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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.
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()
.
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.
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.
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
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.