Retrograde Calculator

Figuring out what's in retrograde and when

Overview

This project produces a JSON file the identifies if a planet (or the moon) is in retrograde on any given day between Jan. 1, 2000 and Dec. 31, 2100.

The calculations were made for midnight UTC time each day.

Data File Keys

The data comes from the de440s.bsp file. The keys in that file are:

1 MERCURY BARYCENTER
2 VENUS BARYCENTER
3 EARTH BARYCENTER
4 MARS BARYCENTER
5 JUPITER BARYCENTER
6 SATURN BARYCENTER
7 URANUS BARYCENTER
8 NEPTUNE BARYCENTER
9 PLUTO BARYCENTER
10 SUN
199 MERCURY
99 EARTH
299 VENUS
301 MOON
499 MARS

The project uses the BARYCENTER for each planent. The moon is pulled from the only available source (key: 301).

Validation

The code identifies Mercury as being in retrograde from:

• 2022-12-30 ~ 2023-01-18

Other sources list the following

• 2022-12-29 ~ 2023-01-18 - Source

• 2022-12-22 ~ 2023-01-18 - Source

• 2022-12-28 ~ 2023-01-18 - Source

The specific dates float around each other, but the overall blocks of time line up. Given that there doesn't seem to be a definitive source this takes the validaiton as far as I know how.

References

I didn't use all of these, but they're what I ended up having open so dropping them here.

• Mathematically calculate if a Planet is in Retrograde - This is the page that I used to figure out the formula from.
• https://www.popsugar.com/smart-living/what-planets-are-retrograde-right-now-48669539
• https://rhodesmill.org/skyfield/api.html#osculating-orbital-elements
• https://towardsdatascience.com/space-science-with-python-quite-around-the-sun-6faa206a1210
• https://rhodesmill.org/skyfield/planets.html
• https://rhodesmill.org/skyfield/positions.html
• https://rhodesmill.org/skyfield/api-topos.html#skyfield.toposlib.GeographicPosition
• https://astronomy.stackexchange.com/q/24012
• https://github.com/G-Street/is-mercury-in-retrograde
• https://rhodesmill.org/skyfield/
• https://rhodesmill.org/skyfield/api.html
• https://rhodesmill.org/skyfield/api-iokit.html
• https://rhodesmill.org/skyfield/api-ephemeris.html
• https://rhodesmill.org/skyfield/planets.html
• https://rhodesmill.org/skyfield/planets.html#ephemeris-download-links
• https://docs.python.org/3/library/calendar.html