The Moon's Phases

 

The light we see emanating from the Moon on any moonlit night is the result of reflected sunlight being reflected back to Earth. The familiar phases the Moon passes through in a month are caused by the different angles from which we see the sunlit side. The dark far side of the Moon only reflects 7% of the incoming light. Because of the Moon's reflective properties and since it illuminates no light of its own, it is said by mystics to be "receptive" (borrowing its light from the sun).

The Moon moves in an elliptical path tilted at a 5 degree angle to the ecliptic, and moves westward in its orbit across the sky. It takes about 12 hours for the Moon to travel from horizon to horizon. As it crosses the sunrise terminator, where the Sun is rising, the Moon is in its waxing phase, where it is increasing in light. As it crosses the sunset terminator, where the Sun is setting, the Moon is in its waning phase and is decreasing in light. If you look up in the sky on any given night and the Moon's right side is brightly lit, it is in its waxing phase. If the Moon's left side is brightly lit, then it is in its waning phase.

The Period of Rotation

It takes 27.3 days to complete one orbit. It also rotates on its axis in 27.3 days, so it always keeps the same face pointing toward earth. A complete cycle from one new Moon to the next takes 29.5 days or one synodic month to complete.

Because the Moon's period of rotation is locked in phase with its orbit around the Earth, we usually only see the one side. However, the Moon has a slightly non-circular orbit, moving quicker when closest the Earth and more slowly when far away. This causes the Moon to appear to wobble a bit, and the usual face we see drifts sideways, to one side or the other, so that a few degrees of the far side can occasionally be seen. While its speed in orbit changes rhythmically, its axis rotation remains uniform. When the Moon is closest to Earth, the axial rotation falls behind a bit, turning its face to the East. While at a greater distance, the Moon moves ahead, turning its face to the West.

The tides, eclipses, and phases of the Moon are in direct correlation to the movement of the sun, moon, and earth. When the Moon is full, earthquakes, storms and floods increase in frequency.

The Moon begins its rhythmic cycle as the Moon's nearside crosses the sunrise terminator, from east to west, to produce a crescent Moon. The Moon waxes to a 1st quarter Moon, where the Moon is half illuminated, to a gibbous Moon. Then when the Earth, Sun and Moon are nearly in a straight line, with the Earth in the middle, a Full Moon occurs. This occurs when the Moon is in opposition to the position of the Sun. From the Full Moon, the Moon begins its waning cycle, crossing the sunset terminator to produce a waning gibbous Moon, to the last quarter. Finally we see only a waning crescent.

The complicated motion of the Moon results in a difference in the way we measure the monthly cycles. Time is measured by a reference point in the sky. We measure the time between one New Moon and the time it takes to return to the same position with respect to a fixed position in the sky. This is called a Sidereal month and is equal to 27.32166 days. The time between the transits of the Moon through one of its nodes is called the Draconic month; this equals 27.212220 days. The time between one New Moon to the next New Moon is called the Synodic month; this equals 29.530589 days and the time the Moon moves from equinox to equinox is called a Tropical month; this equals 27.321.58 days.

The Moon's perigee and nodes also move east and west with periods of 8.85 years and 18.61 years respectably. This results in different lengths of the Sidereal and Draconic months.

Computations and Phases

USNO Master Clock Time, Calculate any time zone

Moon Age Calendar Sunrise and Sunset Computations iG's Moon Tool

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