pyEphem - Calculating Positions of non-Earthy Moons

Question

I'm trying to get the Earth distance and the right ascension (relative to my observer point in Earth) of a satellite not orbiting the Earth, but pyEphem isn't returning the same properties as other solar bodies.

With Ganymede (the largest moon of Jupiter), for instance:

import math, ephem

Observer = ephem.city('London')
Observer.date = '2013-04-23'
Observer.pressure, Observer.elevation = 0, 100

moonGanymede = ephem.Ganymede(Observer)

print math.cos(moonGanymede.ra) # right ascension
print moonGanymede.earth_distance * ephem.meters_per_au # distance

I get this error:

AttributeError: 'Ganymede' object has no attribute 'earth_distance'

The ra attribute exists, but is it relative to my Observer or to Jupiter?

Seems to be relative to the Observer, since if I change the location, the value changes too.

I've read the documentation and I know that these properties are not defined for moons, but I have no idea how to compute those relative to the Earth given the additional defined properties of moon bodies:

On planetary moons, also sets:

Position of moon relative to planet (measured in planet radii)

x — offset +east or –west
y — offset +south or –north
z — offset +front or –behind

Doing:

print moonGanymede.x, moonGanymede.y, moonGanymede.z

Outputs:

-14.8928060532 1.52614057064 -0.37974858284

Since Jupiter has an average radius of 69173 kilometers, those values translate to:

moonGanymede.x = 1030200 kilometers (west)
moonGanymede.y = 105570 kilometers (south)
moonGanymede.z = 26268 kilometers (behind)

Given that I know the distance and right ascension of Jupiter relative to the Observer, how can I calculate the distance and right ascension of moonGanymede (also relative to the Observer)?

I'm using pyEphem 3.7.5.1 (with Python 2.7).

Answer 1

I'm still trying to figure it out (if anyone spots something, please do tell), but it seems that if I do:

sqrt((-14.8928060532)^2 + (1.52614057064)^2 + (-0.37974858284)^2) = 14.9756130481

I'll always get a value that always falls within the min/max distance from orbit center (14.95 - 14.99).

Since that's specified in orbit center radii, I'll need to multiply it by 69173 * 1000 to get the SI unit:

14.9756130481 * 69173 * 1000 = 1.0359080813762213 * 10^9 meters

Since pyEphem deals in distances with AU:

print (1.0359080813762213 * 10**9) / ephem.meters_per_au # 0.00692461785302

At the same time, the Earth-Jupiter distance was 5.79160547256 AU.

Now, to get the distance, I should either add or subtract depending on the sign of the z coordinate:

5.79160547256 - 0.00692461785302 = 5.78468085470698 AU

Running the same code for today (now) returns 6.03799937821 which seems to very close to the value of 6.031 that WolframAlpha is returning at the present time, it doesn't match 100% but perhaps that could be accounted for by some different underlying ephemeris library or data source. Not sure...

Answer 2

Just some thoughts; You probably need to do it two steps.

1. Get location of satellite relative to parent planet
2. Get location of planet relative to observer
3. Trigonometry calculation; get the location of satellite relative to observer.

You already did 1, and can easily do 2. Convert all values to x,y,z and add then back to angular. Or I'm sure you / ephym can do this for you directly.

HTH

Answer 3

Looks like right ascension, declination, azimuth, etc are computed correctly:

In [31]: g = ephem.Ganymede(Observer)

In [32]: j = ephem.Jupiter(Observer)

In [33]: g.ra, g.az, g.dec
Out[33]: (1.3024204969406128, 5.586287021636963, 0.38997682929039)

In [34]: j.ra, j.az, j.dec
Out[34]: (1.303646765055829, 5.5853118896484375, 0.39010250333236757)

Values for Ganimede and Jupiter are close enough, it looks like you get correct results for everything except distance to object.