons.cgi this is something you can easily investigate. The timescale is UTC. I’d be interested in the conclusions.
You wrote: “there seems to be a systematic error somewhere”
Interesting observation. I had thought that a likely cause might have been a mismatch between UT1 and the time in NA but it I don't think that the time differences implied by R.A. and declination line up well enough for that. The fact that the numbers track well over short interval means that both calculations get the Moon’s “velocity” pretty right but I’m not sure that excuses a systematic error in the position.
You wrote: “I think you and many others think of the Nautical Almanac as a refrence book for accurate astronomical positions. It is not.”
Your statement characterizes the modern Nautical Almanac and Lars is certainly very well aware of that. Here we are looking at “The Nautical Almanac and Astronomical Ephemeris for the year 1915” in which positions are given to fractions of an arc second. Not something you see in today’s NA!
As I noted in my original post Skyfield, JPL Horizons and MICA all agree with each other and disagree with the 1915 Almanac,