36 lines
1.3 KiB
Python
36 lines
1.3 KiB
Python
"""Routines for dealing with Julian dates."""
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def compute_calendar_date(jd_integer, julian_before=None):
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"""Convert Julian day ``jd_integer`` to ``(year, month, day)``.
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Uses the proleptic Gregorian calendar unless ``julian_before`` is
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set to a specific Julian day, in which case the Julian calendar is
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used for dates older than that.
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"""
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use_gregorian = (julian_before is None) or (jd_integer >= julian_before)
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# See the Explanatory Supplement to the Astronomical Almanac 15.11.
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f = jd_integer + 1401
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f += use_gregorian * ((4 * jd_integer + 274277) // 146097 * 3 // 4 - 38)
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e = 4 * f + 3
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g = e % 1461 // 4
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h = 5 * g + 2
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day = h % 153 // 5 + 1
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month = (h // 153 + 2) % 12 + 1
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year = e // 1461 - 4716 + (12 + 2 - month) // 12
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return year, month, day
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def compute_julian_date(year, month=1, day=1.0):
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"""Given a proleptic Gregorian date, return a Julian date float."""
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return compute_julian_day(year, month, day) - 0.5
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def compute_julian_day(year, month=1, day=1):
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"""Given a proleptic Gregorian date, return a Julian day int."""
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janfeb = month < 3
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return (+ 1461 * (year + 4800 - janfeb) // 4
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+ 367 * (month - 2 + janfeb * 12) // 12
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- 3 * ((year + 4900 - janfeb) // 100) // 4
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- 32075
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+ day)
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