Moon

Moon

The Moon is Earth's only natural satellite . Reflecting light from the Sun , the Moon is often the brightest object in the night sky.

The Moon orbits Earth at an average distance of approximately 240,000 miles (385,000 km). With revolution and rotation periods of approximately 27.32 Earth days, the Moon is in synchronous orbit about the earth. This synchronous orbit maintains a "near side" and "far side" of the Moon. The "near side" faces Earth, while the far side is not visible from Earth. Although Russian space probes—and later many American probes—took the first pictures of the far side of the Moon years earlier, it was not until the flight of Apollo 8 that United States astronauts became the first humans to directly view the far side of the Moon.

Orbital dynamics between the Sun, Moon, and Earth cause different patterns of illumination on the surface of the Moon as seen from Earth. As the Moon revolves about the earth, it appears to go through a series of illumination phases. The Sun constantly illuminates one-half of the lunar surface. The changing orientation in the three body system (Sun, Earth, and Moon), changes to what extent that solar illumination covers areas on the surface of the Moon that are visible from Earth.

Because the earth is revolving about the Sun, the displacement of the earth along it's orbital path establishes the time it takes to complete a cycle of lunar phases—a synodic month—and return the Sun, Earth, and Moon to the same starting alignment. This synodic month is approximately 29.5 days, and is longer than the 27.32-day sideral month.

A waxing moon is one where the area illuminated increases each night. A waning moon describes a decreasing area of illumination.

The Moon's phases are a cyclic repetition of illumination patterns described as: new moon, waxing crescent moon, waxing half moon, waxing gibbous moon, full moon, waning gibbous moon, waning half moon, waning crescent moon, followed by a return to the new moon phase.

A new moon occurs when the Moon's orbital path places it between the earth and the Sun. Only the side of the Moon not visible to Earth is illuminated and the Moon is lost in the bright sunlight. Occasionally when the Moon is also in the proper plane of alignment, it may provide a full or partial solar eclipse over portions of Earth's surface.

Relative to the Sun and starfield, the Moon appears to move eastward. Following the new moon, the next night, a small sliver or crescent becomes illuminated. The waxing crescent moon is low on the western horizon and is visible just after sunset (i.e., the Moon "sets" shortly after sunset). As the orbital dynamics shift, the crescent grows larger—and the Moon sets later—each night following sunset. Approximately one week following the new moon, the Moon is one quarter of the way through it's orbital revolution of Earth, and one half of the lunar surface is illuminated as a waxing half moon. Depending upon latitude , the waxing half moon appears nearly directly overhead (at the zenith of the celestial meridian) at sunset. The waxing half moon will set about midnight local time. During the next week, the area of the Moon reflecting sunlight to Earth covers more than half of the visible lunar surface, and is described as a waxing gibbous moon.

Approximately two weeks after the new moon, the visible surface of the Moon becomes fully illuminated because the Moon is on the opposite side of Earth relative to the Sun. If the earth and Moon are in the proper plane, Earth may actually block the Sun's light over a portion of the lunar surface and cause a partial to full lunar eclipse. The full moon rises at sunset and sets at dawn.

Following the full moon, the Moon begin to progressively darken through waning gibbous phases until about a week following the full moon it forms a waning half moon. The waning half moon rises about midnight and sets about noon the next day. Continued darkening over the last week of the lunar cycle provides a waning crescent moon that finally returns full cycle to the new moon state, where the Moon and Sun, on the same side of Earth's orbit about the Sun, appear to rise and set together.

The phases of the Moon proved one of the most fundamental astronomical calendars for ancient peoples and the ancient Greek astronomers asserted that the Moon reflected the Sun's light. Phases of the Moon remain critical in determining the date and timing of many religious observances (e.g., Passover, Easter, Ramadan, Visakha Puja, etc.)

Because the earth is larger than the Moon and relatively close to the Moon, it casts a large shadow that causes lunar eclipses. Solar eclipses (where the Moon blocks the Sun) are less frequent and are only possible because, although the Sun is much larger than the Moon, the Moon is much closer to Earth. The present set of orbital dynamics and distances allow solar eclipses because the Sun and Moon have the same angular size (approximately 0.5°) when viewed from Earth. The average human thumb, held out at arm's length obscures approximately 0.5° degrees and will thus, block both the Sun and Moon. (Warning: Direct viewing of the Sun may cause blindness or optic injury and should not be attempted. Solar observation requires special protective goggles that filter and reduce the intensity of sunlight. )

The Moon appears to shift its position eastward on the celestial sphere by approximately 13° per night (i.e., appears to move 13° to the east from its prior position if observed at the same time on successive nights).

The Moon is nearly spherical with polar and equatorial radii varying by about a mile. The equatorial radius of the Moon is approximately 1,080 miles (1,738 km). The diurnal temperatures (the day/night temperatures) on the Moon range from approximately −280°F to +260°F (−173°C to +126°C). Contrary to popular belief, the Moon does have a thin atmosphere that consists of helium, argon, methane, minute amounts of oxygen , and other trace elements. The density of the lunar atmosphere is only approximately 2 × 10⁵ particles/cm³ and results in a lunar atmospheric pressure of only 8.86 × 10⁻¹⁴ inHg (3 × 10⁻¹² mb) in contrast to Earth's average surface atmospheric pressure of 29.92 inHg (1,014 mb).

The thin and dry lunar atmosphere provides no substantial weathering agents (e.g., wind, water , etc.) and so erosional processes are greatly slowed—essentially reduced to heating, cooling, and slow geochemical changes. The thin atmosphere also offers no protection from meteor impacts and the combination of lack of protection and lack of Earth-like erosion produces a heavily cratered lunar landscape that preserves billions of years of accumulated impact craters.

Although the Moon is a quarter of Earth's size, it has only approximately 1.2% of Earth's mass. The gravitational attraction at the surface of the Moon is about one-sixth that of the gravitational attraction at Earth's surface. Accordingly, neglecting air friction (something easily accomplished on the Moon but not on Earth) an object in freefall near Earth's surface accelerates at 9.8 m/s², but near the lunar surface, the acceleration due to gravity is approximately 1.62 m/s².

See also Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Diurnal cycles; Earth (planet); History of manned space exploration; Gravity and the gravitational field; Solar system