PyEphem

Calculating the transit of the first point of Aries

Asked by John Donovan

I'm using PyEphem to generate a nautical almanac, and so far it's doing a good job. However, one of the values found on the "day pages" is the transit, or meridian passage, of the first point of Aries.
So what I thought I'd do is to create a "body" who's parameters are such that it lies at 0h ra:
FPA = ephem.readdb('FPA,f,0:0:0|0.0,0:0:0|0.0,0,2000,0')

then use next_transit():
FPA.compute(greenwich)
FPA_transit = greenwich.next_transit(FPA)

This gets me a pretty close answer, but when I compare it to nautical almanacs, it is often several minutes out. What do I need to do to get a more accurate answer? I thought perhaps it needs a value in the RA proper motion field, but I'm not sure what value it should be.

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Solved by:
Brandon Rhodes
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Best Brandon Rhodes (brandon-rhodes) said : 		#1

The First Point of Ares moves every day as the Earth slowly precesses. So instead of asking when the 2000 FPA is transiting today, ask when today's FPA is transiting:

FPA._epoch = ephem.now()

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John Donovan (mersey-viking) said : 		#2

Ah yes, of course! Thanks Brandon.