The Nautical Almanac --- The Moon

The familiar face ('light side") of the moon. The lunar crust is thin, and the dark lava plains (maria or "seas") indicate seismic activity in the moon's distant past.

 

The unfamiliar far side ("dark side") of the moon. The crust on the dark side is thicker, and the dark side has almost no lava seas. Scientists consider this side to be largely a remnant of long-vanished Theia.

As any user of the Nautical Almanac can tell you, for celestial navigation sightings Earth's Moon is the second most-visualized celestial object, after the Sun. The moon is particularly convenient for astronavigators as it is bright (so bright that a full moon can obscure all but the brightest stars) and visible in haze and light cloud cover, can be seen during the day at times, and can be used in crescent phases to obtain compass headings (in the Northern Hemisphere, an imaginary line drawn from the horns of the moon to the horizon will indicate true South; the same is true in reverse in the Southern Hemisphere). Due to differences in motion, the navigator has to use separate moon tables to do the proper calculations, but this is a minor inconvenience.

To the ancients the moon was a deity, and almost always a female, sometimes the consort of the Sun, sometimes the Sun's sister or rival. It did not take long for even earliest Man to recognize the pareidolic "Man in the Moon" (unusual for being male). and though different cultures saw "him" differently --- in the Pacific basin, "she" was the Woman Who Weaves The Clouds --- that beneficent face has smiled down on humanity since before there was a humanity (the poet Sylvia Plath saw it different: "She is quiet with the O-gape of complete despair"). 

The Moon has always been a sign of the Divine Feminine. Moon goddesses and high priestesses were usually represented with a crescent moon bound on their brows, or as three-faced women (representing the phases) or with hunting bows. In the earliest human societies, the Moon Cult was dominant and women were the leadership.

Ancient Mankind also recognized meaning in the phases ("Faces") of the moon --- the full moon marked time (in lunar calendar systems the full moon marked the new "month," the word itself coming from "moon"), the waxing moon was a time of industriousness, the waning moon a time to complete the work, and the "dark of the moon" was a time not to embark on new endeavors (though as the "new" moon, this idea got turned completely around). 

The Moon moves through an orbital cycle every 29.5 days, giving us a Waxing Crescent, Waxing Half, Waxing Gibbous, Full, Waning Gibbous, Waning Half, Waning Crescent, and Dark (New) Moon every five days or so. A month with two Full Moons is said to have a "Blue Moon."

It did not take long for primitive humans to make the connection between the moon and the tides, nor the seeming connection between a woman's reproductive cycle and the moon's own orbital cycle ("menses" is another word derived from "moon"). 

Thus, Mother, water, darkness, silver, bow hunting (symbolic of the crescent moon), and night were all things that became associated with the moon. Father, fire, light, gold, spear hunting, and day were, in contradistinction, solar attributes. Given that the earliest human societies were matriarchal, the "moon cult" is doubtlessly older than the "sun cult."

This can be seen in the multiplicity of goddesses associated with the moon. In Greece and Rome alone, they include Artemis, Diana, Cynthia, Selene, Luna, Hecate, and Phoebe. The Celts associated the moon with Epona the horse goddess, and Danu the goddess of waters. 

The interacting orbital mechanics of the Earth, Sun, and Moon can cause one body to cast shadows on the other (occulting). When the Moon occults the Sun we call it a solar eclipse. When the Earth occults the Sun we call it a lunar eclipse

Oddly, the moon does not have a "formal" name as does the Sun (Sol), but the names of the moon goddesses are used as adjectives --- one can speak properly of "selenic" rocks. 

"Apocynthion" and "pericynthion" are used to describe the furthest and nearest points in a lunar, er, dianic, um, phoebic orbit. 

For sheer regular usage, "Luna" is probably most recognizable --- and it also gave rise to the words "lunatic" and "lunacy" and "loon" for the mentally ill, for the moon was said to have negatively influenced them. 

The moon was born out of the same nebula that birthed the Sun and its planets. Not long after the Sun first ignited 4.5 billion years ago, the new star's heat began to cause its disk of dust and gas to rotate faster, and as the heat reached further and further outward, eddies and whirlpools in the cloud began resolving themselves into planets. One of these was a planet that scientists now call Gaia; Gaia wasn't quite as large as the Earth, but it occupied the same orbit. 

The destruction of Gaia and Theia gave rise to the Earth and the Moon.

The early solar system was a wild place --- there were probably more than 20 planets, and their orbits went every which way, rising high above the ecliptic plane and falling low below, and orbiting the poles of the Sun and its equator, and sharing orbits with other planets, and having intersecting orbits. Some of these early planets were snatched up by the gas giants Jupiter and Saturn and became moons. Others were so far out from the Sun that they stayed far out --- perhaps a few even got lost. 

Gaia had an orbital partner, Theia. Who knows for how long the two planets --- one not quite Earth-sized and the other Mars-sized --- revolved in their intersecting orbits around the Sun, but it was long enough for them to cool and form rocky surfaces. 

