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Functions for converting dates to/from JD and MJD
"""
Functions for converting dates to/from JD and MJD. Assumes dates are historical
dates, including the transition from the Julian calendar to the Gregorian
calendar in 1582. No support for proleptic Gregorian/Julian calendars.
:Author: Matt Davis
:Website: http://github.com/jiffyclub
"""
import math
import datetime as dt
# Note: The Python datetime module assumes an infinitely valid Gregorian calendar.
# The Gregorian calendar took effect after 10-15-1582 and the dates 10-05 through
# 10-14-1582 never occurred. Python datetime objects will produce incorrect
# time deltas if one date is from before 10-15-1582.
def mjd_to_jd(mjd):
"""
Convert Modified Julian Day to Julian Day.
Parameters
----------
mjd : float
Modified Julian Day
Returns
-------
jd : float
Julian Day
"""
return mjd + 2400000.5
def jd_to_mjd(jd):
"""
Convert Julian Day to Modified Julian Day
Parameters
----------
jd : float
Julian Day
Returns
-------
mjd : float
Modified Julian Day
"""
return jd - 2400000.5
def date_to_jd(year,month,day):
"""
Convert a date to Julian Day.
Algorithm from 'Practical Astronomy with your Calculator or Spreadsheet',
4th ed., Duffet-Smith and Zwart, 2011.
Parameters
----------
year : int
Year as integer. Years preceding 1 A.D. should be 0 or negative.
The year before 1 A.D. is 0, 10 B.C. is year -9.
month : int
Month as integer, Jan = 1, Feb. = 2, etc.
day : float
Day, may contain fractional part.
Returns
-------
jd : float
Julian Day
Examples
--------
Convert 6 a.m., February 17, 1985 to Julian Day
>>> date_to_jd(1985,2,17.25)
2446113.75
"""
if month == 1 or month == 2:
yearp = year - 1
monthp = month + 12
else:
yearp = year
monthp = month
# this checks where we are in relation to October 15, 1582, the beginning
# of the Gregorian calendar.
if ((year < 1582) or
(year == 1582 and month < 10) or
(year == 1582 and month == 10 and day < 15)):
# before start of Gregorian calendar
B = 0
else:
# after start of Gregorian calendar
A = math.trunc(yearp / 100.)
B = 2 - A + math.trunc(A / 4.)
if yearp < 0:
C = math.trunc((365.25 * yearp) - 0.75)
else:
C = math.trunc(365.25 * yearp)
D = math.trunc(30.6001 * (monthp + 1))
jd = B + C + D + day + 1720994.5
return jd
def jd_to_date(jd):
"""
Convert Julian Day to date.
Algorithm from 'Practical Astronomy with your Calculator or Spreadsheet',
4th ed., Duffet-Smith and Zwart, 2011.
Parameters
----------
jd : float
Julian Day
Returns
-------
year : int
Year as integer. Years preceding 1 A.D. should be 0 or negative.
The year before 1 A.D. is 0, 10 B.C. is year -9.
month : int
Month as integer, Jan = 1, Feb. = 2, etc.
day : float
Day, may contain fractional part.
Examples
--------
Convert Julian Day 2446113.75 to year, month, and day.
>>> jd_to_date(2446113.75)
(1985, 2, 17.25)
"""
jd = jd + 0.5
F, I = math.modf(jd)
I = int(I)
A = math.trunc((I - 1867216.25)/36524.25)
if I > 2299160:
B = I + 1 + A - math.trunc(A / 4.)
else:
B = I
C = B + 1524
D = math.trunc((C - 122.1) / 365.25)
E = math.trunc(365.25 * D)
G = math.trunc((C - E) / 30.6001)
day = C - E + F - math.trunc(30.6001 * G)
if G < 13.5:
month = G - 1
else:
month = G - 13
if month > 2.5:
year = D - 4716
else:
year = D - 4715
return year, month, day
def hmsm_to_days(hour=0,min=0,sec=0,micro=0):
"""
Convert hours, minutes, seconds, and microseconds to fractional days.
Parameters
----------
hour : int, optional
Hour number. Defaults to 0.
min : int, optional
Minute number. Defaults to 0.
sec : int, optional
Second number. Defaults to 0.
micro : int, optional
Microsecond number. Defaults to 0.
Returns
-------
days : float
Fractional days.
Examples
--------
>>> hmsm_to_days(hour=6)
0.25
"""
days = sec + (micro / 1.e6)
days = min + (days / 60.)
days = hour + (days / 60.)
return days / 24.
def days_to_hmsm(days):
"""
Convert fractional days to hours, minutes, seconds, and microseconds.
Precision beyond microseconds is rounded to the nearest microsecond.
