The Moon is Earth's only natural satellite and revolves around Earth in about one month. It is closely related to the phases of the Moon and the rise and fall of the tides.
Basic Knowledge
The Moon completes one orbit around Earth in about 27.3 days (a sidereal month), but because Earth also moves around the Sun, the period for the Moon to return to the same appearance as seen from Earth (the synodic month) is about 29.5 days.
Phases of the Moon:
① New moon: The Moon is in the same direction as the Sun and cannot be seen.
② Crescent moon (in the western sky in the evening)
③ First-quarter moon (due south in the evening)
④ Full moon (rises in the east in the evening and sets in the west in the morning)
⑤ Third-quarter moon (rises around midnight and is due south in the morning)
The Moon does not produce its own light; it shines by reflecting sunlight.
Tides are caused mainly by the Moon's universal gravitation and the centrifugal force of the Earth–Moon system. The tidal range is large around full and new moons, producing spring tides, and small around the first- and third-quarter moons, producing neap tides.
Synodic month (from one new moon to the next; about 29.5 days)
Sidereal month (the time for the Moon to return to the same fixed star; about 27.3 days)
New moon, first quarter, full moon, and third quarter (the four main lunar phases)
Tides (periodic changes in sea level caused by the gravitational forces of the Moon and Sun)
Spring tides and neap tides (spring tides at full and new moons; neap tides at half moons)
Lunar and solar eclipses (the Moon enters Earth's shadow / the Moon comes in front of the Sun)
Further Study (Background and Significance)
Because the Moon's orbital and rotational periods are both about 27.3 days and are synchronized, the Moon always keeps the same side facing Earth. The far side of the Moon was first photographed in 1959 by the Soviet probe “Luna 3.”
A solar eclipse occurs on a new-moon day when the Sun, Moon, and Earth line up. A lunar eclipse occurs on a full-moon day when the Sun, Earth, and Moon line up in that order. Both are relatively rare phenomena, and total solar eclipses occur only a few times per century for people living in a particular location.
Tides are important for navigation, fishing, land reclamation, and tidal power generation. France's Rance Tidal Power Station (from 1966) was the world's first large-scale tidal power station, with an output of about 240MW. The Moon's existence also affects life on Earth, including the stabilization of Earth's axis.
- Synodic month = 29.5 days (phase cycle)
- Sidereal month = 27.3 days (orbital period relative to the fixed stars)
- The Moon shines by reflecting sunlight
- New moon → crescent → first quarter → full moon → third quarter → new moon
- A full moon rises in the east in the evening
- Spring tides = full and new moons; neap tides = half moons
- The Moon always keeps the same side facing Earth (rotation and revolution are synchronized)
Points of Caution (Easily Confused)
① Do not confuse the synodic month (29.5 days) with the sidereal month (27.3 days). ② The first quarter is due south in the evening, while the third quarter is due south at dawn. ③ A solar eclipse occurs at new moon, while a lunar eclipse occurs at full moon. ④ The Moon does not produce its own light; it reflects sunlight.
Practice
- What is the period of the Moon's phases called, and approximately how many days is it?
- In what order are the Sun, Earth, and Moon arranged at full moon?
- During which lunar phases do spring tides occur?