The Moon

Ancient cultures of the past were very superstitious about the Moon, often worshipping it as a god. The ancient Greeks and Romans thought of the Sun and the Moon as male and female gods respectively. Even in modern times, the Moon is sometimes viewed in a superstitious or religious context, including the practice of astrology. From an aesthetic point of view, the night sky of Earth wouldn’t be the same without it. It is easily the brightest object in the night sky and also the largest. Our modern culture is permeated with various references to the Moon, in a religious, superstitious, and yes, even romantic context. Humans have looked up at the Moon in the night sky, wondering about it and admiring its beauty since the beginning of civilization, and quite possibly even before.

The Moon is the only natural satellite of the Earth, orbiting at a distance of 225309 miles at the closest approach to 251904 miles at the furthest distance – the average distance is 238606 miles. It is the fifth largest satellite in the Solar System and is larger in relation to the planet that it orbits than any other. At this average distance, it takes about 1.2814 seconds for electromagnetic waves, light or radio waves, for example, to travel the distance to the moon. According to the most widely accepted theory at the present time, it was thought to have formed about 4.5 billion years ago after a collision of the Earth with an ancient planet about the size of Mars.

The Moon orbits the Earth approximately every 27.32 days at an average orbital speed of 2286.149 miles per hour in an almost circular orbit – eccentricity is only .0549. During its orbit of Earth, the Moon always has the same side, the near side, locked towards the Earth; this is called being in synchronous rotation. Thus one Moon day is slightly more than 27 Earth days, half that time, or about 2 weeks in sunlight, and the other 2 weeks in darkness.

As mentioned before, the Moon is the brightest and largest object in Earth’s night sky, with an apparent magnitude range of -2.5 to -12.9 – the mean full moon is -12.74 – and an angular diameter of 29.3 to 34.1 arcminutes, where an arcminute is one-sixtieth of a degree and a degree is one-three hundred sixtieth of a complete circle. The Moon has a mean radius of 1079.383 miles with an equatorial radius of 1080.005 miles, meaning that the Moon has a very slight bulge of about .0576 percent due in large part to the centrifugal force of its rotation, also meaning the moon has a slight fluidity. The polar radius is 1078.70 miles giving the moon a very slight flattening effect of .0632 percent. The circumference of the Moon around its equator is about 6785.995 miles(.2725 Earth’s) while it has a surface area of roughly 14644850 square miles(.074 Earth’s) and a volume of approximately 5268004353(5.628 billion) cubic miles(.020 Earth’s).

The Moon has a mass of about 161863400000000000000000(161.8634 million quadrillion) pounds – about .0123 of Earth’s. It rotates at about 10.3503 miles per hour at its equator – note that this is 10.3503 times 24 hours times 27.3 days which comes to 6781.5193 miles. Compare this to the circumference of the Moon at its equator, 6785.995 miles, showing it is very close to the same, the difference being accounted for by the fact that the Moon has a slight wobble. With this mass, the moon has a surface gravity of.1654g which is roughly one-sixth that of Earths, and which gives it an escape velocity at the surface of about 5323.9084 miles per hour. The Moon has an axial tilt of about 6.687 degrees with respect to its orbit of the Earth.

Although the Moon appears very bright in Earth’s sky, its surface is actually quite dark with an albedo of about .136, which is only slightly more than that of an asphalt road. Albedo is the measure of diffuse solar electromagnetic radiation, in this case the visible light part of the spectrum, reflecting back into space – it will always have a value between 0 and 1. The temperature of the Moon will vary from -279.67 degrees Fahrenheit in the darkness(shadow) to 242.33 degrees Fahrenheit in the sunlight. Since the Moon has almost no atmosphere – the surface pressure in daylight is .0000001 pascals and at night .0000000001 pascals – the temperature variance in the daytime of the Moon is over 500 degrees Fahrenheit(shadow areas versus sunlight areas). The Moon’s extremely thin atmosphere is composed by volume in descending order of Helium, Argon, Neon, Hydrogen, and Radon. Of course, with virtually no atmosphere and these temperature extremes, the moon is completely inhospitable to life of any known kind.

The Soviet Unions Luna 2 was the first unmanned spacecraft to reach the surface of the Moon in September 1959, and in fact, the first to reach the surface of any other celestial body besides the Earth. The United States followed that with Ranger 4 which made a hard landing on the Moon in 1962. Since then there have been a number of Soviet/Russian and U.S.A. missions to the Moon.

