Planets, and Choosing an Ephemeris¶

If you are interested in observing the planets, the Jet Propulsion Laboratory (JPL) has prepared long tables that predict the positions of the planets both in the the distant past and out into the future. A table of positions is called an ephemeris and those supplied by the JPL are of very high accuracy.

You can ask Skyfield to download an ephemeris from the JPL by giving load() a filename. Or you can load an ephemeris that you’ve already saved to disk with load_file().

A popular choice of ephemeris is DE421. It is recent, has good precision, was designed for general-purpose use, and is only 17 MB in size:

from skyfield.api import load
planets = load('de421.bsp')

Once an ephemeris file has been downloaded to your current directory, re-running your program will simply reuse the copy on disk instead of downloading it all over again.

After you have loaded an ephemeris and have used a statement like:

mars = planets['Mars']

— to retrieve a planet, consult the chapter Positions and Coordinates to learn about all the positions that you can use it to generate.

Making an excerpt of an ephemeris¶

Several of the ephemeris files listed below are very large. While most programmers will follow the example above and use DE421, if you wish to go beyond its 150-year period you will need a larger ephemeris. And programmers interested in the moons of Jupiter will need JUP310, which weighs in at nearly a gigabyte.

What if you need data from a very large ephemeris, but don’t require its entire time span?

When you installed Skyfield another library named jplephem will have been installed. When invoked from the command line, it can build an excerpt of a larger ephemeris without needing to download the entire file, thanks to the fact that HTTP supports a Range: header that asks for only specific bytes of a file. For example, let’s pull two weeks of data for Jupiter’s moons (using a shell variable $u for the URL only to make the command less wide here on the screen and easier to read):

$ u=https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/satellites/jup310.bsp
$ python -m jplephem excerpt 2018/1/1 2018/1/15 $u jup_excerpt.bsp

The resulting file jup_excerpt.bsp weighs in at only 0.2 MB instead of 932 MB but supports all of the same objects as the original JUP310 over the given two-week period:

$ python -m jplephem spk jup_excerpt.bsp
File type DAF/SPK and format LTL-IEEE with 13 segments:
2458119.75..2458210.50  Jupiter Barycenter (5) -> Io (501)
2458119.50..2458210.50  Jupiter Barycenter (5) -> Europa (502)
2458119.00..2458210.50  Jupiter Barycenter (5) -> Ganymede (503)
2458119.00..2458210.50  Jupiter Barycenter (5) -> Callisto (504)
...

You can load and use it directly off of disk with load_file().

Popular ephemerides¶

Here are several popular ephemerides that you can use with Skyfield. The filenames matching de* predict the positions of many or all of the major planets, while jup310.bsp focuses on Jupiter and its major moons:

Ephemeris	Size	Years	Issued
de405.bsp	63 MB	1600 to 2200	May 1997
de406.bsp	287 MB	−3000 to 3000	May 1997
de421.bsp	17 MB	1900 to 2050	February 2008
de422.bsp	623 MB	−3000 to 3000	September 2009
de430t.bsp	128 MB	1550 to 2650	February 2010
jup310.bsp	932 MB	1900 to 2100	December 2013

You can think of negative years, as cited in the above table, as being almost like years BC except that they are off by one. Historians invented our calendar back before zero was a counting number, so AD 1 was immediately preceded by 1 BC without a year in between. But astronomers count backwards AD 2, AD 1, 0, −1, −2, and so forth.

So if you are curious about the positions of the planets back in 44 BC, when Julius Caesar was assassinated, be careful to ask an astronomer about the year −43 instead.

How segments are linked to predict positions¶

You can print() an ephemeris to learn which objects it supports.

print(planets)

SPICE kernel file 'de421.bsp' has 15 segments
  JD 2414864.50 - JD 2471184.50  (1899-07-28 through 2053-10-08)
-> 1    SOLAR SYSTEM BARYCENTER -> MERCURY BARYCENTER
-> 2    SOLAR SYSTEM BARYCENTER -> VENUS BARYCENTER
-> 3    SOLAR SYSTEM BARYCENTER -> EARTH BARYCENTER
-> 4    SOLAR SYSTEM BARYCENTER -> MARS BARYCENTER
-> 5    SOLAR SYSTEM BARYCENTER -> JUPITER BARYCENTER
-> 6    SOLAR SYSTEM BARYCENTER -> SATURN BARYCENTER
-> 7    SOLAR SYSTEM BARYCENTER -> URANUS BARYCENTER
-> 8    SOLAR SYSTEM BARYCENTER -> NEPTUNE BARYCENTER
-> 9    SOLAR SYSTEM BARYCENTER -> PLUTO BARYCENTER
-> 10   SOLAR SYSTEM BARYCENTER -> SUN
-> 301  EARTH BARYCENTER -> MOON
-> 399  EARTH BARYCENTER -> EARTH
-> 199  MERCURY BARYCENTER -> MERCURY
-> 299  VENUS BARYCENTER -> VENUS
-> 499  MARS BARYCENTER -> MARS

Bodies in JPL ephemeris files are each identified by an integer, but Skyfield translates them so that you do not have to remember that a code like 399 stands for the Earth and 499 for Mars.

Each ephemeris segment predicts the position of one body with respect to another. Sometimes several segments sometimes have to be combined to generate a complete position. The DE421 ephemeris shown above, for example, can produce the position of the Sun directly. But if you ask it for the position of Earth then it will have to add together two distances:

From the Solar System’s center (0) to the Earth-Moon barycenter (3)
From the Earth-Moon barycenter (3) to the Earth itself (399)

This happens automatically behind the scenes. All you have to say is planets[399] or planets['Earth'] and Skyfield will put together a solution using the segments provided.

earth = planets['earth']
print(earth)

Sum of 2 vectors:
 + Segment 'de421.bsp' 0 SOLAR SYSTEM BARYCENTER -> 3 EARTH BARYCENTER
 + Segment 'de421.bsp' 3 EARTH BARYCENTER -> 399 EARTH

Each time you ask this earth object for its position at a given time, Skyfield will compute both of these underlying vectors and add them together to generate the position.