ssapy.ephemeris

Planetary ephemeris selection and long-span kernels.

SSAPy evaluates Sun, Moon and planet positions from JPL development ephemerides (DE). The ssa-data-* packages ship the supported kernels:

Ephemeris	Shipped kernel	Coverage
de440	de440s.bsp	1849-12-26 to 2150-01-22
de430	de430_1900_2150.bsp	1900-01-01 to 2150-01-01

DE440 is the default. Select DE430, for example to reproduce results made with SSAPy <= 1.1.10, with set_planetary_ephemeris() or the SSAPY_EPHEMERIS environment variable.

For epochs outside the shipped kernel, SSAPy uses a longer kernel of the same ephemeris (de440.bsp or de430.bsp, 1550-2650) and, for DE440 beyond that, DE441 (-13200 to +17191). These are 120 MB to 1.65 GB, so they are not shipped: SSAPy looks for them in the working directory, the ssa-data-* packages, and the cache directory, and otherwise downloads them from NAIF, checks their SHA-256, and caches them, warning with EphemerisDownloadWarning before each download. Set SSAPY_EPHEMERIS_DOWNLOAD=0 to forbid downloads (e.g. on compute nodes without network access; prefetch on a login node with fetch()) and SSAPY_DATA_CACHE to choose the cache directory.

The shipped and long kernels of one ephemeris hold the same solution, so positions are continuous where SSAPy switches between them. DE441 is a different solution from DE440 (no lunar core-mantle damping; less accurate in the current century), so using it raises EphemerisRangeWarning.

Functions

set_planetary_ephemeris(name)	Select the planetary ephemeris, "de440" (default) or "de430".
get_planetary_ephemeris()	Name of the selected planetary ephemeris.
fetch([name, include_de441])	Download (if needed) the long-span kernels of an ephemeris and return their paths.
cache_dir()	Directory where downloaded kernels are kept (SSAPY_DATA_CACHE).

Classes

EphemerisDownloadWarning	A long-span ephemeris kernel is being downloaded.
EphemerisRangeWarning	An epoch is evaluated with a different solution than the one selected.
EphemerisUnavailableError	No kernel of the selected ephemeris covers the requested epoch.

Class Inheritance Diagram