def visible_crescent(self, date):
"""Return S. K. Shaukat's criterion for likely
visibility of crescent moon on eve of date 'date',
at location 'location'."""
tee = self.universal_from_standard(self.dusk(date - 1, mpf(4.5)))
phase = Lunar.lunar_phase(tee)
altitude = self.lunar_altitude(tee)
arc_of_light = arccos_degrees(cos_degrees(Lunar.lunar_latitude(tee)) * cos_degrees(phase))
return ((Lunar.NEW < phase < Lunar.FIRST_QUARTER) and
(mpf(10.6) <= arc_of_light <= 90) and
(altitude > mpf(4.1)))
def refraction(self, tee):
"""Return refraction angle at location 'location' and time 'tee'."""
h = max(0, self.elevation)
cap_R = 6.372E6
dip = arccos_degrees(cap_R / (cap_R + h))
return angle(0, 50, 0) + dip + secs(19) * math.sqrt(h)
