Module: Astro::Moon

Defined in:

lib/astro/moon.rb

Overview

Provides functions for lunar phases and lunar month dates (eg. new/full moon)

This file is a thoughtless manual compilation of Astro::MoonPhase perl module (which, in turn, is influenced by moontool.c). I don’t know how it all works.

Defined Under Namespace

Classes: Phase, PhaseHunt

Constant Summary

EPOCH =

Astronomical constants. 1980 January 0.0

2444238.5

ELONGE =

Constants defining the Sun’s apparent orbit.

ecliptic longitude of the Sun at epoch 1980.0

278.833540

ELONGP =

ecliptic longitude of the Sun at perigee

282.596403

ECCENT =

eccentricity of Earth’s orbit

0.016718

SUNSMAX =

semi-major axis of Earth’s orbit, km

1.495985e8

SUNANGSIZ =

sun’s angular size, degrees, at semi-major axis distance

0.533128

MMLONG =

moon’s mean longitude at the epoch

64.975464

MMLONGP =

mean longitude of the perigee at the epoch

349.383063

MLNODE =

mean longitude of the node at the epoch

151.950429

MINC =

inclination of the Moon’s orbit

5.145396

MECC =

eccentricity of the Moon’s orbit

0.054900

MANGSIZ =

moon’s angular size at distance a from Earth

0.5181

MSMAX =

semi-major axis of Moon’s orbit in km

384401.0

MPARALLAX =

parallax at distance a from Earth

0.9507

SYNMONTH =

synodic month (new Moon to new Moon)

29.53058868

Class Method Summary

• .phase(dt = nil) ⇒ Object

  Finds the lunar phase for the specified date.

• .phasehunt(date = nil) ⇒ Object

  Finds the key dates of the specified lunar month.

Class Method Details

.phase(dt = nil) ⇒ Object

Finds the lunar phase for the specified date. Takes a DateTime object (or creates one using now() function). Returns a Phase struct instance. (see Constants section)

# find the current moon illumination, in percents
Astro::Moon.phase.illumination * 100     # => 63.1104513958699

# find the current moon phase (new moon is 0 or 100,
# full moon is 50, etc)
Astro::Moon.phase.phase * 100            # => 70.7802812241989

# File 'lib/astro/moon.rb', line 124

def phase(dt = nil)
    dt = DateTime.now if !dt
    pdate = dt.ajd

    # Calculation of the Sun's position.

    day = pdate - EPOCH			# date within epoch
    n = ((360 / 365.2422) * day) % 360.0
    m = (n + ELONGE - ELONGP) % 360.0	# convert from perigee
    # co-ordinates to epoch 1980.0
    ec = kepler(m, ECCENT)		# solve equation of Kepler
    ec = Math.sqrt((1 + ECCENT) / (1 - ECCENT)) * Math.tan(ec / 2)
    ec = 2 * todeg(Math.atan(ec))		# true anomaly
    lambdasun = (ec + ELONGP) % 360.0	# Sun's geocentric ecliptic
    # longitude
    # Orbital distance factor.
    f = ((1 + ECCENT * Math.cos(torad(ec))) / (1 - ECCENT * ECCENT))
    sundist = SUNSMAX / f		# distance to Sun in km
    sunang = f * SUNANGSIZ		# Sun's angular size in degrees

    # Calculation of the Moon's position.

    # Moon's mean longitude.
    ml = (13.1763966 * day + MMLONG) % 360.0

    # Moon's mean anomaly.
    mm = (ml - 0.1114041 * day - MMLONGP) % 360.0

    # Moon's ascending node mean longitude.
    mn = (MLNODE - 0.0529539 * day) % 360.0

    # Evection.
    ev = 1.2739 * Math.sin(torad(2 * (ml - lambdasun) - mm))

    # Annual equation.
    ae = 0.1858 * Math.sin(torad(m))

    # Correction term.
    a3 = 0.37 * Math.sin(torad(m))

    # Corrected anomaly.
    mmp = mm + ev - ae - a3

    # Correction for the equation of the centre.
    mec = 6.2886 * Math.sin(torad(mmp))

    # Another correction term.
    a4 = 0.214 * Math.sin(torad(2 * mmp))

    # Corrected longitude.
    lp = ml + ev + mec - ae + a4

    # Variation.
    v = 0.6583 * Math.sin(torad(2 * (lp - lambdasun)))

    # True longitude.
    lpp = lp + v

    # Corrected longitude of the node.
    np = mn - 0.16 * Math.sin(torad(m))

    # Y inclination coordinate.
    y = Math.sin(torad(lpp - np)) * Math.cos(torad(MINC))

    # X inclination coordinate.
    x = Math.cos(torad(lpp - np))

    # Ecliptic longitude.
    lambdamoon = todeg(Math.atan2(y, x))
    lambdamoon += np

    # Ecliptic latitude.
    betam = todeg(Math.asin(Math.sin(torad(lpp - np)) *
                            Math.sin(torad(MINC))))

    # Calculation of the phase of the Moon.

    # Age of the Moon in degrees.
    moonage = lpp - lambdasun

    # Phase of the Moon.
    moonphase = (1 - Math.cos(torad(moonage))) / 2

    # Calculate distance of moon from the centre of the Earth.

    moondist = (MSMAX * (1 - MECC * MECC)) /
        (1 + MECC * Math.cos(torad(mmp + mec)))

    # Calculate Moon's angular diameter.

    moondfrac = moondist / MSMAX
    moonang = MANGSIZ / moondfrac

    # Calculate Moon's parallax.

    moonpar = MPARALLAX / moondfrac

    pphase = moonphase
    mpfrac = (moonage % 360) / 360.0
    mage = SYNMONTH * mpfrac
    dist = moondist
    angdia = moonang
    sudist = sundist
    suangdia = sunang
    Phase.new(mpfrac, pphase, mage, dist, angdia, sudist, suangdia)
end

.phasehunt(date = nil) ⇒ Object

Finds the key dates of the specified lunar month. Takes a DateTime object (or creates one using now() function). Returns a PhaseHunt struct instance. (see Constants section)

# find the date/time of the full moon in this lunar month
Astro::Moon.phasehunt.moon_full.strftime("%D %T %Z") \
# => "03/04/07 02:17:40 +0300"

# File 'lib/astro/moon.rb', line 81

def phasehunt(date = nil)
    date = DateTime.now if !date
    sdate = date.ajd

    adate = sdate - 45
    ad1 = DateTime.jd(adate)

    k1 = ((ad1.year + ((ad1.month - 1) *
                       (1.0 / 12.0)) - 1900) * 12.3685).floor

    adate = nt1 = meanphase(adate,  k1)

    while true
        adate += SYNMONTH
        k2 = k1 + 1
        nt2 = meanphase(adate, k2)
        break if nt1 <= sdate && nt2 > sdate
        nt1 = nt2
        k1 = k2  
    end

    return PhaseHunt.new(*[
                         truephase(k1, 0.0),
                         truephase(k1, 0.25),
                         truephase(k1, 0.5),
                         truephase(k1, 0.75),
                         truephase(k2, 0.0)
    ].map {
        |_| _.new_offset(date.offset)
    })
end