Getting started

The eph package provides some useful functions, classes and tools to retrieve, parse and manipulate ephemerides in an astropy-compatible way.

See eph-howto (jupyter notebook) for more info.

Basic Usage

import eph

req = eph.JplReq() # create the request
req.read('eph.ini', section='jplparams') # read parameters from 'jplparams' section in 'eph.ini'
req.set({
    'COMMAND': 'venus',
    'START_TIME': '2007-11-17',
    'STOP_TIME': '2017-4-22'
    'STEP_SIZE': '10d'
    }) # set parameters from dictionary
req['OBJ_DATA'] = False # set parameter dict-like
req.csv = True # set parameter as attributes
req.set(
    TABLE_TYPE='V',
    VEC_LABELS=False,
    VEC_TABLE=1
) # set position vectors output

res = req.query() # perform the request obtaining a response from Jpl Horizons service
e = res.parse() # parse the ephemeris in an astropy QTable

from astropy.io import ascii

ascii.write(e, format='csv') # write output data

The content of eph.ini can be something like this (see ftp://ssd.jpl_process.nasa.gov/pub/ssd/horizons_batch_example.long for a complete description of JPL parameters)

[jplparams]
MAKE_EPHEM=YES
REFERENCE_PLANE=ECLIPTIC
REF_SYSTEM=J2000
OUT_UNITS=AU-D

Shortcuts

eph package defines also some useful shortcut functions to easily access Jpl Horizons data. Instead of building a JplReq and get back a JplRes to parse, you can get an astropy QTable with

from eph import *
from datetime import datetime

e = get('venus', dates=['2000-1-1', datetime.now()])

Shortcut functions accept also one-date queries (non-interval) and multiple target objects. Behind the scenes eph makes multiple calls to JPL Horizons system and merge the results in one table. In this case non-key (used to join) columns are renamed with a prefix referring to the object (e.g. column X for object venus becomes venus_X). Meta info are listified and collapsed in a single value only if they take the same value for all objects.

from eph import *

e = get(['venus', 'mars'], dates=datetime.now())

Dates kwarg have the datetime.now() default value so it can be omitted if you want present data.

from eph import *

e = get(['venus', 'mars'], table_type='V', vec_table=1) # present vector positional data for Venus and Mars

There are other shortcut functions like vec, pos, vel, elem, obs, radec, altaz, etc.. to simplify parameter settings.

For example, if you want vectors, type

e = vec('2000-1-1', '2018-1-1', 299, step=100).

Command line tool

eph package also provides a command line tool:

$ eph venus --dates 2007-11-07 2017-04-22

This command gives you an ephemeris table of Venus starting from 2007-11-17 to 2017-4-22. You can also change the reference frame, the time-step size, the output etc.. through the options provided or setting up a config file. Check available options typing

$ eph --help