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Further reading on the concepts and technology behind Skyfield.

USNO Circular 179 by George H. Kaplan, October 2005

My favorite introduction to how the International Astronomical Union responded in 1997 and 2000 to the problems with the old J2000 system by adopting the ICRS and a new Earth rotation model. Thanks to Kaplan’s explanations, I finally understood the maneuvers of the USNO’s freely available NOVAS library (which comes with a Python interface!) when dealing with dates and coordinate systems, and I was able to perform the same calculations correctly in Skyfield. It also provided a useful introduction to the JPL ephemerides that NOVAS uses.

User’s Guide to NOVAS Version F3.1, March 2011

There were also several useful tidbits of information in the United States Naval Observatory’s guide to using NOVAS. It tends towards more practical and user oriented advice than does the Circular, and of course was also my guide to the library’s concepts when I used it as my model for Skyfield’s internal astrometry routines.

SPICE Required Reading documents

When implementing Skyfield’s sister library jplephem, these documents were invaluable for their detailed description of the .bsp binary file format as well as the description of how the SPICE system combines multiple ephemeris segments to learn the displacement between two Solar System objects.

JPL Solar System Dynamics directory of ephemeris PDFs

I cannot always find the official documentation describing a particular JPL ephemeris, but this directory is always a good starting point, as it collects several of the PDFs together in one place.

Earth Rotation — the Change in the Length of Day and ΔT Plot showing lod from -2000 to +2500

This page offers the most up-to-date ΔT tables from Morrison, Stephenson, Hohenkerk, and Zawilski, which Skyfield uses to predict the Earth’s orientation for the years preceding and following the more detailed numbers published for 1973 through the current day by the IERS (see below). It also links to their academic papers, which provide a wealth of information about the historical events — primarily eclipses — that provide us with evidence for which direction the Earth was pointing in past centuries.

Historical values of the Earth’s clock error ∆T and the calculation of eclipses by Morrison & Stephenson, 2004

Detailed discussion of the fact that ∆T has the shape of a parabola over history because, omitting short-term variations, the shortening length of Earth’s day makes the difference between modern clock time and actual sunrise and sunset skew ever longer over the centuries as the tiny error introduced each year gradually accumulates. Ancient eclipses for which records survive are our one point-source of data about how far the error had accumulated each century.

IERS “Earth orientation data” page

There used to be a United States Naval Observatory web page dedicated to Earth orientation, but these days I look for data files here: