Astro::MoonPhase - Information about the phase of the Moon


use Astro::MoonPhase;

        ( $MoonPhase,
          $SunAng ) = phase($seconds_since_1970);

        @phases  = phasehunt($seconds_since_1970);

        ($phase, @times) = phaselist($start, $stop);


MoonPhase calculates information about the phase of the moon at a given time.



        ( $MoonPhase,
          $SunAng )  = phase($seconds_since_1970);

          $MoonPhase = phase($seconds_since_1970);

The argument is the time for which the phase is requested, expressed as a time returned by the time function. If $seconds_since_1970 is omitted, it does phase(time).

Return value in scalar context is $MoonPhase, the terminator phase angle as a percentage of a full circle (i.e., 0 to 1).

Return values in array context:

the terminator phase angle as a percentage of a full circle (i.e., 0 to 1)


the illuminated fraction of the Moon's disc


the Moon's age in days and fraction


the distance of the Moon from the centre of the Earth


the angular diameter subtended by the Moon as seen by an observer at the centre of the Earth.


the distance from the Sun in km


the angular size of Sun in degrees


   ( $MoonPhase,
     $SunAng ) = phase();

     print "MoonPhase  = $MoonPhase\n";
     print "MoonIllum  = $MoonIllum\n";
     print "MoonAge    = $MoonAge\n";
     print "MoonDist   = $MoonDist\n";
     print "MoonAng    = $MoonAng\n";
     print "SunDist    = $SunDist\n";
     print "SunAng     = $SunAng\n";

could print something like this:

     MoonPhase  = 0.598939375319023
     MoonIllum  = 0.906458030827876
     MoonAge    = 17.6870323368022
     MoonDist   = 372479.357420033
     MoonAng    = 0.534682403555093
     SunDist    = 152078368.820205
     SunAng     = 0.524434538105092


     @phases = phasehunt($seconds_since_1970);

Finds time of phases of the moon which surround the given date. Five phases are found, starting and ending with the new moons which bound the current lunation.

The argument is the time, expressed as a time returned by the time function. If $seconds_since_1970 is omitted, it does phasehunt(time).


    @phases = phasehunt();
    print "New Moon      = ", scalar(localtime($phases[0])), "\n";
    print "First quarter = ", scalar(localtime($phases[1])), "\n";
    print "Full moon     = ", scalar(localtime($phases[2])), "\n";
    print "Last quarter  = ", scalar(localtime($phases[3])), "\n";
    print "New Moon      = ", scalar(localtime($phases[4])), "\n";

could print something like this:

    New Moon      = Wed Jun 24 06:51:47 1998
    First quarter = Wed Jul  1 21:42:19 1998
    Full moon     = Thu Jul  9 19:02:47 1998
    Last quarter  = Thu Jul 16 18:15:18 1998
    New Moon      = Thu Jul 23 16:45:01 1998


    ($phase, @times) = phaselist($start, $stop);

Finds times of all phases of the moon which occur on or after $start but before $stop. Both the arguments and the return values are expressed as seconds since 1970 (like the time function returns).

$phase is an integer indicating the phase of the moon at $times[0], as shown in this table:

    0  New Moon
    1  First quarter
    2  Full Moon
    3  Last quarter

The remaining values in @times indicate subsequent phases of the moon (in ascending order by time). If there are no phases of the moon between $start and $stop, phaselist returns the empty list.


    @name = ("New Moon", "First quarter", "Full moon", "Last quarter");
    ($phase, @times) = phaselist($start, $stop);

    while (@times) {
      printf "%-14s= %s\n", $name[$phase], scalar localtime shift @times;
      $phase = ($phase + 1) % 4;

could produce the same output as the phasehunt example above (given the appropriate start & stop times).


The algorithms used in this program to calculate the positions of Sun and Moon as seen from the Earth are given in the book Practical Astronomy With Your Calculator by Peter Duffett-Smith, Second Edition, Cambridge University Press, 1981. Ignore the word "Calculator" in the title; this is an essential reference if you're interested in developing software which calculates planetary positions, orbits, eclipses, and the like. If you're interested in pursuing such programming, you should also obtain:

Astronomical Formulae for Calculators by Jean Meeus, Third Edition, Willmann-Bell, 1985. A must-have.

Planetary Programs and Tables from -4000 to +2800 by Pierre Bretagnon and Jean-Louis Simon, Willmann-Bell, 1986. If you want the utmost (outside of JPL) accuracy for the planets, it's here.

Celestial BASIC by Eric Burgess, Revised Edition, Sybex, 1985. Very cookbook oriented, and many of the algorithms are hard to dig out of the turgid BASIC code, but you'll probably want it anyway.

Many of these references can be obtained from Willmann-Bell, P.O. Box 35025, Richmond, VA 23235, USA. Phone: (804) 320-7016. In addition to their own publications, they stock most of the standard references for mathematical and positional astronomy.


This program is in the public domain: "Do what thou wilt shall be the whole of the law".


The moontool.c Release 2.0:

    A Moon for the Sun
    Designed and implemented by John Walker in December 1987,
    revised and updated in February of 1988.

Initial Perl transcription:

    Raino Pikkarainen, 1998

The moontool.c Release 2.4:

    Major enhancements by Ron Hitchens, 1989


    Brett Hamilton
    Bug fix, 2003
    Second transcription and bugfixes, 2004

    Christopher J. Madsen
    Added phaselist function, March 2007