An Etymological Dictionary of Astronomy and Astrophysics
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One of the two points of intersection of the orbit of the Moon with the plane of → ecliptic. Indeed, the lunar orbit is tilted by about 5 degrees relative to the ecliptic. The revolution period of a lunar node in ecliptic is 18.61 years. Due to perturbation by the Sun, the lunar nodes slowly regress westward by 19.3° per year. See also → ascending node; → descending node.

→ lunar; → node.

Gereh, → node; gowzahri, related to gowzahr, → draconic month.

Same as → lunar node.

→ lunar; → orbit; → node.

The apparent shift in the → Moon's position relative to the background stars when observed from different places on Earth. The first parallax determination was for the Moon, by Hipparchus (150 B.C.). He determined that one-fifth of the Sun's angular diameter corresponded to the lunar parallax between Hellespont and Alexandria.

→ lunar; → parallax.

One of the various changes in the apparent shape of the Moon, because as the Moon orbits the Earth different amounts of its illuminated part are facing us. The phases of the Moon include: the → new moon, → waxing crescent, → first quarter, → waxing gibbous, → full moon, → waning gibbous, → last quarter, → waning crescent, and → new moon again.

→ lunar; → phase.

A probe for exploring and reporting on conditions on or about the Moon.

→ lunar; → probe.

The process whereby the → Moon gradually moves out into a slightly larger orbit. The → gravitational attraction of the Moon on the → Earth creates two ocean → tidal bulges on the opposite sides of our planet. The Earth rotates faster than the Moon revolves about the Earth. Therefore, the tidal bulge facing the Moon advances the Moon with respect to the line joining the centers of the Earth and the Moon. The Moon's gravity pulls on the bulge and slows down the → Earth's rotation. As a result, the Earth loses → angular momentum and the days on Earth are gradually increasing by 2.3 milliseconds per century. Since the angular momentum in the → Earth-Moon system is conserved, the Earth must impart the loss in its own angular momentum to the Moon's orbit. Hence, the Moon is being forced into a slightly larger orbit which means it is receding from the Earth. However, eventually this process will come to an end. This is because the Earth's own rotation rate will match the Moon's orbital rate, and it will therefore no longer impart any angular momentum to it. In this case, the planet and the Moon are said to be tidally locked (→ tidal locking). This is a stable situation because it minimises the energy loss due to friction of the system. Long ago, the Moon's own rotation became equal to its orbital period about the Earth and so we only see one side of the Moon. This is known as → synchronous rotation and it is quite common in the solar system. The Moon's average distance from Earth in increasing by 3.8 cm per year. Such a precise value is possible due to the Apollo laser reflectors which the astronauts left behind during the lunar landing missions (Apollo 11, 14, and 15). Eventually, the Moon's distance will increase so much that it will be to far away to produce total eclipses of the Sun.

→ lunar; → recession.

The loose, fragmentary material on the Moon's surface. The lunar regolith has resulted from → meteorite collisions all along the Moon's history. It is the → debris thrown out of the → impact craters. The composition of the lunar regolith varies from place to place depending on the rock types impacted. Generally, the older the surface, the thicker the regolith. Regolith on young → maria may be only 2 meters thick; whereas, it is perhaps 20 meters thick in the older → highlands.

→ lunar; → regolith.

The Moon's motion around its axis, which takes place in 27.321 661 days (→ sidereal month). Since the Moon and the Earth are → tidally locked our satellite has a → synchronous rotation. This means that it rotates once on its axis in the same length of time it takes to revolve around Earth. That is why the Moon always shows the same face to us. However, over time we can see up to 59 percent of the lunar surface because the Moon does not orbit at a constant speed (→ libration in longitude) and its axis is not perpendicular to its orbit (→ libration in latitude). The Moon also creates tides in Earth oceans. As the Earth rotates, the rising and falling sea waters bring about friction within the liquid itself and between the water and solid Earth. This removes energy from Earth's rotation and causes it to spin more slowly. As a result, days are getting longer, at about 2 milliseconds per century. On the other hand, since the → angular momentum of the → Earth-Moon system must be conserved, the Moon gradually moves away from the Earth. This, in turn, requires its orbital period to increase and, because the Moon is tidally locked to Earth, to spin more slowly.

→ lunar; → rotation.

Same as → sidereal month.

→ lunar; → sidereal; → orbital; → period.

→ lunar highland.

→ lunar; terra "earth," → terrestrial.

A year based solely on the Moon's motion, containing 12 synodic months, each of 29.5306 days, that is a year of 354.3672 days. Used by Hebrews, Babylonians, Greeks, and Arabs.

→ lunar; → year.

The rocks that make up the bright portions of the lunar surface.

From → lunar + ite a suffix used to form the names of minerals, such as hematite and malachite.

A → lunar eclipse when the Earth's → umbra passes over only part of the Moon, causing only moderate darkening of the full Moon. See also → penumbral lunar eclipse.

→ partial; → lunar; → eclipse.

A lunar eclipse that occurs when the Moon passes through the Earth's → penumbra, but misses the darker umbral shadow. Because the Moon is only partially dimmed, a penumbral eclipse is not impressive. Total penumbral eclipses are rare, and when these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.

→ penumbral; → eclipse.

A dense disk of liquid and vapor supposed to have formed around the Earth below the Roche limit after the proto-Earth was impacted by a Mars-sized object. The Moon was accreted from the Earth's mantle material froming the disk. See also → Theia (S. Charnoz and C. Michaut, 2015, arxiv.1507.05658).

→ proto-; → lunar; → disk.

A → lunar eclipse when the entire → Moon passes through the Earth's → umbra. The maximum duration of a total lunar eclipse is 1h 47m. It happens when the Moon crosses the umbra at its → apogee, where it moves the most slowly, and the Earth is at its → aphelion. The longest total lunar eclipse of the 21st century, lasting 1h 42m 59s, occurred on the night of 27 to 28 July 2018 (Europe, Africa, Asia, Australia, and New Zealand). See also → tetrad.

→ total; → lunar; → eclipse.

A short-lived change in the brightness of patches on the face of the Moon. The TLPs last from a few seconds to a few hours and can grow from less than a few to a hundred kilometers in size. They have been reported by many observers since the invention of the telescope. However, the physical mechanism responsible for creating a TLP is not well understood. Several theories have been proposed, among which lunar outgassing, that is, gas being released from the surface of the Moon.

→ transient; → lunar; → phenomenon.

The space beyond the orbit of the Moon. Compare to → cislunar.

→ trans- + → lunar.

Ânsumâhi, ânsumângi, from ânsu "the other side," from ân "that" + su "side" + mâhi, mângi, → lunar; tarâ-, → trans-.