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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.

If you want to examine the segments that make up the ephemeris, you can loop over its segments list. You can print() a segment to see a textual description, or access segment attributes that give its center, target, and the dates over which it provides valid positions:

ts = load.timescale()

segment = planets.segments[0]
start, end = segment.time_range(ts)

print('Center:', segment.center_name)
print('Target:', segment.target_name)
print('Date range:', start.tdb_strftime(), '-', end.tdb_strftime())
Center: 0 SOLAR SYSTEM BARYCENTER
Target: 1 MERCURY BARYCENTER
Date range: 1899-07-29 00:00:00 TDB - 2053-10-09 00:00:00 TDB

For example, you can see above that the first segment of the ephemeris DE421 provides the position of Mercury relative to the center of the Solar System over the entire twentieth century and half of the twenty-first.

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().

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)
      0 -> 1    SOLAR SYSTEM BARYCENTER -> MERCURY BARYCENTER
      0 -> 2    SOLAR SYSTEM BARYCENTER -> VENUS BARYCENTER
      0 -> 3    SOLAR SYSTEM BARYCENTER -> EARTH BARYCENTER
      0 -> 4    SOLAR SYSTEM BARYCENTER -> MARS BARYCENTER
      0 -> 5    SOLAR SYSTEM BARYCENTER -> JUPITER BARYCENTER
      0 -> 6    SOLAR SYSTEM BARYCENTER -> SATURN BARYCENTER
      0 -> 7    SOLAR SYSTEM BARYCENTER -> URANUS BARYCENTER
      0 -> 8    SOLAR SYSTEM BARYCENTER -> NEPTUNE BARYCENTER
      0 -> 9    SOLAR SYSTEM BARYCENTER -> PLUTO BARYCENTER
      0 -> 10   SOLAR SYSTEM BARYCENTER -> SUN
      3 -> 301  EARTH BARYCENTER -> MOON
      3 -> 399  EARTH BARYCENTER -> EARTH
      1 -> 199  MERCURY BARYCENTER -> MERCURY
      2 -> 299  VENUS BARYCENTER -> VENUS
      4 -> 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:

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:
 'de421.bsp' segment 0 SOLAR SYSTEM BARYCENTER -> 3 EARTH BARYCENTER
 'de421.bsp' segment 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.

Closing the file automatically

If you need to close files as you finish using them instead of waiting until the application exits, each Skyfield ephemeris offers a close() method. It can either be called manually when you are done with an ephemeris, or you can use Python’s closing() context manager to call the method automatically at the completion of a with statement:

from contextlib import closing

ts = load.timescale()
t = ts.J2000

with closing(planets):
    planets['venus'].at(t)  # Ephemeris can be used here

planets['venus'].at(t)  # But it’s closed outside the “with”
Traceback (most recent call last):
  ...
ValueError: seek of closed file

Type 1 and Type 21 ephemeris formats

If you generate an ephemeris with a tool like NASA’s HORIZONS system, it might be in a format not yet natively supported by Skyfield. The first obstacle to opening the ephemeris might be its lack of a recognized suffix:

load('wld23593.15')
Traceback (most recent call last):
  ...
ValueError: Skyfield does not know how to open a file named 'wld23593.15'

A workaround for the unusual filename extension is to open the file manually using Skyfield’s JPL ephemeris support. The next obstacle, however, will be a lack of support for Type 21 ephemerides in Skyfield:

from skyfield.jpllib import SpiceKernel
kernel = SpiceKernel('wld23593.15')
Traceback (most recent call last):
  ...
ValueError: SPK data type 21 not yet supported

Older files with a similar format might instead generate the complaint “SPK data type 1 not yet supported.”

Happily, thanks to Shushi Uetsuki, a pair of third-party libraries exist that offer preliminary support for Type 1 and Type 21 ephemerides!

Their documentation already includes examples of generating raw coordinates, but many Skyfield users will want to use them in conjunction with standard Skyfield methods like observe(). To integrate them with the rest of Skyfield, you will want to define a new vector function class that calls the third-party module to generate coordinates:

from skyfield.constants import AU_KM
from skyfield.vectorlib import VectorFunction
from spktype21 import SPKType21

t = ts.utc(2020, 6, 9)

eph = load('de421.bsp')
earth = eph['earth']

class Type21Object(VectorFunction):
    def __init__(self, kernel, target):
        self.kernel = kernel
        self.center = 0
        self.target = target

    def _at(self, t):
        k = self.kernel
        r, v = k.compute_type21(0, self.target, t.whole, t.tdb_fraction)
        return r / AU_KM, v / AU_KM, None, None

kernel = SPKType21.open('wld23593.15')
chiron = Type21Object(kernel, 2002060)

ra, dec, distance = earth.at(t).observe(chiron).radec()
print(ra)
print(dec)
00h 27m 38.99s
+05deg 57' 08.9"

Hopefully this third-party support for Type 1 and Type 23 SPK ephemeris segments will be sufficient for projects that need them, until there is time for a Skyfield contributor to integrate such support into Skyfield itself.