For my natal Moon, Stellarium gives a topocentric longitude of 59°32'42.4" or 29°32'42.4" Taurus. Astrolog gives a geocentric figure of 29°26'22" Taurus. This is a difference of slightly more than six minutes of arc. This isn't much but it could constitute a difference of sign if applied to my natal Saturn in 0°00'47" Virgo. Fortunately, this is not the case. Stellarium reports a topocentric longitude of 0°00'41.9" for Saturn. This is a difference of only about 5 seconds of arc.
I'll list the differences comprehensively below with Astrolog listed first and Stellarium second (measurements are rounded to the nearest second):
Sun --> 12°47'13" versus 12°47'51"
Moon --> 29°26'22" versus 29°32'42"
Mercury --> 2°14'36" versus 2°15'11"
Venus --> 9°09'42" versus 9°10'17"
Mars --> 9°15'54" versus 9°16'27"
Jupiter --> 28°47'00" versus 28°47'15"
Saturn --> 0°00'47" versus 0°00'42"
Uranus --> 26°52'14" versus 26°52'29"
Neptune -->13°15'24" versus 13°15'12"
Pluto --> 14°16'26" versus 14°16'24"
Chiron --> 8°57'05" versus NOT FOUND
Ceres --> 11°34'23" versus 8°41'52"
The glaring difference in positions is apparent for Ceres. I've checked carefully and that discrepancy of almost three degrees is very real and very disconcerting. What's gone wrong? What program is in error and why? Given that Ceres is between Mars and Jupiter, the difference should be measured in seconds of arc. Let's check the differences between the two program's for Ceres today (14th February 2018): Astrolog gives 8°13'07" Leo whereas Stellarium gives 8°54'06" Leo, which is a smaller discrepancy than in my natal chart.
For the time being, leaving out Ceres, the differences between the two sets of measurements is slight but significantly greatest where the position of the Moon is concerned. For a lunar position that is greater than 29°50" or less than 0°10" of a sign, the geocentric position should be checked against the topocentric to determine whether a change of sign is involved. If so, I would say that the topocentric measurement that should be used and not the geocentric.
As for Ceres, it will be useful to check some other asteroids as well, namely Juno, Vesta and Pallas. In my natal chart, Pallas is located at 29°09'00" Gemini according to Astrolog, while Stellarium puts it as 29°38'17" (almost a half degree different and much larger than the discrepancy between the Moon's positions). Vesta is located at 23°42'21" Sagittarius using Astrolog but 21°42"00' using Stellarium (a difference of a full two degrees). Juno is located at 27°16'47" Taurus using Astrolog but at 28°23'11" using Stellarium (a difference of more than a degree). So for the asteroids at least, there are significant differences between the celestial longitudes as reported by the two programs. This is not due to a differences between topocentric and geocentric positions. These should be slight. The difference is due to a software bug.
What is happening with Ceres on a day to day basis according to Stellarium? As mentioned earlier, it is currently in 8°54'06" Leo, so I'll track its weekly progress:
21st February: Stellarium 7°32'20" Leo versus Astrolog 7°01'01" retrograde
28th February: Stellarium 6°26'41" Leo versus Astrolog 5°54'55" retrograde
7th March: Stellarium 5°40'08" Leo versus Astrolog 5°07'49" retrograde
15th March: Stellarium 5°12'42" Leo versus Astrolog 4°39'22" retrograde
19th March: Stellarium 5°08'57" Leo versus Astrolog 4°35'35" Leo stationary direct
22nd March: Stellarium 5°10'58" Leo versus Astrolog 4°37'18" direct
29th March: Stellarium 5°30'25" Leo versus Astrolog 4°56'06" direct
Stellarium and Astrolog both report Ceres turning stationary direct on the 19th March, it's only that there is a consistent difference of about half a degree between positions on any given date. I suspect that the Astrolog software is in error with the error increasing as one travels back in time. For the time, it is sufficient to add about 30 minutes to the position as reported by Astrolog. I'll try to find out more about the source of this problem.
Very informative. Thank you.
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