histastro.datetime module

Date and time functions for HistAstro.

histastro.datetime.DeltaT(jd)[source]

Return the value of DeltaT through interpolation.

For the date range -700 - now, the value for Delta T is obtained by interpolation of known historical: values. Outside this range, a lenghtening of the day of 1.8 ms/century is assumed, as well as that the minimum of the parabola is DeltaT=12s in 1820.

Parameters:: jd (float) – Julian day (days).
Returns:: Delta T (s).
Return type:: float

References

[International Earth Rotation and Reference Systems Service](ftp://maia.usno.navy.mil/ser7/deltat.data) of the U.S. Naval Observatory.
[Robert van Gent’s website on Delta T](https://www.staff.science.uu.nl/~gent0113/deltat/deltat.htm).
[Extrapolation of Delta T](http://hemel.waarnemen.com/Computing/deltat.html).

histastro.datetime.DeltaT1820(jd)[source]

Return a rough estimate for the value of Delta T.

A lenghtening of the day of 1.8 ms/century is assumed, as well as and that the minimum of the parabola is DeltaT=12s in 1820.

Parameters:: jd (float) – Julian day (days).
Returns:: Delta T (s).
Return type:: float

References

[Extrapolation of Delta T](http://hemel.waarnemen.com/Computing/deltat.html).

histastro.datetime.gmst(jd)[source]

Calculate Greenwich Mean Sidereal Time for any instant, in radians.

Parameters:: jd (float) – Julian day (days).
Returns:: gmst: Greenwich mean sidereal time (rad).
Return type:: float

References

Explanatory Supplement to the Astronomical Almanac, 3rd ed, Eq. 6.66 (2012).

histastro.datetime.jd2cal(jd, julian=None)[source]

Compute the calendar date from a given Julian Day.

Notes

Date and time are expressed in UT.
Decimals can be returned in the day to indicate the time of day, e.g. 1.0 for midnight and 1.5 for noon on the first day of the month.

Parameters:

jd (float) – Julian day (days).
julian (bool) – Force Julian (True) or Gregorian calendar (False). Default: None: Julian before 1583, else Gregorian.

Returns:

Tuple containing (year, month, day):

year (int): Year CE (UT). Note that year=0 indicates 1 BCE.

month (int): Month number of year (UT; 1-12).

day (float): Day of month with fraction (UT; 1.0-31.999).

Return type:

tuple (int,int,float)

histastro.datetime.jd2tjc(jd)[source]

Compute the time in Julian centuries since 2000.0.

Parameters:: jd (float) – Julian day (days).
Returns:: tjc: Time in Julian centuries since 2000.0 (UT).
Return type:: float

histastro.datetime.jd2tjm(jd)[source]

Compute the time in Julian millennia since 2000.0.

Parameters:: jd (float) – Julian day (days).
Returns:: tjm: Time in Julian millennia since 2000.0 (UT).
Return type:: float

histastro.datetime.jd2year(jd)[source]

Compute a year with fraction from a given Julian Day.

Parameters:: jd (float) – Julian day (days).
Returns:: Year CE, with decimals. Note that year=0 indicates 1 BCE.
Return type:: float

histastro.datetime.julianDay(year, month, day, julian=None)[source]

Compute the Julian Day for a given year, month and day.

Notes

Date and time are expressed in UT.
Decimals can be used in the day to take into account the time of day other than midnight, e.g. 1.5 for noon on the first day of the month.

Parameters:

year (int) – Year CE (UT). Note that year=0 = 1 BCE.
month (int) – Month number of year (UT; 1-12).
day (float) – Day of month with fraction (UT; 1.0-31.999).
julian (bool) – Force Julian (True) or Gregorian calendar (False). Default: None: Julian before 1583, else Gregorian.

Returns:

jd: Julian day (days).

Return type:

float