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Changed the formula used to calculate moon phase.

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Tom Keffer
2018-09-17 06:11:01 -07:00
parent b989eabeab
commit 0220656bc8
2 changed files with 32 additions and 24 deletions
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52
bin/weeutil/Moon.py
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@@ -1,42 +1,46 @@
#
# Copyright (c) 2009-2015 Tom Keffer <tkeffer@gmail.com>
# Copyright (c) 2009-2018 Tom Keffer <tkeffer@gmail.com>
#
# See the file LICENSE.txt for your full rights.
#
"""Determine the phase of the moon phase given a date.
CF: http://en.wikipedia.org/wiki/Lunar_phase
"""
"""Determine the phase of the moon phase given a date."""
import datetime
import time
import math
import decimal
dec = decimal.Decimal
moon_phases = ["new (totally dark)",
"waxing crescent (increasing to full)",
"in its first quarter (increasing to full)",
"waxing gibbous (increasing to full)",
"full (full light)",
"waning gibbous (decreasing from full)",
"in its last quarter (decreasing from full)",
moon_phases = ["new (totally dark)",
"waxing crescent (increasing to full)",
"in its first quarter (increasing to full)",
"waxing gibbous (increasing to full)",
"full (full light)",
"waning gibbous (decreasing from full)",
"in its last quarter (decreasing from full)",
"waning crescent (decreasing from full)"]
# First new moon of 2018: 16-Jan-2018 at 1017UTC
new_moon_2018 = 1516097820
def moon_phase(year, month, day):
def moon_phase(year, month, day, hour=12):
"""Calculates the phase of the moon, given a year, month, day.
returns: a tuple. First value is an index into an array
of moon phases, such as Moon.moon_phases above. Second
value is the percent fullness of the moon.
"""
time_dt = datetime.datetime(year, month, day)
diff = time_dt - datetime.datetime(2001, 1, 1)
days = dec(diff.days) + (dec(diff.seconds) / dec(86400))
lunations = dec('0.20439731') + (days / dec('29.530589'))
# Convert to UTC
time_ts = time.mktime((year, month, day, hour, 0, 0, 0, 0, -1))
# How many days since the first moon of 2018
delta_days = (time_ts - new_moon_2018) / 86400
# Number of lunations
lunations = delta_days / 29.530589
# The fraction of the lunar cycle
position = float(lunations) % 1.0
fullness = int(100.0*(1.0 - math.cos(2.0 * math.pi * position))/2.0 + 0.5)
# The percent illumination, rounded to the nearest integer
fullness = int(100.0 * (1.0 - math.cos(2.0 * math.pi * position)) / 2.0 + 0.5)
index = int((position * 8) + 0.5) & 7
return (index, fullness)
return index, fullness

4
docs/changes.txt
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@@ -34,6 +34,10 @@ Guard against a negative value for 'interval' in WMR200 stations.
Fixed bug that caused older, "type A" Vantage Pro2, to crash.
Fixes issue #343.
Changed the formula used to calculate percentage illumination of the moon to
something more accurate around 2018. This formula is only used if pyephem is not
installed.
3.8.2 08/15/2018