DateTime::Julian

Provides several modules to handle astronomical Julian dates (sometimes called Julian day numbers) as well as conversions between Gregorian and Julian calendar dates.

SYNOPSIS

use DateTime::Julian;
my $jd  = nnnn.nnnn; # Julian Date for some event
my $mjd = nnnn.nnnn; # Modified Julian Date for some event
my $utc  = DateTime::Julian.new: :julian-date($jd);
my $utc2 = DateTime::Julian.new: :modified-julian-date($mjd);
say $utc.j2000; # Julian date for epoch J2000
say $utc.j1900; # Julian date for epoch J1900
say $utc.jcent;
say $utc.mjd;   # Modified Julian date
my $lon = -86.234; # local observer's longitude;
# Get the local sidereal time for the UTC at the current Julian date
my $lst = $utc.lst: :$lon; 

DESCRIPTION

Module DateTime::Julian defines a class (inherited from a Raku DateTime class) that is usually instantiated from a Julian date, although it can also be instantiated by any of the methods described in the Raku documentation for a DateTime class;

Following are some pertinent definitions from Wikipedia topic [Julian day](https://en.m.wikipedia.org/wiki/Julian_day):

• The Julian day is the continuous count of the days since the beginning of the Julian period, and is used primarily by astronomers....

• The Julian day number (JDN) is the integer assigned to a whole solar day count starting from noon Universal time, with Julian day number 0 assigned to the day starting at noon on Monday, January 1, 4713 BC, proleptic [Note 1] Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar), a date at which three multi-year cycles started (which are: indiction, Solar, and Lunar cycles) and which preceded any dates in recorded history. For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2451545.

• The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. Julian dates are expressed as a Julian day number with a decimal fraction added. For example, the Julian date for 00:30:00.0 UT January 1, 2013, is 2456293.520833.

The following methods and routines were developed from the descriptions of code in References 1 and 2. The author of Ref. 3 has been very helpful with this author's questions about astronomy and the implementation of astronomical routines.

The main purpose of this module is to simplify time and handling for this author who still finds Julian dates to be somewhat mysterious, but absolutely necessary for dealing with astronomy and predicting object positions, especially the Sun and Moon, for local observation and producing astronomical almanacs.

This module will play a major supporting role in this author's planned Raku module Astro::Almanac;

Class DateTime::Julian methods

DateTime::Julian routines

Notes

1. A proleptic calendar, according to Wikipedia, "is a calendar that is applied to dates before its introduction."

References

1. Astronomy on the Personal Computer, Oliver Montenbruck and Thomas Pfleger, Springer, 2000.

2. Celestial Calculations: A Gentle Introduction to Computational Astronomy, J. L. Lawrence, The MIT Press, 2019.

3. Astro::Montenbruck, Sergey Krushinsky, CPAN.

AUTHOR

Tom Browder <[email protected]>

COPYRIGHT AND LICENSE

Copyright © 2021 Tom Browder