A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robin Stuart
Date: 2018 Oct 22, 14:14 -0700
I recently found an application that makes it easy to generate positions of celestial bodies to the full accuracy of the JPL Ephemerides without the overhead of figuring out the file format and setting up the interpolation. I assume that there are others on Navlist who might find this useful. It’s called Skyfield https://rhodesmill.org/skyfield/ and it can also load the entire Hipparcos catalog with positions, visual magnitudes, proper motion and parallax which are incorporated when star positions are calculated. It’s written in Python. I’ve never understood why such a big deal is made about Python as to me it doesnt really seem to offer anything very much is new and that hasn’t been around in other languages for many years. Be that as it may, Skyfield makes things easy and generally does a nice job. Calculating the apparent geocentric position of the Moon for the equator of date is as simple as
MoonRA, MoonDec, MoonDist = earth.at(t).observe(moon).apparent().radec(epoch=t)
Results agree with those from the https://ssd.jpl.nasa.gov/horizons.cgi and MICA.
As an exercise I generated a lunar almanac for 24 June, 1915 and compared it to the published Nautical Almanac. The hour here is astronomical time with 0h corresponding to 12h civil time. The time scale is UT1. I’ll leave it to you to decide whether the the NA of the time quotes more significant figures than are justified.