The Moon formed about 4.5 billion years ago, only around 60 million years after the formation of our Solar System. The most widely accepted current theory, which emerged from a conference in Hawaii on lunar origins in October 1984, is that the Earth was larger 4.5 billion years ago than it is today and also hotter with more fluidity. As the theory goes an ancient planet about the size of Mars, which is called Theia, collided with the larger proto-Earth and a huge chunk of material was blasted into Earth’s orbit. This material gradually attained its spherical shape and cooled while in orbit around the Earth becoming the Moon as we know it today. It should be noted that the far side of the Moon has a crust about 25 miles thicker than the near side which supports this theory.

The internal structure of the Moon consists of the crust, mantle, and core. The inner core of the Moon is believed to be liquid and made mostly of iron and to be about 150 miles thick(300 mile diameter) while the outer core is semi-solid with some fluidity and made mostly of iron and is roughly 190 miles thick(radius). Around the core is the major part of the internal structure of the Moon known as the mantle, which is actually in two parts – the lower mantle and the upper mantle. The lower mantle of the Moon which is above the core is called the athenosphere and is hot enough to be somewhat fluid and be able to flow. The upper mantle, called the lithosphere, is about 620 miles thick and not as hot as the lower mantle(athenosphere) and so is not fluid. It is the mantle that transmits seismic waves that can cause moonquakes. The composition of the mantle is largely olivine, orthopyroxene, and clinopyroxene minerals. The crust of the Moon varies in thickness from 38 miles on the near side to 63 miles on the far side and is believed to be composed of a dusty rock layer which is fused together.

The surface of the Moon is characterized by lunar plains, volcanic features, impact craters, strange enigmatic lunar swirls, and quite possibly even the presence of water.

The Moon has large plains which look dark and are more or less featureless and which can even be seen with the naked eye. They are called maria, which is Latin for ‘seas‘, since they were once believed to be filled with water. Today they are known to be vast expanses filled with lava, probably from volcanic activity in the past.

Almost all of the plains of the Moon are on the near side, about 31 percent coverage, as opposed to the far side of the Moon, where plains cover only about 2 percent of the surface. As mentioned before, volcanic activity on the Moon seems to account for these plains, and since the crust of the Moon is substantially thicker(roughly 25 miles on the average) on the far side than on the near side, volcanic eruptions would be much closer to the surface and more prominent on the near side. Most of these volcanic eruptions are believed to be in the distant past of the Moon, but even recently some moonquakes have been recorded with the resultant tectonic activity.

Besides volcanoes, impact craters are the other major geologic process that has affected the moonscape. These are basically formed from the impact of asteroids, comets, and meteorites with the lunar surface. Just on the near side of the Moon it is estimated that there are over 300000 craters wider than half a mile. Of course, there is a greater number of craters on the near side in large part because of the larger coverage of moon plains, or maria(31 percent versus 2 percent). The near side has also been studied much more intensely since it is easily observed with Earth-based telescopes in great detail. Some of the larger impact craters include Nectaris, Imbrium, and Orientale which are from hundreds to over a thousand miles in diameter and with multiple rings of impact material visible. Another very important point is that since the Moon has no significant atmosphere there are no weathering effects on moon craters and they are extremely well preserved.

Lunar swirls are a strange and very enigmatic feature on the Moon’s surface that actually resembles the cream swirls that you might find in your coffee. They are characterized by high reflectivity(albedo) swirls that appear brighter with low reflectivity areas in between which are darker. The swirls of moon dust can expand out dozens of miles in some cases. They are also often associated with magnetic anomalies such as high-level magnetic fields and high impact events such as those mentioned in the moon crater section are believed to be a causal factor in their formation.

At first, the idea of water on the surface of the Moon seems ridiculous since under normal circumstances water simply can not exist in liquid form on the surface of the Moon. With no significant atmosphere(and virtually no pressure to speak of), the Moon has an almost perfect vacuum, and liquid water does not exist under these conditions. It just can not be, the laws of physics will not permit the existence of liquid water. And yet, it is now believed that water DOES exist on the Moon, not liquid water, but water in the form of ice!

Ever since the 1960s scientists have hypothesized that ice can exist in the deep shadows inside craters on the moon as a result of the impact of comets with the Moon eons ago. In fact today this is even past the point of belief – in 1994 the radar experiment on the Clementine spacecraft seemed to indicate small pockets of water close to the Moon’s surface. Although these were later shown to be inconclusive, in August 2018 analysis of the results of the Moon Mineralogy Mapper did show definitive evidence for the first time of water in the form of ice on the lunar surface. This finding is certainly very important in its implications for possible future colonies on the Moon since transporting water to the lunar surface from Earth would be extremely difficult and untenable, if not impossible in the quantities that would be needed.

