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Corrected the B1875 epoch for determining constellation boundaries.

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It turns out I was off by nearly 18 hours in the B1875 epoch.
This has a tiny effect on the orientation of the Earth's axis.
Instead of:           ut = 1875-01-01T12:00:00.000Z
the correct epoch is: ut = 1874-12-31T18:12.21.950Z

See the comments in the Constellation functions in
each of the source files for more info.
This commit is contained in:
Don Cross
2020-05-04 21:30:36 -04:00
parent 4b4c637468
commit 6c3655c1c6
9 changed files with 102 additions and 10 deletions
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13
source/python/astronomy.py
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@@ -6537,7 +6537,18 @@ def Constellation(ra, dec):
# Lazy-initialize rotation matrix.
if _ConstelRot is None:
_ConstelRot = Rotation_EQJ_EQD(Time.Make(1875, 1, 1, 12, 0, 0))
# Need to calculate the B1875 epoch. Based on this:
# https://en.wikipedia.org/wiki/Epoch_(astronomy)#Besselian_years
# B = 1900 + (JD - 2415020.31352) / 365.242198781
# I'm interested in using TT instead of JD, giving:
# B = 1900 + ((TT+2451545) - 2415020.31352) / 365.242198781
# B = 1900 + (TT + 36524.68648) / 365.242198781
# TT = 365.242198781*(B - 1900) - 36524.68648 = -45655.741449525
# But the Time constructor wants UT, not TT.
# Near that date, I get a historical correction of ut-tt = 3.2 seconds.
# That gives UT = -45655.74141261017 for the B1875 epoch,
# or 1874-12-31T18:12:21.950Z.
_ConstelRot = Rotation_EQJ_EQD(Time(-45655.74141261017))
_Epoch2000 = Time(0.0)
# Convert coordinates from J2000 to B1875.