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Phases of the Moon

Moon phases depend on the position of both the Sun and Moon with respect to the Earth. The 4 primary phases of the Moon are: new, first quarter, full and third quarter.

Time lapse picture of Moon phases
The phases of the Moon.
©bigtockphoto.com/Petr Jilek

The intermediate phases between the primary phases are, waxing crescent, waxing gibbous, waning gibbous, and waning crescent.

Why does the Moon have phases?

Position of Moon, Earth and Sun at New Moon
New Moon―Positions of the Sun (left), the Moon (middle), and the Earth (right) at new moon, as seen from the northern direction of the solar system. The arrows show the direction of the sun's rays, and the bright part of the Moon and Earth shows the part illuminated by the sun. Note that the true distance between the Sun, Earth, and Moon is much larger than showed in these illustrations.

New Moon

A new moon is the moment when the Sun and Moon are in conjunction, meaning that the Sun and Earth are on the opposite sides of the Moon. New moon cannot normally be seen from the Earth, however, since only the dark side of the Moon faces the Earth. Sometimes, if the Moon lies directly between the Earth and the Sun during the new moon phase, a solar eclipse occurs as the Earth enters the Moon's shadow. In most locations on Earth, only a fraction of the Sun will be obscured, in a partial solar eclipse. Locations where the center of the shadow passes over experience a total eclipse in which the Moon completely obscures the Sun, or an annular or ring eclipse, in which the apparent size of the Moon is smaller than the Sun.

Waxing Crescent Moon

A few days after the new moon phase, the Moon will be visible again in a phase that lasts until the first quarter, called waxing crescent moon. The initial period, just after the Moon becomes visible, is sometimes called new moon, although it has another definition. Although only a small part of the Moon may be illuminated by the Sun, the rest of the Moon may also be faintly visible, due to a reflection from the Earth to the Moon, called earthshine. The waxing crescent moon is most visible after sunset. The first visual sight of the waxing crescent moon determines the beginnings of months in the Muslim calendar.
Position of Moon, Earth and Sun at First Quarter
First Quarter Moon―The Moon has now moved 90 degrees from the SunEarth axis, in a counter-clockwise direction (as seen from the northern part of the solar system).

First Quarter Moon

During the first quarter, half of the Moon is illuminated, as seen from the Earth. The Moon rises near the middle of the day and sets near the middle of the night. In northern regions of the world, the right part will be visible, while the left part will be visible in the southern regions. Near the equator, the upper part is bright after moonrise, and the lower part is bright before moonset (the bright part appears and disappears first).

Waxing Gibbous Moon

The waxing gibbous moon occurs between the first quarter and the full moon. The sun illuminates more than half of the Moon's surface during this period.
Position of Moon, Earth and Sun at Full Moon
Full Moon―The Moon has completed 180 degrees of its path around the Earth, and the bright part of it faces the Earth.

Full Moon

Full moon appears when the Sun and the Moon are on opposite sides of the Earth. As seen from Earth, all of the Moon's surface will be visible. The full moon is visible approximately from sunset to sunrise. When observed from Earth, the Moon can appear to be full for a couple of days, since more than 98 percent of the Moon's disc is illuminated a day before or after the full moon. During full moon, the Moon may pass through Earth's shadow causing a lunar eclipse. If the whole moon is in the Earth's shadow, or umbra, a total lunar eclipse occurs. If only a part of the Moon enters the umbra, we see a partial lunar eclipse.

Waning Gibbous Moon

The period between full moon and third quarter is called waning gibbous moon. The portion of the visible half of the Moon illuminated goes down from 100 percent to 50 percent during this period.
Position of Moon, Earth and Sun at Third Quarter
Third Quarter Moon―The Moon is now in a position 270 degrees from the original, as observed from the Earth.

Third Quarter Moon

The third quarter moon occurs when the other half of the Moon is illuminated compared to the first quarter. On the day of third quarter, the Moon rises approximately in the middle of the night and sets in the middle of the day.

Waning Crescent Moon

The waning crescent moon is the period between the third quarter moon and the next new moon. It is most visible before sunrise. The Sun illuminates less than half the Moon during this period. When only a small part of the Moon is visible, it may be possible to see earthshine on the dark side of the Moon.

Lunation

Lunation is a cycle of the Moon. It starts at new moon and lasts until the next new moon. On average, it takes the Moon 29 days 12 hours 44 minutes to go from one new Moon to the next. This time frame is called a synodic month. The duration of a synodic month varies from one lunation to another, most importantly because the orbit of the Earth and Moon are ellipses rather than circles, where the orbit speed depends on how close the orbiting object is to the mass center. For example, the Moon moves faster when it is closest to the Earth. Some years, such as 2004, have relatively small duration differences throughout the year (five hours difference between minimum and maximum duration), while the year 2008 will have larger differences (more than twelve hours).

Topics: Astronomy, Moon, Sun, Earth

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