different RA/Decs returned by pyEphem -


i using pyephem calculate ra/decs of satellites , i'm confused different values computed , described on

http://rhodesmill.org/pyephem/radec.html

this bit of code

sat=ephem.readtle("satname ", \               "1 38356u 12030a   14148.90924578  .00000000  00000-0  10000-3 0  5678",\               "2 38356   0.0481  47.9760 0002933 358.9451 332.7970  1.00270012  3866") gatech = ephem.observer() gatech.lon, gatech.lat = '-155.47322222', '19.82561111' gatech.elevation = 4194 gatech.date = '2014/01/02 07:05:52' sat.compute(gatech) print 'a_ra=',sat.a_ra,'a_dec=',sat.a_dec,'g_ra=',sat.g_ra,'g_dec=',sat.g_dec,'ra=',sat.ra,'dec=',sat.dec

gives

a_ra= 0:52:40.75 a_dec= -3:15:23.7 g_ra= 1:14:10.55 g_dec= 0:06:09.8 ra= 0:53:23.57 dec= -3:10:50.5

if change observers location gatech.lon, gatech.lat = '-5.47322222', '19.82561111'

i get

a_ra= 1:15:36.95 a_dec= -2:32:29.9 g_ra= 1:14:10.55 g_dec= 0:06:09.8 ra= 1:16:19.75 dec= -2:28:04.6

i thought observers position came calculation of sat.ra , sat.dec suprised see a_ra , a_dec had changed.

what missing?

thanks ad

per last paragraph of “body.compute(observer)” section of quick reference:

http://rhodesmill.org/pyephem/quick.html#body-compute-observer

for earth satellite objects, astrometric coordinates [meaning a_ra , a_dec] topocentric instead of geocentric, since there little point in figuring out satellite appear on j2000 (or whatever epoch using) star chart observer sitting @ center of earth.

and in issue has been opened behavior, project open suggestions text can appear more prominently prevent future confusion users:

https://github.com/brandon-rhodes/pyephem/issues/55


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