One strange day though, Gaia and Theia met at the precise spot where their orbits crossed, and a cataclysm unlike any other occurred. The collision of the two planets rocked the solar system. Theia hit Gaia with such force that Theia's surface layers were torn off and her core was driven down to the heart of the larger planet, where the two cores melded into one. Billions of tons of Gaia-material was ejected into space to mix with the ruins of Theia, for the smaller planet was effectively destroyed. 

The smashed Gaia was knocked off its axis --- 23.5 degrees off its 90 degree poles --- and the planet was sent spinning, so that a day on Gaia was only five hours long for hundreds of thousands of years. 

A solar eclipse. We are lucky to live at a time in joint Selenic-Solarian history when the Moon is just distant enough from the Earth to appear the same size of the Sun. Due to orbital mechanics, the Sun can block the Moon's reflected light (a lunar eclipse) and the Moon can block the Sun's emitted light (a solar eclipse). Solar eclipses are rare and dramatic, and we get a very good view of the energy the Sun gives off.

During a lunar eclipse light refracted through the Earth's atmosphere makes the Moon turn a dark copper color, not usually deep black. The effect is called a "Blood Moon."

Gaia settled down surprisingly fast in cosmic terms due in large part to the ruins of Theia. 

The mixed stuff of Gaia and Theia was trapped in orbit around the newly-remade planet, drawn by the strong gravity of the newly-enlarged core. Eventually, the large mass of material attained stability, and, as much of the solar system's raw material did, formed a sphere. The sphere of this new world was surprisingly large, about 25% of the size of the planet it was orbiting, and its orbit was close in. As it orbited, its own gravity acted as a brake on the planet's rapid spinning, lengthening the planet's day. As the speed lessened, the sphere began moving further away from the planet to maintain its own equilibrium. It also became "tidally locked," so that its revolution time and its rotation time became identical. It would never again show any but one face to the new and improved planet, which we now call Earth. And we call the sphere the Moon. It is the only celestial body we have visited in person.

Until 1959, no one had ever seen the dark side --- the far side --- of the moon. The first view was a muzzy photograph, and it would be another nine years before Man would orbit the moon, and another before we walked on it. When we studied the moon's rocks we discovered that much of Earth was on the moon and in the moon. The two very different sides of the Moon gave a clue to its creation from two very different worlds. We live in a binary planet system made up of three planets. 

Four billion years ago the Moon was less than 20,000 miles from Earth. It has receded to more than ten times that distance. But if it were still that close it would appear something like this. It would cross the sky about once every fifteen minutes.

If the Moon were still 20,000 miles from Earth it's likely life would be very different, if it existed at all. The Earth's day would be just 20% as long as it is now, the tides would rise and fall hundreds of feet every hour, the Earth would be rocked by gravitationally induced earthquakes, the friction induced by the force exerted by the Moon would probably heat the Earth to unlivable temperatures, and the Moon would probably be shattered by the force exerted by the Earth.

When the moon was formed it was only about 18,000 miles from the Earth. Today it is about 239,000 miles away, and is backing away (still) at a rate of about 1.5 inches per year. It is difficult to imagine the moon being twelve times larger in our skies, stretching from horizon to horizon, but it was so. How bright the moon appeared to be at that time is hard to estimate, but the Sun is 398,110 times as bright as the moon, which of course only reflects the Sun's light. 

The moon not only slowed the Earth down eventually to a 24 hour day, but it also acted to restrain the new planet's wild gyrations, so that a precession cycle, the wobble of Earth's axis, takes 26,000 years, not mere hours or days. This allowed regular seasons to establish themselves. 

Today, the Earth rotates in 24 hours; the Sun rotates in 27 days; and the Moon both rotates and revolves in 29 days. 

A moon colony, possibly in ten years.

The moon also protected the Earth by absorbing tens of thousands of hits from meteorites left over from the formation of the solar system and from the breakup of Theia. Though the Earth was battered badly, the real scars of those periods of massive bombardment can still be seen on the moon's cratered surface. 

The moon has a very weak magnetic field that affects the orbits of spacecraft circling it. It has a tiny molten core, and in the far past had vulcanism, but present-day moonquakes are caused by the gravitational pull of the Earth. It also has an evanescent atmosphere that weighs about 11 tons total. 

A footprint on the Moon

The radius of the moon is 1,079 miles, its diameter is 2,159 miles, and its circumference is 6,783 miles. The moon is just 1/81th the mass of the Earth, but it is 60% the density of the Earth. The moon's gravity is weak, just 16% that of the Earth's. A 200 pound astronaut weighs only 32 pounds on the moon. 

The moon is very cold. The average temperature is 64 degrees below zero Fahrenheit. Highs of 242 degrees F. occur in sunlight. Lows of -280 F. occur in darkness. Since the moon is tilted a little more than one degree on its axis, it has seasons of a sort. The coldest temperatures in the solar system occur at the bottom of certain craters where the sun never shines. Winter Solstice temperatures in these places of eternal darkness can thud down to 413 degrees below zero on the Fahrenheit scale. As brutal as that may be, such craters are excellent sites for remote telescopes of various types used in the study of cosmology.

Shackleton is a "crater of eternal darkness" where the sun never shines.