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The lunar phase or phase of the moon is the shape of the illuminated (sunlit) portion of the Moon as seen by an observer on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing positions of the Moon and Sun relative to the Earth. The Moon and the Earth are tidally locked, therefore the same lunar surface always faces Earth. This face is variously sunlit depending on the position of the Moon in its orbit. Therefore, the portion of this hemisphere that is visible to an observer on Earth can vary from about 100% (full moon) to 0% (new moon). The lunar terminator is the boundary between the illuminated and darkened hemispheres. Each of the 4 lunar phases is roughly 7 days (~7.4 days) each but varies slightly due to lunar apogee and perigee. Aside from some craters near the lunar poles such as Shoemaker, all parts of the Moon see around 14.77 days of sunlight, followed by 14.77 days of "night" (the "dark side" of the Moon is a reference to radio communication darkness, not visible light darkness). == Phases of the moon == In Western culture, the four principal lunar phases are first quarter, full moon, third quarter (also known as last quarter), and new moon. These are the instants when the Moon's apparent geocentric celestial longitude minus the Sun's apparent geocentric celestial longitude is 0°, 90°, 180° and 270°, respectively. Each of these phases is roughly 7.38 days long, but the durations vary slightly because the Moon's orbit is slightly elliptical, and thus its speed in orbit is not constant. The eight principal and intermediate phases are given the following names, in sequential order: When the Sun and Moon are aligned on the same side of the Earth, the moon is "new", and the side of the Moon facing 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 through new moon, crescent moon, first-quarter moon, gibbous moon, and full moon. The moon is then said to ''wane'' as it passes through the gibbous moon, third-quarter moon, crescent moon and back to new moon. 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, while the term 'quarter' refers to the extent of the moon's cycle around the Earth, not its shape. 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, Origin: 1350–1400; Middle English < Latin gibbōsus humped, equivalent to gibb ( a ) hump + -ōsus -ous〔(【引用サイトリンク】work=Dictionary.com )〕 ), whereas if the half-ellipse is concave with respect to the half-circle, then the shape will be a crescent. When a crescent Moon occurs, the phenomenon of Earthshine may be apparent, where the night side of the Moon faintly reflects light from the Earth. 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 toward 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 (past full and moving toward 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 (i.e., if the right side is dark, the Moon is growing darker; if the right side is lit, the Moon is growing lighter). In the southern hemisphere the Moon is observed from a perspective inverted to that of the northern hemisphere, so the opposite sides appear to grow (wax) and shrink (wane). The above descriptions of the lunar phases apply for observers at temperate or high latitudes on the Earth. Observers in tropical latitudes see the Moon with its terminator apparently horizontal during the morning and evening. The crescent Moon can open upward or downward, with the "horns" of the crescent pointing up or down, respectively. When the Sun appears above the Moon in the sky, the crescent opens downward; when the Moon is above the Sun, the crescent opens upward. The crescent Moon is most clearly and brightly visible when the Sun is below the horizon, which implies that the Moon must be above the Sun, and the crescent must open upward. This is therefore the orientation in which the crescent Moon is most often seen from the Earth's tropics. The waxing and waning crescents look very similar. The waxing crescent appears in the western sky in the evening, and the waning crescent in the east, in the morning. When the Moon, as seen from Earth, is a narrow crescent, the Earth as seen from the Moon is almost fully lit by the Sun. Often, the part of the Moon that is not directly lit by the Sun is sufficiently brightly lit by light reflected from the Earth to be easily visible from Earth. This phenomenon is called "earthshine", and is sometimes picturesquely described as "the old moon in the new moon's arms". Non-Western cultures may use a different number of Moon phases, for example traditional Hawaiian culture has a total of 30 different Moon phases.〔(【引用サイトリンク】work=Imiloa, Hilo Attractions )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lunar phase」の詳細全文を読む スポンサード リンク
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