Orbiting spacecraft have given us good information about the Moon’s gravitational field through the observation of the Doppler shift from the emission of radio signals. One fascinating thing about lunar gravity is the existence of mascons which are large gravitational anomalies associated with the large basins and which can greatly affect the orbit of spacecraft around the Moon. Although the lava flows in these basins may be partially responsible for these effects, they can not completely explain them and the mascon gravitational anomalies remain a mystery.

The Moon has only a weak magnetic field which is about one-hundredth the strength of the Earths, and most of this magnetic field is believed to emanate from the Moons crust since the Moon has no global dipolar magnetic field, north and south, like that of Earths.

Tides on the Earth are caused by the gravitational force of the Moon on the varying distance between the near side of the Earth to the Moon and the far side of the Earth. This really happens because of the inverse square law of gravitational attraction – the force of gravity decreases inversely proportional to the distance, so in the case of the Moon and Earth the near side of the Earth to the Moon is subject to a significantly greater gravitational force than the far side of the Earth, the diameter of which is 7926 miles at the equator. This will cause a bulge in oceans on the near side of Earth and another bulge in oceans on the far side. Since the Earth, of course, spins on its axis a high tide will occur about every 12 hours and 25 minutes, slightly off from 12 hours because of the Moon’s orbital period around the Earth. It’s interesting to note here that the Sun also affects tides on the Earth; although it is far more massive than the Moon, it’s gravitational effect on the tides is only about 40 percent that of the Moons because of the much greater distance of the Sun from the Earth – a mean distance of 93 million miles versus a mean distance of 238600 miles. Also, there are a number of other factors in play that affect the tides on Earth, such as the friction of the water in the Earth’s oceans with the ocean floors because of the Earth’s rotation, the inertia of the huge amount of water itself, the fact that ocean depths decrease closer to land, etc. As a result of this predicting tides is not precise and is based on both observation and theory.

Something else that should be mentioned here is that the Moon’s(and the Sun’s) gravitational force not only effects water but also the Earth’s crust as well. The effects on water are much more evident because of the great fluidity of the water, but there is also some plasticity and fluidity present in the Earth’s crust and mantle as well. There is some current thought that the gravitational effect of the Moon, and to a lesser extent the Sun, may be a factor in causing earthquakes, but this idea is still somewhat controversial and the extent to which this may be the case has yet to be determined.

There are both solar and lunar eclipses – a solar eclipse occurs when the Earth is in the Moon’s shadow, and a lunar eclipse occurs when the Moon is in the Earth’s shadow.

As mentioned above, an eclipse of the Sun occurs when the Earth passes through the Moon’s shadow. Usually this is a partial eclipse – the Sun is not completely covered by the Moon. On rare occasions a total eclipse of the Sun will occur where the Sun is completely covered by the Moon; this will only last about 7 and 1/2 minutes in a very narrow range of the path of the Moon’s shadow – outside this range but still in the Moons shadow a partial eclipse of the Sun will occur. These solar eclipses only happen during a new moon when the Sun, Moon, and Earth are in a three-dimensional straight line with the Moon between the Sun and the Earth and the Moon is close to the ecliptic plane of the Sun.

A lunar eclipse occurs when the Moon passes through the Earth’s shadow – this only occurs during a full moon and when the Sun, Earth, and Moon are aligned three-dimensionally in a straight line with the Earth between the Sun and the Moon. The plane of the Moon’s orbit around the Earth is inclined about 5.145 degrees from the plane of the Earth’s orbit around the Sun, so for a lunar eclipse to occur at the full moon, or a solar eclipse to occur at the new moon, the Moon has to be close to the intersection of these two planes. A lunar eclipse is also a rare event but is seen over a much wider area because the Earth’s shadow is much larger. When it does occur it is a beautiful reddish color as seen in the photo above.

The phase of the Moon is the visible sunlight portion of the Moon as seen from Earth(see illustration above). The various phases of the Moon will gradually and cyclically change over the course of every 29.53 days as the relationship between the orbit of the Moon around the Earth and the orbit of the Earth around the Sun changes. One side of the Moon, the ‘near’ side, is locked continuously towards the Earth(there is a small amount of ‘wobble’), so the percentage of the near side of the Moon which is illuminated by the Sun varies from 0 percent(new moon) to 100 percent(full moon). The Moon has what is called a lunar terminator which is the boundary that divides the illuminated part of the Moon from the dark part.

There are 4 main phases of the Moon which last about 7.4 days, although this will vary some since the Moon’s orbit around the Earth is elliptical and not perfectly circular. These four main phases are the new moon, first quarter, full moon, and third quarter(also known as last quarter). When the Moon’s ecliptic longitude varies 0 degrees in relation to the Sun’s ecliptic longitude, there is a new moon. Likewise, when this relationship varies by 90 degrees there is a first quarter, 180 degrees full moon, and 270 degrees third quarter(last moon). The ecliptic longitude and latitudes are a coordinate system to show the positions of celestial bodies in the Solar System. There are intermediate phases between the four main moon phases when the moon will have a crescent or gibbous shape as seen from Earth.

