Using Ephem¶
As shown above (How to use Data Containers),
Ephem objects can be created on the fly. However,
Ephem can also be used to access ephemerides information
from remote services with a largely uniform API.
For instance, the following few lines will query
ephemerides for asteroid Ceres on a given date and for the position of
Mauna Kea Observatory (IAU observatory code 568) from the JPL Horizons service:
>>> from sbpy.data import Ephem
>>> from astropy.time import Time
>>> epoch = Time('2018-08-03 14:20', scale='utc') # time in UT
>>> eph = Ephem.from_horizons('Ceres',
... location='568',
... epochs=epoch)
>>> eph
<QTable masked=True length=1>
targetname H G solar_presence ... PABLon PABLat epoch
mag ... deg deg
str7 float64 float64 str1 ... float64 float64 object
---------- ------- ------- -------------- ... ------- ------- -----------------
1 Ceres 3.34 0.12 ... 171.275 9.3473 2458334.097222222
>>> eph.field_names
['targetname', 'H', 'G', 'solar_presence', 'flags', 'RA', 'DEC', 'RA_app', 'DEC_app', 'RA*cos(Dec)_rate', 'DEC_rate', 'AZ', 'EL', 'AZ_rate', 'EL_rate', 'sat_X', 'sat_Y', 'sat_PANG', 'siderealtime', 'airmass', 'magextinct', 'V', 'surfbright', 'illumination', 'illum_defect', 'sat_sep', 'sat_vis', 'ang_width', 'PDObsLon', 'PDObsLat', 'PDSunLon', 'PDSunLat', 'SubSol_ang', 'SubSol_dist', 'NPole_ang', 'NPole_dist', 'EclLon', 'EclLat', 'r', 'r_rate', 'delta', 'delta_rate', 'lighttime', 'vel_sun', 'vel_obs', 'elong', 'elongFlag', 'alpha', 'lunar_elong', 'lunar_illum', 'sat_alpha', 'sunTargetPA', 'velocityPA', 'OrbPlaneAng', 'constellation', 'TDB-UT', 'ObsEclLon', 'ObsEclLat', 'NPole_RA', 'NPole_DEC', 'GlxLon', 'GlxLat', 'solartime', 'earth_lighttime', 'RA_3sigma', 'DEC_3sigma', 'SMAA_3sigma', 'SMIA_3sigma', 'Theta_3sigma', 'Area_3sigma', 'RSS_3sigma', 'r_3sigma', 'r_rate_3sigma', 'SBand_3sigma', 'XBand_3sigma', 'DoppDelay_3sigma', 'true_anom', 'hour_angle', 'alpha_true', 'PABLon', 'PABLat', 'epoch']
from_horizons uses one or more target names, an
observer location in the form of an IAU observatory code, and a list
of discrete epochs or a range of epochs defined in a dictionary (see
from_horizons) to query the JPL Horizons
service. Epochs have to be provided in the form of Time
objects. The column names in the data table can be inquired using
field_names.
from_horizons is actually a wrapper around
ephemerides. This function
conveniently combines the creation of a
HorizonsClass query and the actual
ephemerides information retrieval into a single function. Additional
optional parameters provided to from_horizons are
directly passed on to
ephemerides, maintaining the
full flexibility of the latter function:
>>> import astropy.units as u
>>> epoch1 = Time('2018-08-03 14:20', scale='utc')
>>> epoch2 = Time('2018-08-04 07:30', scale='utc')
>>> eph = Ephem.from_horizons('Ceres',
... location='568',
... epochs={'start': epoch1,
... 'stop': epoch2,
... 'step': 10*u.minute},
... skip_daylight=True)
>>> eph
<QTable masked=True length=26>
targetname H G ... PABLon PABLat epoch
mag ... deg deg
str7 float64 float64 ... float64 float64 object
---------- ------- ------- ... -------- ------- -----------------
1 Ceres 3.34 0.12 ... 171.275 9.3473 2458334.097222222
1 Ceres 3.34 0.12 ... 171.2774 9.3472 2458334.104166667
1 Ceres 3.34 0.12 ... 171.2798 9.3471 2458334.111111111
1 Ceres 3.34 0.12 ... 171.2822 9.347 2458334.118055556
1 Ceres 3.34 0.12 ... 171.2846 9.3469 2458334.125
1 Ceres 3.34 0.12 ... 171.2869 9.3468 2458334.131944444
... ... ... ... ... ... ...
1 Ceres 3.34 0.12 ... 171.5076 9.3369 2458334.777777778
1 Ceres 3.34 0.12 ... 171.5099 9.3368 2458334.784722222
1 Ceres 3.34 0.12 ... 171.5123 9.3367 2458334.791666667
1 Ceres 3.34 0.12 ... 171.5147 9.3366 2458334.798611111
1 Ceres 3.34 0.12 ... 171.5171 9.3365 2458334.805555556
1 Ceres 3.34 0.12 ... 171.5195 9.3364 2458334.8125
Note that skip_daylight is an optional parameter of
ephemerides and it can be used
here as well. An additional feature of
from_horizons is that you can automatically
concatenate queries for a number of objects:
>>> eph = Ephem.from_horizons(['Ceres', 'Pallas', 12893, '1983 SA'],
... location='568',
... epochs=epoch1)
>>> eph
<QTable masked=True length=4>
targetname H G ... PABLat epoch
mag ... deg
str26 float64 float64 ... float64 object
-------------------------- ------- ------- ... -------- -----------------
1 Ceres 3.34 0.12 ... 9.3473 2458334.097222222
2 Pallas 4.13 0.11 ... -20.1396 2458334.097222222
12893 Mommert (1998 QS55) 13.9 0.15 ... -2.0567 2458334.097222222
3552 Don Quixote (1983 SA) 12.9 0.15 ... 13.3365 2458334.097222222
Please be aware that these queries are not simultaneous. The more
targets you query, the longer the query will take. Furthermore, keep
in mind that asteroids and comets have slightly different table
layouts (e.g., different magnitude systems: T-mag and N-mag
instead of V-mag), which will complicate the interpretation of the
data. It might be safest to query asteroids and comets separately.
