Lunar phase: Wikis

  
  

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Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent wobbling of the Moon is known as libration.

A lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun. One half of the lunar surface is always illuminated by the Sun (except during lunar eclipses), and hence is bright, but the portion of the illuminated hemisphere that is visible to an observer can vary from 100% (full moon) to 0% (new moon). The boundary between the illuminated and unilluminated hemispheres is called the terminator.

Contents

Overview

The lunar phase depends on the Moon's position in orbit around the Earth and the Earth's position in orbit around the sun. This diagram looks down on Earth from north. Earth's rotation and the Moon's orbit are both counter-clockwise here. Sunlight is coming in from the right, as indicated by the yellow arrows. From this diagram we can see, for example, that the full moon will always rise at sunset and that the waning crescent moon is high overhead around 9:00 AM local time.

Lunar phases are the result of looking at the illuminated half of the Moon from different viewing geometries; they are not caused by shadows of the Earth on the Moon that occur during a lunar eclipse. The Moon exhibits different phases as the relative geometry of the Sun, Earth and Moon changes, appearing as a full moon when the Sun and Moon are on opposite sides of the Earth, and as a new moon (also named dark moon, as it is not visible at night) when they are on the same side. The phases of full moon and new moon are examples of syzygies, which occur when the Earth, Moon and Sun lie (approximately) in a straight line. The time between two full moons (and between successive occurrences of the same phase) is about 29.53 days (29 days, 12 hours, 44 minutes) on average (hence, the concept of a timeframe of a period of time of an approximated month was derived). This synodic month is longer than the time it takes the Moon to make one orbit about the Earth with respect to the fixed stars (the sidereal month), which is about 27.32 days. This difference is caused by the fact that the Earth-Moon system is orbiting about the Sun at the same time the Moon is orbiting about the Earth.

The actual time between two syzygies or two phases is quite variable because the orbit of the Moon is elliptic and subject to various periodic perturbations, which change the velocity of the Moon. When the moon is closer to the earth, it moves faster; when it is farther, it moves slower. The orbit of the Earth around the Sun is also elliptic, so the speed of the Earth also varies, which also affects the phases of the Moon.[1]

It might be expected that once every month when the Moon passes between Earth and the Sun during a new moon, its shadow would fall on Earth causing a solar eclipse. Likewise, during every full moon we might expect the Earth's shadow to fall on the Moon, causing a lunar eclipse. We do not observe a solar and lunar eclipse every month because the plane of the Moon's orbit around the Earth is tilted by about 5 degrees with respect to the plane of Earth's orbit around the Sun. Thus, when new and full moons occur, the Moon usually lies to the north or south of a direct line through the Earth and Sun. Although an eclipse can only occur when the Moon is either new or full, it must also be positioned very near the intersection of Earth's orbit plane about the Sun and the Moon's orbit plane about the Earth (that is, at one of its nodes). This happens about twice per year, and so there are between 4 and 7 eclipses in a calendar year. Most of these are quite insignificant; major eclipses of the Moon or Sun are relatively rare.

Names of lunar phases

Phases of the Moon, as seen from the Northern Hemisphere. The Southern Hemisphere will see each phase rotated through 180°.

The phases of the Moon have been given the following names, in sequential order:

Phase Northern Hemisphere Southern Hemisphere Visibility
New moon Not visible, traditionally Moon's first visible crescent after sunset
Waxing crescent moon . Right 1-49% visible Left 1-49% visible afternoon and post-dusk
First quarter moon Right 50% visible Left 50% visible afternoon and early night
Waxing gibbous moon Right 51-99% visible Left 51-99% visible afternoon and most of night
Full moon Fully visible Fully visible sunset to sunrise (all night)
Waning gibbous moon Left 51-99% visible Right 51-99% visible most of night and morning
Last quarter moon Left 50% visible Right 50% visible late night and morning
Waning crescent moon Left 1-49% visible Right 1-49% visible pre-dawn and morning
Dark moon Not visible, traditionally Moon's last visible crescent before sunrise

When the Sun and Moon are aligned on the same side of the Earth the Moon is "new", and the side of the Moon visible from Earth is not illuminated by the Sun. As the Moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress from new moon, crescent moon, first-quarter moon, gibbous moon and full moon phases, before returning through the gibbous moon, third-quarter moon, crescent moon and new moon phases. The terms old moon and new moon are interchangeable, although new moon is more common. Half moon is often used to mean the first- and third-quarter moons.

When a sphere is illuminated on one hemisphere and viewed from a different angle, the portion of the illuminated area that is visible will have a two-dimensional shape defined by the intersection of an ellipse and circle (where the major axis of the ellipse coincides with a diameter of the circle). If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous (bulging outwards), whereas if the half-ellipse is concave with respect to the half-circle, then the shape will be a crescent.

In the northern hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing (moving towards a full moon). If the right side of the Moon is dark then the light part is shrinking, and the Moon is referred to as waning (moving towards a new moon). Assuming that the viewer is in the northern hemisphere, the right portion of the Moon is the part that is always growing.

Calendar

May-June 2005 calendar of lunar phases

The average calendrical month, which is 1/12 of a year, is about 30.4 days, while the Moon's phase (synodic) cycle repeats every 29.53 days. Therefore the timing of the Moon's phases shifts by an average of about one day for each successive month. If you photographed the Moon's phase every day for a month, starting in the evening after sunset, and repeating approximately 25 minutes later each successive day, ending in the morning before sunrise, you could create a composite image like the example calendar from May 8, 2005 to June 6, 2005. Note that there is no picture on May 20 since a picture would be taken before midnight on May 19, and after midnight on May 21. Similarly, if you look at a calendar listing moon rise or set times, some days will appear to be skipped. When the Moon rises just before midnight one night it will rise just after midnight the next (so too with setting). The 'skipped day' is just a calendar artifact and not the Moon behaving oddly.

See also

References

  1. ^ Timing of moon phases

External links

Educational aids


Simple English


Throughout the month,the moon goes through lunar phases. A lunar phase is a "transformation" that the moon goes through. Each lunar phase can be identified by the appearance of the moon and the time of the month.

Phases

There are 8 total phases that the moon undergoes(does). At first, the moon is invisible, because it is a new moon. This starts out the cycle. The moon, however, soons shows itself after a couple of days. The next phase is a Waxing Cresent. After that, the moon comes out of the darkness and becomes a half moon. The next step is a gibbous, which is about 3/4, or 75%, of the moon showing. The last waxing phase is the full, complete moon. When that is all done, the moon shrinks, and every phase listed is repeated again, except the moon is getting smaller as opposed to larger. Finally, it restarts the cycle again.

Explanation

These changes happen because the moon revolves, or moves, slowly around the moon. When the moon is shown as a cresent,half moon,etc., it is because the 2 different hemispheres of Earth are looking at different, illuminated parts of the moon. So, astronomers (people who look at the sky)in the Northern Hemisphere may see a Waxing Gibbous, but people in the Southern Hemisphere will look up and probably see a Waxing Crescent.

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