- Natural satellite of the Earth. Mass 7.35 x 1025 g = 1/81 or
0.0123 Earth’s. Mean radius 1740 km = ~ 1/4 the Earth’s; this relatively small
size ratio makes the Earth-Moon
system unique in the → solar system.
Mean density 3.34 g cm-3.
Mean distance from Earth 384,400 km.
→ Escape velocity 2.38 km s-1.
→ Surface gravity 162.2 cm s-2 = 0.165 Earth’s.
→ Sidereal period 27d 7h 43m 11s. → Eccentricity
0.0549. → Inclination of → orbital plane
to → ecliptic 5° 8’ 43’’.
→ Obliquity 6° 41’.
→ Synodic period 29d 12h 44m 2s.9.
→ Orbital velocity 1.02 km s-1.
The Moon’s average visual → Albedo is 0.12, a factor of
three smaller than that of Earth.
The Moon’s → center of mass is displaced about 2 km in the
direction of Earth.
The average temperature on the surface of the Moon during the day is
107 °C. During the night, the average temperature drops to -153 °C.
Studies of lunar rock have shown that melting and separation must have begun
at least 4.5 x 109 years ago, so the → crust
of the Moon was beginning to form
a very short time after the → solar system itself.
Thickness of crust ~ 60 km; of mantle ~ 1000 km. Temperature of core ~ 1500 K.
It would have taken only 107 years to slow the Moon’s rotation into
its present lock with its → orbital period.
Because of this → synchronous rotation,
the Moon revolves once on its axis
each time it orbits the Earth, thus always presenting the same face,
the nearside, toward Earth.
The Moon may have formed during a collision between the early
Earth and a Mars-sized rocky planet about 4.6 billion years ago;
→ Theia.
- A large body orbiting a planet.
Etymology (EN): O.E. mona, from P.Gmc. *mænon- (cf. O.S., O.H.G. mano,
O.Fris. mona, O.N. mani, Du. maan, Ger. Mond,
Goth. mena “moon”), cognate with Pers. mâh, as below,
from PIE *me(n)ses- “moon, month.”
Etymology (PE): Mâh and mâng in Pers. are variants of the same term, the dominant form being
mâh, while mâng (Av. from, see below) is used in classical literature
as well as in some dialects: Tabari, Kurd. mâng, Laki, Tâti, Taelši
mong, Šahmirzâdi,
Sangesari mung; Mid.Pers. mâh “moon, month;” O.Pers. māha-
“moon, month;” Av. māh- “month, moon,” also māwngh-; cf.
Skt. mās- “moon, month;” Gk.
mene “moon,” men “month;” L. mensis “month;”
O.C.S. meseci, Lith. menesis “moon, month;” O.Ir. mi,
Welsh mis, Bret. miz “month;”
O.E. mona; E. moon, month; Ger. Mond, Monat;
Du. maan; PIE base *me(n)ses- “moon, month.”
Note: In Persian the same term, mâh, is used for two different,
but related, concepts: moon and month.
This was also the case for other IE languages, as shows the above
etymology. However, other IE languages have evolved toward more accuracy by
using different forms of the same initial term, as in E. moon / month or
Ger. Mond / Monat. The Latin family uses two unrelated
words, as in Fr. lune “moon” / mois “month” and Sp. luna / mes.
An additional difficulty in present Pers. is that the adj. mâhi not only
means “lunar” and “monthly” it also denotes “fish.” For the sake of clarity
and precision, this dictionary uses mângi for “lunar.”