Also note that the two examples shown above use different ways to
define epochs. The first example uses a dictionary that defines a
start and stop epoch, as well as a step size (see
from_horizons for
details). Instead of a step size, the user can also provide a
number of steps as an integer; step is then automatically
derived from the interval and number in units of full minutes. The
second example uses a specific epoch as input. The
Time object provided to epochs can also be
initialized with a list of epochs, querying multiple epochs at the
same time. Note that the total number of epochs queried using this
option should be less than a few hundred to prevent corruption of the
query (see ephemerides for
details).
Observer locations can be defined as strings using official IAU
observatory codes or
using EarthLocation as shown in the following
example:
>>> from astropy.coordinates import EarthLocation
>>> lowell = EarthLocation.of_site('Lowell Observatory')
>>> eph = Ephem.from_horizons(1, epochs=Time('2018-01-01', format='iso'),
... location=lowell)
>>> eph
<QTable masked=True length=1>
targetname H G solar_presence ... PABLon PABLat epoch
mag ... deg deg
str7 float64 float64 str1 ... float64 float64 object
---------- ------- ------- -------------- ... -------- ------- ---------
1 Ceres 3.34 0.12 * ... 130.4303 9.2004 2458119.5
Offering almost identical functionality, the
from_mpc method will retrieve ephemerides from the
Minor Planet Center:
>>> eph = Ephem.from_mpc('2P', location='568',
... epochs={'start': Time('2018-10-22'),
... 'stop': Time('2018-10-26'),
... 'step': 1*u.day})
>>> eph
<QTable length=5>
Targetname Date ... Moon distance Moon altitude
... deg deg
str2 object ... float64 float64
---------- ----------------------- ... ------------- -------------
2P 2018-10-22 00:00:00.000 ... 28.0 -33.0
2P 2018-10-24 00:00:00.000 ... 54.0 -48.0
2P 2018-10-25 00:00:00.000 ... 67.0 -53.0
2P 2018-10-26 00:00:00.000 ... 81.0 -56.0
2P 2018-10-23 00:00:00.000 ... 41.0 -41.0
Finally, from_miriade will retrieve ephemerides
from the Miriade ephemeris generator at IMCCE:
>>> eph = Ephem.from_miriade('2P', objtype='comet', location='568',
... epochs={'start': Time('2018-10-22'),
... 'stop': Time('2018-10-26'),
... 'step': 1*u.day})
>>> eph
<QTable masked=True length=5>
target epoch RA ... DEC_rate delta_rate
deg ... arcsec / min km / s
bytes20 object float64 ... float64 float64
------- --------- ------------------ ... ------------ ------------
2P 2458413.5 329.99213124999994 ... -0.063365 24.7933113
2P 2458414.5 329.91132124999996 ... -0.059361 25.0280603
2P 2458415.5 329.83517041666664 ... -0.055369 25.253586
2P 2458416.5 329.76366666666667 ... -0.051392 25.4700287
2P 2458417.5 329.6967958333333 ... -0.04743 25.677518
Ephemerides can also be derived from Orbit objects using
sbpy’s interface to pyoorb with the function
from_oo. The following example computes
ephemerides for the next ten days in steps of 1 hr for Ceres as seen
from the Discovery Channel Telescope:
>>> import numpy as np
>>> from sbpy.data import Orbit, Ephem
>>> from astropy.time import Time
>>> epochs = Time(Time.now().jd + np.arange(0, 10, 1/24), format='jd')
>>> ceres = Orbit.from_horizons('1')
>>> eph = Ephem.from_oo(ceres, epochs, 'G37')
>>> eph
<QTable length=240>
targetname RA ... trueanom epoch
deg ... deg
str7 float64 ... float64 object
---------- ------------------ ... ------------------ ------------------
1 Ceres 238.56187075007446 ... 105.8270438687299 2458694.6423231447
1 Ceres 238.564318627966 ... 105.83566067245822 2458694.683989811
1 Ceres 238.56680284927273 ... 105.8442772820886 2458694.725656478
1 Ceres 238.56933812666867 ... 105.8528936974433 2458694.7673231447
1 Ceres 238.57193638137088 ... 105.8615099186335 2458694.808989811
1 Ceres 238.57460592776462 ... 105.87012594577034 2458694.850656478
... ... ... ... ...
1 Ceres 239.4677754274348 ... 107.83811369526742 2458704.3923231447
1 Ceres 239.4726928414698 ... 107.846685468736 2458704.433989811
1 Ceres 239.47756694312102 ... 107.85525705166283 2458704.475656478
1 Ceres 239.48240809475683 ... 107.8638284438719 2458704.5173231447
1 Ceres 239.48722955376766 ... 107.87239964547449 2458704.558989811
1 Ceres 239.49204656314026 ... 107.88097065658197 2458704.600656478
The properties computed by pyoorb and listed in the resulting table
are defined in the pyoorb documentation. Note that this function requires pyoorb to be installed, which is not a requirement for sbpy.