Date::MSD - conversion between flavours of Mars Sol Date
use Date::MSD qw(js_to_msd msd_to_cmsdn cmsdn_to_js); $msd = js_to_msd($js); ($cmsdn, $cmsdf) = msd_to_cmsdn($msd, $tz); $js = cmsdn_to_js($cmsdn, $cmsdf, $tz); # and 33 other conversion functions
For date and time calculations it is convenient to represent dates by a simple linear count of days, rather than in a particular calendar. This module performs conversions between different flavours of linear count of Martian solar days ("sols").
Among Martian day count systems there are also some non-trivial differences of concept. There are systems that count only complete days, and those that count fractional days also. There are some that are fixed to Airy Mean Time (time on the Martian prime meridian), and others that are interpreted according to a timezone. The functions of this module appropriately handle the semantics of all the non-trivial conversions.
The day count systems supported by this module are Mars Sol Date, Julian Sol, and Chronological Mars Solar Date, each in both integral and fractional forms.
In the interests of orthogonality, all flavours of day count come in both integral and fractional varieties. Generally, there is a quantity named "XYZ" which is a real count of days since a particular epoch (an integer plus a fraction) and a corresponding quantity named "XYZN" ("XYZ Number") which is a count of complete days since the same epoch. XYZN is the integral part of XYZ. There is also a quantity named "XYZF" ("XYZ Fraction") which is a count of fractional days since the XYZN changed. XYZF is the fractional part of XYZ, in the range [0, 1). This quantity naming pattern is derived from the naming of Terran day counts.
All calendar dates given are in the Darian calendar for Mars. An hour number is appended to each date, separated by a "T"; hour 00 is midnight at the start of the day. An appended "Z" indicates that the date is to be interpreted in the timezone of the prime meridian (Airy Mean Time), and so is absolute; where any other timezone is to be used then this is explicitly noted.
days elapsed since 0140-19-26T00Z (approximately MJD 5521.50 in Terrestrial Time). This epoch is the most recent near coincidence of midnight on the Martian prime meridian with noon on the Terran prime meridian. MSD is defined by the paper at http://pubs.giss.nasa.gov/docs/2000/2000_Allison_McEwen.pdf.
days elapsed since 0000-01-01T00Z (MSD -94129.0) (approximately MJD -91195.22 in Terrestrial Time). This epoch is an Airy midnight approximating the last northward equinox prior to the first telescopic observations of Mars. The same epoch is used for the Darian calendar for Mars. JS is defined (but not explicitly) by the document describing the Darian calendar, at http://pweb.jps.net/~tgangale/mars/converter/calendar_clock.htm.
days elapsed since -0608-23-20T00 in the timezone of interest. CMSD = MSD + 500000.0 + Zoff, where Zoff is the timezone offset in fractional days. CMSD is defined by the memo at http://www.fysh.org/~zefram/time/define_cmsd.txt.
A day count has meaning only in the context of a particular definition of "day". Potentially several time scales could be expressed in terms of a day count, just as Terran day counts such as MJD are used in the timescales UT1, UT2, UTC, TAI, TT, TCG, and others. For a day number to be meaningful it is necessary to be aware of which kind of day it is counting. Conversion between the different time scales is out of scope for this module.
Day counts in this API may be native Perl numbers or Math::BigRat objects. Both are acceptable for all parameters, in any combination. In all conversion functions, the result is of the same type as the input, provided that the inputs are of consistent type. If native Perl numbers are supplied then the conversion is subject to floating point rounding, and possible overflow if the numbers are extremely large. The use of Math::BigRat is recommended to avoid these problems. With Math::BigRat the results are exact.
Math::BigRat
There are conversion functions between all pairs of day count systems. This is a total of 36 conversion functions (including 6 identity functions).
When converting between timezone-relative counts (CMSD) and absolute counts (MSD, JS), the timezone that is being used must be specified. It is given in a ZONE argument as a fractional number of days offset from the base time scale (typically Airy Mean Time). Beware of floating point rounding when the offset does not have a terminating binary representation; use of Math::BigRat avoids this problem. A ZONE parameter is not used when converting between absolute day counts (e.g., between MSD and JS) or between timezone-relative counts (e.g., between CMSD and CMSDN).
Conversions between fractional day counts principally involve a change of epoch. The input identifies a point in time, as a fractional day count of input flavour. The function returns the same point in time, represented as a fractional day count of output flavour.
These conversion functions go from a fractional count to an integral count. The input identifies a point in time, as a fractional day count of input flavour. The function determines the day number of output flavour that applies at that instant. In scalar context only this integral day number is returned. In list context a list of two values is returned: the integral day number and the day fraction in the range [0, 1). The day fraction, representing the time of day, is relative to midnight.
These conversion functions go from an integral count to a fractional count. The input identifies a point in time, as an integral day number of input flavour plus day fraction in the range [0, 1). The day fraction, representing the time of day, is relative to midnight. The identified point in time is returned in the form of a fractional day number of output flavour.
These conversion functions go from an integral count to another integral count. They can be used either to convert only a day number or to convert a point in time using integer-plus-fraction form. The output convention is identical to that for msd_to_msdn et al, including the variation depending on calling context.
msd_to_msdn
If converting a point in time, the input identifies it as an integral day number of input flavour plus day fraction in the range [0, 1). The day fraction, representing the time of day, is relative to midnight. The same point in time is (in list context) returned as a list of integral day number of output flavour and the day fraction in the range [0, 1).
If it is desired only to convert integral day numbers, it is still necessary to consider time of day, because in the general case the days are delimited differently by the input and output day count flavours. A day fraction must be specified if there is such a difference, and the conversion is calculated for the point in time thus identified. To perform a conversion for a large part of the day, give a representative time of day within it. If converting between systems that delimit days identically (e.g., between MSD and JS), the day fraction is optional and defaults to zero.
Date::Darian::Mars, Date::JD
Andrew Main (Zefram) <zefram@fysh.org>
Copyright (C) 2007, 2009, 2010 Andrew Main (Zefram) <zefram@fysh.org>
This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
To install Date::MSD, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Date::MSD
CPAN shell
perl -MCPAN -e shell install Date::MSD
For more information on module installation, please visit the detailed CPAN module installation guide.