Phases

  • The Moon and planets do not emit their own light - we see them in the sky only because they reflect sunlight. Depending on the relative positions of the Earth, Sun and a planet or the Moon, varying amounts of the surface appear illuminated. The amount of illumination is known as the phase.

    moonorbit.jpg
    The Moon's motion around the Earth, with the Sun illuminating only one side of the Earth and Moon.

    phases.jpg
    The Moon goes through a cycle of phases that repeats every 29.531 days (a synodic month). We see these phase changes occur with the Moon rising between 20-70 minutes later each day.

    The phases of the Moon

    PhaseRise, Transit and Set timeDiagram Position
    New MoonRises at sunrise, transits meridian at noon, sets at sunsetA
    Waxing CrescentRises before noon, transits meridian before sunset, sets before midnightB
    First QuarterRises at noon, transits meridian at sunset, sets at midnightC
    Waxing GibbousRises after noon, meridian after sunset, sets after midnightD
    Full MoonRises at sunset, transits meridian at midnight, sets at sunriseE
    Waning GibbousRises after sunset, transits after midnight, sets after sunriseF
    Last QuarterRises at midnight, transits meridian at sunrise, sets at noonG
    Waning CrescentRises after midnight, transits after sunrise, sets after noonH
    New MoonThe cycle repeatsA

    A complete cycle of phases is observable for the inferior planets, Mercury and Venus. Early telescopic observations of Venus by Galileo were used to support the Copernican view that the Sun was at the centre of the Solar System (a heliocentric model). Mars is observed to have a gibbous phase when it is near quadrature (elongation = 90o and 270o).

    See also: gibbous moon, waxing crescent moon, first quarter moon, waxing gibbous moon, waning gibbous moon, last quarter moon, waning crescent moon.


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