Parameters
----------
days : float
A fractional number of days. Must be less than 1.
Returns
-------
hour : int
Hour number.
min : int
Minute number.
sec : int
Second number.
micro : int
Microsecond number.
Raises
------
ValueError
If `days` is >= 1.
Examples
--------
>>> days_to_hmsm(0.1)
(2, 24, 0, 0)
"""
hours = days * 24.
hours, hour = math.modf(hours)
mins = hours * 60.
mins, min = math.modf(mins)
secs = mins * 60.
secs, sec = math.modf(secs)
micro = round(secs * 1.e6)
return int(hour), int(min), int(sec), int(micro)
def datetime_to_jd(date):
"""
Convert a `datetime.datetime` object to Julian Day.
Parameters
----------
date : `datetime.datetime` instance
Returns
-------
jd : float
Julian day.
Examples
--------
>>> d = datetime.datetime(1985,2,17,6)
>>> d
datetime.datetime(1985, 2, 17, 6, 0)
>>> jdutil.datetime_to_jd(d)
2446113.75
"""
days = date.day + hmsm_to_days(date.hour,date.minute,date.second,date.microsecond)
return date_to_jd(date.year,date.month,days)
def jd_to_datetime(jd):
"""
Convert a Julian Day to an `jdutil.datetime` object.
Parameters
----------
jd : float
Julian day.
Returns
-------
dt : `jdutil.datetime` object
`jdutil.datetime` equivalent of Julian day.
Examples
--------
>>> jd_to_datetime(2446113.75)
datetime(1985, 2, 17, 6, 0)
"""
year, month, day = jd_to_date(jd)
frac_days,day = math.modf(day)
day = int(day)
hour,min,sec,micro = days_to_hmsm(frac_days)
return datetime(year,month,day,hour,min,sec,micro)
def timedelta_to_days(td):
"""
Convert a `datetime.timedelta` object to a total number of days.
Parameters
----------
td : `datetime.timedelta` instance
Returns
-------
days : float
Total number of days in the `datetime.timedelta` object.
Examples
--------
>>> td = datetime.timedelta(4.5)
>>> td
datetime.timedelta(4, 43200)
>>> timedelta_to_days(td)
4.5
"""
seconds_in_day = 24. * 3600.
days = td.days + (td.seconds + (td.microseconds * 10.e6)) / seconds_in_day
return days
class datetime(dt.datetime):
"""
A subclass of `datetime.datetime` that performs math operations by first
converting to Julian Day, then back to a `jdutil.datetime` object.
Addition works with `datetime.timedelta` objects, subtraction works with
`datetime.timedelta`, `datetime.datetime`, and `jdutil.datetime` objects.
Not all combinations work in all directions, e.g.
`timedelta - datetime` is meaningless.
See Also
--------
datetime.datetime : Parent class.
"""
def __add__(self,other):
if not isinstance(other,dt.timedelta):
s = "jdutil.datetime supports '+' only with datetime.timedelta"
raise TypeError(s)
days = timedelta_to_days(other)
combined = datetime_to_jd(self) + days
return jd_to_datetime(combined)
def __radd__(self,other):
if not isinstance(other,dt.timedelta):
s = "jdutil.datetime supports '+' only with datetime.timedelta"
raise TypeError(s)
days = timedelta_to_days(other)
combined = datetime_to_jd(self) + days
return jd_to_datetime(combined)
def __sub__(self,other):
if isinstance(other,dt.timedelta):
days = timedelta_to_days(other)
combined = datetime_to_jd(self) - days
return jd_to_datetime(combined)
elif isinstance(other, (datetime,dt.datetime)):
diff = datetime_to_jd(self) - datetime_to_jd(other)
return dt.timedelta(diff)
else:
s = "jdutil.datetime supports '-' with: "
s += "datetime.timedelta, jdutil.datetime and datetime.datetime"
raise TypeError(s)
def __rsub__(self,other):
if not isinstance(other, (datetime,dt.datetime)):
s = "jdutil.datetime supports '-' with: "
s += "jdutil.datetime and datetime.datetime"
raise TypeError(s)
diff = datetime_to_jd(other) - datetime_to_jd(self)
return dt.timedelta(diff)
def to_jd(self):
"""
Return the date converted to Julian Day.
"""
return datetime_to_jd(self)
def to_mjd(self):
"""
Return the date converted to Modified Julian Day.
"""
return jd_to_mjd(self.to_jd())

kynan commented Feb 22, 2017

@jiffyclub could you please add a license so this code becomes (re)usable?

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