MOON

The Moon is Earth’s only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia),[15] it is the largest natural satellite in the Solar System relative to the size of its planet,[f] the fifth largest satellite in the Solar System overall, and is larger than any known dwarf planet. Orbiting Earth at an average distance of 384,400 km (238,900 mi),[16] or about 30 times Earth’s diameter, its gravitational influence slightly lengthens Earth’s day and is the main driver of Earth’s tides. The Moon is classified as a planetary-mass object and a differentiated rocky body, and lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth’s (0.1654 g); Jupiter’s moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

Designations

Designation – Earth I
Alternative names – LunaSelene (poetic)

Cynthia(poetic)

Adjectives – Lunar

Selenian (poetic)

Cynthian (poetic)

Moonly (poetic)

Orbital characteristics

Perigee 362600 km
(356400–370400 km)

Apogee 405400 km
(404000–406700 km)

Semi-major axis 384399 km (1.28 ls, 0.00257 AU)

Eccentricity 0.059

Orbital period 27.321661 d
(27 d 7 h 43 min 11.5 s[1])

Synodic period 29.530589 d
(29 d 12 h 44 min 2.9 s)

Average orbital speed 1.022 km/s

Longitude of Regressing by one revolution in

ascending node 18.61 years

Satellite of Earth

Physical characteristics

Mean radius 1737.4 km
(0.2727 of Earth’s)

Equatorial radius 1738.1 km
(0.2725 of Earth’s)

Polar radius 1736.0 km
(0.2731 of Earth’s)

Flattening 0.0012

Circumference 10921 km (equatorial)

Surface area 3.793×107 km2
(0.074 of Earth’s)

Volume 2.1958×1010 km3
(0.020 of Earth’s)

Mass 7.342×1022 kg
(0.012300 of Earth’s)

Surface gravity 1.62 m/s2

Atmosphere

Surface pressure 10−7 Pa (1 picobar) (day)
10−10 Pa (1 femtobar) (night)

Composition by volume He,Ar,Ne,Na,K,Hi,Rn

The Moon’s orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration

The most widely accepted origin explanation posits that the Moon formed about 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria (“seas”), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon’s apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

Both the Moon’s prominence in the earthly sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union’s Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States’ Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon’s origins, internal structure, and subsequent history.

FACTS

1. The Moon is Earth’s only permanent natural satellite

It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits.

2. The Moon is the second-densest satellite

Among those whose densities are known anyway. The first densest is Jupiter’s satellite Io.

3. The Moon always shows Earth the same face

The Moon is in synchronous rotation with Earth. Its near side is marked by large dark plains (volcanic ‘maria’) that fill the spaces between the bright ancient crustal highlands and the prominent impact craters.

4. The Moon’s surface is actually dark

Although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.

5. The Sun and the Moon are not the same size

From Earth, both the Sun and the Moon look about same size. This is because, the Moon is 400 times smaller than the Sun, but also 400 times closer to Earth.

6. The Moon is drifting away from the Earth

The Moon is moving approximately 3.8 cm away from our planet every year.

7. The Moon was made when a rock smashed into Earth

The most widely-accepted explanation is that the Moon was created when a rock the size of Mars slammed into Earth, shortly after the solar system began forming about 4.5 billion years ago.

8. The Moon makes the Earth move as well as the tides

Everyone knows that the Moon is partly responsible for causing the tides of our oceans and seas on Earth, with the Sun also having an effect. However, as the Moon orbits the Earth it also causes a tide of rock to rise and fall in the same way as it does with the water. The effect is not as dramatic as with the oceans but nevertheless, it is a measurable effect, with the solid surface of the Earth moving by several centimetres with each tide.

9. The Moon has quakes too

They’re not called earthquakes but moonquakes. They are caused by the gravitational influence of the Earth. Unlike quakes on Earth that last only a few minutes at most, moonquakes can last up to half an hour. They are much weaker than earthquakes though.

10. There is water on the Moon!

This is in the form of ice trapped within dust and minerals on and under the surface. It has been detected on areas of the lunar surface that are in permanent shadow and are therefore very cold, enabling the ice to survive. The water on the Moon was likely delivered to the surface by comets.