The United States(NASA), Soviet Union/Russia, Europe, Japan, China, and India have all explored the Moon with various unmanned spacecraft in the 20th and 21st centuries.

The Ranger program occurred in the 1960s and gave us closeup pictures of the Moon before the spacecraft crashed into the Moon.

The Lunar Orbiter program was a series of five missions in 1966-1967 that orbited the Moon mapping it’s surface.

The Surveyor program was a series of seven robotic missions making soft landings on the Moon lasting from June 1966 until January 1968.

In 1994 NASA in joint cooperation with the Defense Department put Clementine into lunar orbit to make a global topographic map of the Moon and global multi-spectral images of the Moon’s surface.

In 1998 the Lunar Prospector space probe was put into low polar orbit above the Moon to do further mapping and study the composition of the Moon’s surface in the polar region. It’s instruments actually detected the presence of hydrogen, the most likely cause being water in the form of ice in some permanently shadowed craters.

In June 2009 the Lunar Reconnaissance Orbiter and the LCROSS Impacter were launched together and put into lunar orbit to obtain high-resolution imagery and more telemetry of the Moon. LCROSS was only in orbit a short time until it made a planned crash landing into the crater Cabeus obtaining important data. The Lunar Reconnaissance Orbiter is still in operation in lunar orbit as of this writing in 2019.

Two NASA GRAIL space probes began orbiting the Moon in January 2012 on a mission to learn more about the Moon’s internal structure. The LADEE space probe was put into lunar orbit in October 2012 to learn more about the Moon’s exosphere.

The Luna program was a series of missions launched by the Soviet Union, Luna 1 – Luna 24, that started in 1959 and lasted until 1976. Luna 1 was the first man-made object to escape Earth’s gravity, Luna 2 the first to crash on the Moon’s surface, and Luna 3 the first to photograph the far side of the Moon, up until then nothing was visible – these 3 Luna missions all occurred in 1959. Luna 9 was the first human mission to achieve a soft lunar landing in February 1966. Luna 10 was the first spacecraft to achieve lunar orbit later in 1966 becoming the first artificial satellite of the Moon. Three Luna missions – Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976 brought soil samples back to Earth from the Moon – about two-thirds of a pound altogether. Luna 24 was the last Soviet mission to the Moon.

The Soviet Lunokhod program landed two robotic rovers on the Moon in 1970 and 1973 to further explore the Moon’s surface.

In 1990 Japan successfully put it’s Hiten space probe into lunar orbit, becoming the third country to do so.

Japan put the Selene space probe into lunar orbit from October 2007 to June 2009 which contained a high-definition video camera along with 2 radio transmitter satellites – the video camera obtained very high-resolution movies and still photos of the Moon’s surface while the 2 radio satellites obtained important geophysics data.

Europe(European Space Agency) launched the ion-propelled space probe SMART-1 and placed it into lunar orbit from November 2004 until September 2006 when it crashed into the lunar surface. Until then it made some detailed chemical analysis of the Moon’s surface.

The Chinese Lunar Exploration Program successfully placed Chang’e 1 into lunar orbit from November 2007 to March 2009 obtaining a full image map of the Moon. Chang’e 2 reached the Moon in October 2010 and went into lunar orbit where it obtained a very high-resolution map of the Moon over an eight-month period of time. In December 2013 Chang’e 3 landed on the Moon’s surface and deployed a lunar rover called Yutu, making more important discoveries and photos of the Moon’s surface. Chang’e 4 was landed on the far side of the Moon on January 3rd, 2019 sending back some astonishing photos and other data about the far side of the Moon. Another Chinese space probe, Chang’e 5, is planned for later on.

Chandrayaan 1 was placed by India into lunar orbit from November 2008 until August 2009, when contact was lost. This space probe created a high-resolution geological map of the Moon’s surface and along with other chemical and mineral analysis confirmed the presence of water molecules on the lunar surface. The Chandrayaan 2 space probe is planned for the near future.

Exploration of the Moon has really just begun, the future certainly looks very bright. The near future will bring many more lunar missions, some of which are already being planned. In the more distant future, there is a very real possibility of astronomical telescopes on the Moon which will be able to explore the Cosmos as never before, tourists going to the Moon, a Moon base which could be used as a starting point to explore the rest of the Solar System, and perhaps even lunar colonies which may eventually declare their independence from Earth. Use your imagination – the possibilities are almost limitless in the decades and centuries ahead!