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 lunar_altitude(self, tee):
"""Return the geocentric altitude of moon at moment, tee,
at location, location, as a small positive/negative angle in degrees,
ignoring parallax and refraction. Adapted from 'Astronomical
Algorithms' by Jean Meeus, Willmann_Bell, Inc., 1998."""
lamb = Lunar.lunar_longitude(tee)
beta = Lunar.lunar_latitude(tee)
alpha = Astro.right_ascension(tee, beta, lamb)
delta = Astro.declination(tee, beta, lamb)
theta0 = Astro.sidereal_from_moment(tee)
cap_H = mod(theta0 + self.longitude - alpha, 360)
altitude = arcsin_degrees(
(sin_degrees(self.latitude) * sin_degrees(delta)) +
(cos_degrees(self.latitude) * cos_degrees(delta) * cos_degrees(cap_H)))
return mod(altitude + 180, 360) - 180
