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Hello Moon

September 13, 2009

Did you see my new Moon Phases widget?  Isn’t it the coolest?  You can get one too – from Calculator Cat

Brief Explanation of the Moon Phases
The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth in 27.3 days, or 27 days 7 hours 43 minutes, on average. This measurement is relative to the stars and is called the sidereal period or orbital period. However, because of the earth’s motion around the sun, a complete moon cycle (New Moon to New Moon) appears to earthbound observers to take a couple of days longer: 29.5305882 days to be exact. This number is called the synodic period or "lunation", and is relative to the sun.

The sun always illuminates the half of the moon facing the sun (except during lunar eclipses, when the moon passes through the earth’s shadow). When the sun and moon are on opposite sides of the earth, the moon appears "full" to us, a bright, round disk. When the moon is between the earth and the sun, it appears dark, a "new" moon. In between, the moon’s illuminated surface appears to grow (wax) to full, then decreases (wanes) to the next new moon. The edge of the shadow (the terminator) is always curved, being an oblique view of a circle, giving the moon its familiar crescent shape. 

(Taken from the Calculator Cat website, for more information on moon phases, go here, USNO.)

  • New Moon new moon
  • Waxing Crescent waxing_crescent
  • First Quarter first_quarter
  • Waxing Gibbous waxing_gibbous
  • Full Moon full
  • Waning Gibbous waning_gibbous
  • Last Quarter last_quarter
  • Waning Crescent waning_crescent


    Lunation Movie

    Click to see small movie
    Small movie (134k)

    Click to see large movie
    Large movie (493k)

    If you click on either of the two images above, you will see a time-lapse movie of the appearance of the Moon over one lunation. A lunation is a lunar month, during which time the Moon completely circles the Earth in its orbit. The complete cycle of phases is obvious. Two other effects can be easily seen. First, due to the elliptical shape of the Moon’s orbit, the apparent size of the Moon’s disk changes as its distance from Earth varies (the closest and farthest points do not always occur at the same phases, however). Second, although the Moon’s near side directly faces the Earth on average, we get to view the Moon from slightly different angles as it orbits us. This effect, called libration, is caused partly by the tilt of the Moon’s rotation axis with respect to its orbital plane and partly by the fact that the Moon’s speed in its orbit varies but its rotation rate does not.

    The movie is an animated GIF created by Antonio Cidadao from a sequence of still images taken during March and April 1998.

    Copyright Antonio Cidadao.

    I love this stuff!

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    One Comment leave one →
    1. September 14, 2009 5:34 pm


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