The Galileo spacecraft took these images of the Moon on 7 December 1992 on its way to explore the Jupiter system in 1995-97. The distinct bright ray crater at the bottom of the image is the Tycho impact basin. The dark areas are lava rock filled impact basins: Oceanus Procellarum (on the left), Mare Imbrium (center left), Mare Serenitatis and Mare Tranquillitatis (center), and Mare Crisium (near the right edge).
Source: Courtesy NASA/JPL-Caltech.
Average Distance from the Earth: 384 400 km (0.0026 AU)
Size (Equatorial Diameter): 3475.6 km (0.272 x that of Earth)
Mass: 7.35 x 1022 kg (0.012 x that of Earth)
Rotation Period: 27.3 days
Temperature: 117°C (sunlit side), -163°C (far side)
Gravity: 1.7 m/s2 (0.17 x that of Earth)
We see the Moon in our northern sky and though it may appear large, especially when seen close to the horizon, our view of the Moon is actually small enough to be covered by your thumb.
The mass of the Earth and the Moon are similar when compared to the other planets in the solar system, so the Earth and Moon could be said to be companion planets. The Moon is thought to have formed when a rogue Mars-sized planet collided with the Earth 4.6 billion years ago.
The Moon is a rocky object and its surface consists of rock and dust, all of a uniform grey colour. The dark areas that dominate the near side are called maria, or 'seas', and were formed by lava flows about three billion years ago. The lunar surface is also covered with craters from impacts of meteorites, mostly very small (around 1mm) but occasionally much larger. The Clementine probe found evidence of water ice at the base of a permanently shadowed crater near the Moon's south pole, known as the South Pole-Aitken Basin.
Largest sea: Mare Imbrium (The Sea of Showers),
diameter 1300 km.
Largest crater: Bailly, diameter 295 km.
Deepest crater: Newton, depth 8.85 km.
The Moon has no atmosphere, so there is no erosion or weathering of surface features. The footprints left by the Apollo astronauts will still be clearly visible thousands of years from now.
Phases of the Moon
As the Moon orbits the Earth it appears to change shape as different parts of its illuminated face are visible from Earth. The four phases of the Moon are as follows:
A New Moon cannot be seen from the Earth. It is too close to the Sun in the sky and only the far side is facing us.
At First Quarter we see half of the Moon. It rises around midday and sets around midnight.
A Full Moon rises at Sunset and sets at Sunrise. We see the entire sunlit side.
At Last (or Third) Quarter we see half of the Moon. It rises about midnight and sets around midday.
The Lunar Month
A month is the time taken for the Moon to make one apparent orbit of the Earth, travelling at an average speed of 3680 km/h. It can be measured in the following three ways:
- Synodic Month: 29.5 days
This is the time between successive New Moons and is the basis for our calendar month.
- Sidereal Month: 27.3 days
This is the time taken for the Moon to return to the same spot against the background stars. It so happens that this is the same as the Moon's rotation period, which means that the same side of the Moon always faces the Earth.
- Anomalistic Month: 27.55 days
This is the time between successive perigees or closest approaches to the Earth.
Dates in Lunar Exploration
March 1840. First photograph of the Moon.
2 January, 1959. Luna 1 is the first probe to fly by the Moon.
13 September 1959. Luna 2 is the first probe to crash land on the Moon.
31 January 1966. Luna 9, is launched and is the first to soft-land on the lunar surface.
25 December 1968. Apollo 8 makes the first manned flight around the Moon.
20 July 1969. Apollo 11 makes the first manned landing.
21 July 1969. Neil Armstrong is the first person to walk on the Moon.
December 1972. Eugene Cernan, Apollo 17, is the last person on the surface (so far).
21 February 1994. The Clementine spacecraft goes into orbit around the Moon.
The Moon's Orbit
The Moon's orbit is tilted by 5° to the ecliptic, ie it gets to 5° North and 5° South of the ecliptic. Hence it is as much as 28.5° north or south of the Celestial Equator and as little as 18.5° North or South. This means the Moon can be higher or lower than the Sun. The points where it crosses the Ecliptic are called the Nodes. These points precess (move backwards) with time.
Against the background stars the Moon moves about its own width (0.5°) every hour or a little over 13° per day. The Sun travels 1° per day, so, with respect to the Sun, the Moon travels 12° per day. Each month it moves all the way around the sky and a little more. The Moon moves around the Earth about 13 times in one year.
The Sun is the main offender in disturbances of the Moon's orbit, its gravitational influence being twice that of the Earth on the Moon. The Moon is affected by 150 direct periodic variations and 500 smaller variations in its passage.
The Moon arrives approximately 50 minutes later per day on the meridian (between 38 and 66 minutes). The variations are due to changes in orbital speed, variations in the Sun's apparent speed along the ecliptic and changes in the inclination of the Moon's orbit.
Rising times of the Moon can vary from 13 minutes to 80 minutes on latitude. Greatest delay in rise is near Full Moon close to the Spring Equinox. Least delay in Moonrise is near the Autumn Equinox.
The Moon's distance from the Earth varies by about 5.49%. The Moon is slightly larger and moves faster when it is at perigee.
Due to the constant motion of the Moon about the Earth, Full Moon is only an instant. However, when we view the Moon it does appear full a few days before and after the exact time of the phase. Moon phases are between 6 or 7 days apart due in part to the Moon's elliptical orbit and that the Earth-Moon system is in orbit around the Sun.
The Moon shines only by reflected light. The albedo of the Moon is 0.07, which means it reflects only 7% of the light falling on it. (The albedo of the Earth is 0.39).
The Moon is at magnitude -12.7. The faintest stars seen by our eyes are magnitude 6, the brightest star is -1.58. The Moon is 33,000 times brighter than Sirius. It is also 14 magnitudes dimmer than the Sun, ie sends out approximately 400,000 times less light to us.
Our eyes adapt to light changes whereas cameras don't. Therefore as the sky gets dark it appears that the Moon is dazzling. In fact, it is only a dull grey colour. If we could surround the sky with full moons then the sky would still be only 1/4 as bright as one Sun in the sky even though the sky would be filled with 105,000 full moons. A full moon is 465,000 times fainter than a sunny day. The only reason why we are able to see features outside by moonlight is that the eye is amazingly adaptable. A sheet of white paper by moonlight is 2,000 times darker than black velvet in sunlight. Yet some people are able to read a book by moon light.
The surface brightness of the Moon is not uniform and hence a quarter phase Moon is not half as bright as a Full Moon. The Moon is a sphere, so more light is reflected back when the Moon's face is directly towards us. The Moon is brighter in the middle and greyer towards the terminators. The Moon's surface is covered with valleys and mountains that don't reflect much light back. A first quarter Moon is therefore about 1/10 the brightness of a Full Moon. Approximately 2.4 days before Full Moon results in a brightness that is half of a Full Moon.