Ephem - rhannequin/astronoby GitHub Wiki

Astronoby depends on the gem ephem which provides a Ruby interface for JPL SPICE binary kernels.

These kernels are extremely precise ephemerides produced by the JPL and are the foundation of the data provided by Astronoby. The IMCCE also produces kernels in the same format for their INPOP model.

Download an ephemeris

To download an ephemeris, you can either do it manually or use ephem by providing the name of the file.

Manually

Ephemerides are available for download from:

the JPL public FTP interface: https://ssd.jpl.nasa.gov/ftp/eph/planets/bsp/
the INPOP releases page of the IMCCE: https://www.imcce.fr/inpop

You can download any .bsp file. For the moment, Astronoby only supports the planets of the Solar System, and the Sun and and the Moon, so you need to download a Development Ephemeris, which starts with de (i.e. de421.bsp), or any of the INPOP ephemeris files (i.e. inpop19a.bsp).

With `Ephem`

If you already know the ephemeris you want to use, you can use Ephem to download and store it for you:

Astronoby::Ephem.download(name: "de421.bsp", target: "tmp/de421.bsp")

Load an ephemeris

To compute the position of a Solar System body, you need to provide an ephemeris to extract the data from. You can use Astronoby::Ephem and load the file you downloaded.

ephem = Astronoby::Ephem.load("tmp/de421.bsp")

How to choose the right ephemeris?

JPL produces many different kernels over the years, with different accuracy and ranges of supported years. Here are some that we would recommend to begin with:

de421.bsp: from 1900 to 2050, 17 MB
de440s.bsp: from 1849 to 2150, 32 MB
inpop19a.bsp: from 1900 to 2100, 22 MB

How to limit weight and increase speed?

The Ephem library offers a CLI to produce excerpt ephemerides in order to limit the range of dates or bodies.

Here is an example to produce an excerpt ephemeris for the year 2025:

ruby-ephem excerpt 2024-12-31 2026-01-01 de440s.bsp de440s_2025_excerpt.bsp

You can also specify which bodies of the Solar System (or "targets") you want to include:

ruby-ephem excerpt --targets 10,399 2024-12-31 2026-01-01 inpop19a.bsp inpop19a_2025_sun_earth.bsp

You may want to check how bodies are handled between JPL DE and IMCCE INPOP. For example, the excerpt file from a JPL DE kernel needs to include targets 3 and 399 for the Earth, while the excerpt file from a IMCCE INPOP kernel only needs the target 399.