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Add documentation on computing velocity per second 

This addresses a question raised in #30.
This commit is contained in:
Brandon Rhodes 2019-02-03 07:08:20 -05:00
parent 7dae84d506
commit 48c99ce40c
1 changed files with 14 additions and 7 deletions
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jplephem/__init__.py
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@ -103,7 +103,7 @@ solar system, this ephemeris only requires you to take a single step:
>>> position = kernel[0,4].compute(2457061.5)
>>> print(position)
[ 2.05700211e+08 4.25141646e+07 1.39379183e+07]
[2.05700211e+08 4.25141646e+07 1.39379183e+07]
But learning the position of Mars with respect to the Earth takes three
steps, from Mars to the Solar System barycenter to the Earth-Moon
@ -113,7 +113,7 @@ barycenter and finally to Earth itself:
>>> position -= kernel[0,3].compute(2457061.5)
>>> position -= kernel[3,399].compute(2457061.5)
>>> print(position)
[ 3.16065185e+08 -4.67929557e+07 -2.47554111e+07]
[ 3.16065185e+08 -4.67929557e+07 -2.47554111e+07]
You can see that the output of this ephemeris is in kilometers. If you
use another ephemeris, check its documentation to be sure of the units
@ -126,9 +126,9 @@ returned will itself be a vector as long as your date:
>>> jd = np.array([2457061.5, 2457062.5, 2457063.5, 2457064.5])
>>> position = kernel[0,4].compute(jd)
>>> print(position)
[[ 2.05700211e+08 2.05325363e+08 2.04928663e+08 2.04510189e+08]
[ 4.25141646e+07 4.45315179e+07 4.65441136e+07 4.85517457e+07]
[ 1.39379183e+07 1.48733243e+07 1.58071381e+07 1.67392630e+07]]
[[2.05700211e+08 2.05325363e+08 2.04928663e+08 2.04510189e+08]
[4.25141646e+07 4.45315179e+07 4.65441136e+07 4.85517457e+07]
[1.39379183e+07 1.48733243e+07 1.58071381e+07 1.67392630e+07]]
Some ephemerides include velocity inline by returning a 6-vector instead
of a 3-vector. For an ephemeris that does not, you can ask for the
@ -137,10 +137,17 @@ is delivered as a second return value:
>>> position, velocity = kernel[0,4].compute_and_differentiate(2457061.5)
>>> print(position)
[ 2.05700211e+08 4.25141646e+07 1.39379183e+07]
[2.05700211e+08 4.25141646e+07 1.39379183e+07]
>>> print(velocity)
[ -363896.06287889 2019662.99596519 936169.77271558]
[-363896.06287889 2019662.99596519 936169.77271558]
The velocity will by default be distance traveled per day, in whatever
units for distance the ephemeris happens to use. To get a velocity per
second, simply divide by the number of seconds in a day:
>>> velocity_per_second = velocity / 86400.0
>>> print(velocity_per_second)
[-4.21175999 23.37572912 10.8352983 ]
Details of the API
------------------