def precession(cls, tee):
"""Return the precession at moment tee using 0,0 as J2000 coordinates.
Adapted from "Astronomical Algorithms" by Jean Meeus,
Willmann-Bell, Inc., 1991."""
c = cls.julian_centuries(tee)
eta = mod(poly(c, [0, secs(mpf(47.0029)), secs(mpf(-0.03302)), secs(mpf(0.000060))]), 360)
cap_P = mod(poly(c, [mpf(174.876384), secs(mpf(-869.8089)), secs(mpf(0.03536))]), 360)
p = mod(poly(c, [0, secs(mpf(5029.0966)), secs(mpf(1.11113)), secs(mpf(0.000006))]), 360)
cap_A = cos_degrees(eta) * sin_degrees(cap_P)
cap_B = cos_degrees(cap_P)
arg = arctan_degrees(cap_A, cap_B)
return mod(p + cap_P - arg, 360)
def direction(self, focus):
"""Return the angle (clockwise from North) to face focus when
standing in location, location. Subject to errors near focus and
its antipode."""
y = sin_degrees(focus.longitude - self.longitude)
x = ((cos_degrees(self.latitude) * tan_degrees(focus.latitude)) -
(sin_degrees(self.latitude) * cos_degrees(self.longitude - focus.longitude)))
if x == y == 0 or focus.latitude == 90:
return 0
elif focus.latitude == -90:
return 180
else:
return arctan_degrees(y, x)
def right_ascension(cls, tee, beta, lam):
"""Return right ascension at moment UT 'tee' of object at
latitude 'lam' and longitude 'beta'."""
varepsilon = cls.obliquity(tee)
return arctan_degrees((sin_degrees(lam) * cos_degrees(varepsilon)) - (tan_degrees(beta) * sin_degrees(varepsilon)), cos_degrees(lam))
