Lunar phase

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the Moon

2nd quarter, 79%

Lunar phase 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 geometry of the Earth, Moon, and Sun. One half of the lunar surface is always illuminated by the Sun, and is hence 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.

[edit] 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 the 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 our eyes seeing 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 change, appearing as a full moon when the Sun and Moon are on opposite sides of the Earth, and becoming invisible as a new moon (also named dark moon) 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 (or between successive occurrences of the same phase) is about 29.53 days (29 days, 12 hours, 44 minutes) on average. 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 is variable because the orbit of the Moon is elliptic and subject to various periodic perturbations, which change the velocity of the Moon.

It might be naively 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 at (or very nearly at) 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 rather rare and newsworthy events.

[edit] Names of lunar phases

Phases of the Moon.

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

• Dark Moon - Not visible
• New Moon - Not visible, or traditionally, the first visible crescent of the Moon
• Waxing Crescent Moon - Right 1-49% visible
• First Quarter Moon - Right 50% visible
• Waxing gibbous Moon - Right 51-99% visible
• Full Moon - Fully visible
• Waning gibbous Moon - Left 51-99% visible
• Third Quarter Moon - Left 50% visible
• Waning Crescent Moon - Left 1-49% visible
• New Moon - Not visible

Gibbous (red) and crescent (blue) shapes.

References to left and right are for observers in the northern hemisphere of Earth only; in the southern hemisphere, these should be reversed. For example, a "waxing crescent moon" would have the left portion illuminated in the southern hemisphere.

When the Sun and Moon are aligned on the side of the Earth, the Moon is "new", and 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.

Animation of the Moon as it cycles through its phases. The apparent wobbling of the Moon is known as libration.

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. 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 one is in the northern hemisphere, the right portion of the Moon is the part that is always growing. The acronym mnemonic "DOC" represents this: "D" is the waxing moon; "O" the full moon; and "C" the waning moon. (One phrase that can be used to remember this is "Dog comes; Cat goes".) In the Southern hemisphere, this order is reversed, and the mnemonic is "COD".

[edit] Observing lunar phases

Casual observers will not typically notice a waxing crescent moon until about 60 hours after the passage of a new moon, but some individuals have crafted a hobby out of attempting to view the Moon after a much shorter interval than this. Informal "records" and their confirmability vary; some have claimed to have seen the Moon as little as 12 hours after the moment of conjunction (that is, dark moon). Three factors increase the likelihood of spying a very "young" Moon: First, the angle the Moon makes with the ecliptic must be favourable for the applicable side of the Earth — the optimum scenario for this would be a new moon that falls in mid-March in the northern hemisphere or mid-September in the southern hemisphere; second, the Moon should be at or near perigee, which causes it to move faster across the sky (and hence draw away from the Sun more quickly); third, the New Moon must be at or near its maximum separation from the node in a favourable direction based on the hemisphere of the observer; and fourth, the observer must be at a location with excellent atmospheric conditions: preferably at high elevation. These same principles can be applied to sight a very "old" Moon just before conjunction (with the best time of year being very early autumn for that side of the Earth), but this is far less commonly pursued.

A first-quarter moon follows a daily path in the sky corresponding to that of the Sun after three months. Hence it comes at the highest altitude — or "runs high" — at or near the vernal equinox. Similarly, a full moon comes highest at the winter solstice, a last quarter Moon at the autumnal equinox, and a (almost) new moon at the summer solstice (the opposite of "runs high" is "rides low" — a First-Quarter Moon at or near the autumnal equinox, a Full Moon at the summer solstice, etc.). This also means that a first-quarter moon will not necessarily set at midnight, nor must a last-quarter moon rise at midnight; both would do so at the equator, but north or south of this the time of rising or setting will vary by a progressively wider margin as the latitude increases; indeed, a first-quarter moon in the late winter or early spring would remain constantly above the horizon in the polar regions (as would a last-quarter moon in the late summer or early autumn).

The dates and times of the lunar phases are given in the table of lunar phases.

[edit] Calendar

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 29.5 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 below 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. For a similar reason, if you look at a calendar listing moon rise or set times, there will be days where the moon never rises or sets.

[edit] See also

• Full moon
• Month
• New moon
• Orbit of the Moon
• Planetshine
• Table of lunar phases

[edit] External links

• Telescopic moon photos through the phases by Michael Myers
• The Moon Below the Equator
• Moon calendar for different places in the world by Evolutio Rodurago
• Current Moon Phase
• Virtual Reality Moon Phase
• US Naval Service on Moon Phase
• The Length of the Lunar Cycle (numerical integration analysis)
• Full Moon Names