by Phill Edwards

My interest in astronomonical calculations and predictions took off when Eric Bescher posted the question "I wish to understood why sunsets are delayed 20 minutes in the next 21 days, whereas sunrises remain unchanged. Shouldn't this stuff be symmetrical with respect to noon?" on his Facebook page. This started an investigation into the Equation of Time. I found that papers on the Internet on the subject have errors, are confusing and make unexplained assumptions. Even worse, the so called definitive text book Astronomical Algorithms by Jean Meeus is so out of date, confusing and error riddled as to be practically useless. This started this series of explanations of astronomical events and phenomena.

After much searching and struggling to understand badly written Wikipedia pages, I finally found the definitive source of ephemeris data at NASA JPL. The data sets define the positions of the Sun, Moon and planets to a high degree of accuracy. The data can be found here. One major problem I found was that none of the software to use the data actually works with the later data sets. I chose to work with DE430t data. This is the data set which most accurately describes the lunar orbit and it is used as the basis for the annual Astronomical Almanac. I have written a Java ephemeris library which processes the DE430t data and validates it agains the test data which is provides.

Having got the ephemeris working, I then started converting my software to use it. The current implementation contains the following.

I have downloaded a copy of the Almanac Astronomical Phenomenon for the year 2016. I am using it to write tests for my programs based on the Almanac data. My software agrees with the Almanac to within a minute. Interestingly there are a few sunrise times which disagree by about 3 minutes. I believe that I have found errors in the Almanac.

The software is still under development and the following projects are in progress:

There are still some outstanding issues to be resolved:

Here are some pages which go into more details about the various phenomena: