By Aiden Plumbe
When you think of “moon”, you most likely imagine the grey sphere that orbits our planet. However, Galileo Galilei discovered that moons are not unique to Earth in 1610. Many of the planets we know in our Solar System have more moons than one. Others don’t have any. Furthermore, the Moon has been a significant symbol in many cultures and religions throughout history. The history of the “moon” is rich with a complex history.
The Names Behind Moons
You have probably seen our one moon in the sky many times. We know Earth’s nearest large celestial body and natural satellite as “the Moon”. However, other moons have different names with their own significance.
Starting with the planets closest to the Sun, Mercury and Venus both have zero moons. This is because the planets’ proximity to the Sun makes having moons extremely difficult. Due to the Sun’s great size, its gravitational pull would likely pull any moons out of Venus and Mercury’s orbits.
Moving past Earth, two moons orbit Mars – Phobos (the larger one) and Deimos (the smaller one). Unlike the common misconception of spherical moons, Mars’ moons are lopsided and lumpy. This often occurs because the moons’ gravitational pull is not strong enough. Mars’ name derives from the Roman god of war’s name. Astronomer Asaph Hall saw it fitting to name the moons after the god’s mythological sons (their Greek counterpart) – Phobos (fear) and Deimos (dread).
NASA recognises Jupiter and Saturn having the most moons in our Solar System, 95 and 146 respectively. These moons range in size from being larger than Mercury (Saturn’s Titan) to the size of a football stadium. The two planets have so many moons because of their large size and, therefore, large gravitational pull. Some of their most notable moons include Europa, Ganymede, Io, Callisto, Iapetus, Tethys and many others.
Most are named after Greek mythological figures, while smaller ones don’t have names (e.g. S/2004 S 12).
Uranus has 28 documented moons, its major five being Miranda, Ariel, Umbriel, Titania and Oberon. These moons are often called the “literary moons” because William Heschel decided to stray from the theme of mythology and named Uranus’ moons after magic spirits in English literature, predominantly from the works of Shakespeare and Alexander Pope, a famous English poet.
The last planet in the Solar System, Neptune has 16 known moons. The largest one, Triton, was discovered by William Lassell in 1846 only 17 days after Neptune’s discovery. Only two moons have not been named, the others named after minor sea gods and nymphs in Greek mythology. This is because Neptune is the Roman god of the seas. Triton is the only large moon in the Solar System whose orbit is opposite to its planet’s rotation.
The Science Behind Moons
It is important to know the difference between moons and planets. There are three criteria for a celestial body to be considered a moon. Firstly, it must orbit the Sun. Even here, moons are out of the equation as moons orbit around a planet. This is because their gravitational pull is not strong enough to escape a planet’s gravity. Secondly, a planet must have sufficient mass, and therefore gravity, to be spherical. Again, this is not the case for all moons which can be lopsided, seen with Phobos and Deimos. Thirdly, a moon has to “clear the neighbourhood” around its orbit. Moons, however, often have not cleared their orbit as many other celestial bodies such as moons or asteroids orbit the planet.
Now we know why moons aren’t planets, what exactly are they? Moons are known as natural satellites, meaning they are solid objects in orbit around a planet. Moons have layers, just like the Earth. The Earth’s moon has three distinct layers: a dense iron alloy core, a mantle composed of minerals such as olivine and pyroxene and a rocky crust. Additionally, scientists confirmed in 2018 that there is frozen water in the shadowed craters of the Moon’s poles.
The formation of moons is not factually known although there are several likely theories. Most moons were probably formed from discs of dust that circulated planets in the early Solar System. The leading theory behind the formation of Earth’s moon is called the Giant Impact Hypothesis. This theory claims a Mars-sized celestial body called Theia collided violently with Earth. This would have ejected debris into orbit which eventually coalesced into the Moon due to gravity. However, the moons of the gas giants were most likely taken from surrounding areas due to the planets’ large gravitational pull.
Some moons’ orbits are not stable, including our own. Earth’s moon is moving away from the planet 3.8 centimeters a year. However, this is not a problem because both the Earth and the Moon will be engulfed by the Sun in 7.59 billion years, long before the Moon escapes Earth’s gravity.
The possibility of moons supporting life has interested many astronomists. It has not been discovered yet, though. Astrobiologists search for the three essential conditions for life – liquid water, certain chemical compounds and an energy source. Jupiter’s moon, Europa, is the closest celestial body to possibly fit these criteria. Evidence leads to Europa harbouring liquid water 100km deep beneath its icy crust. Jupiter’s radiation and potential interactions between the water and rocky seafloor heated by tidal forces may mean that Europa has chemical energy sources. We’ll have to see as our knowledge of space develops.
Moons that could exist beyond our Solar System are called exomoons. However, even though 5,600 exoplanets have been documented, there are no exomoons yet to be discovered. This most likely has something to do with the methods used to discover these extrasolar celestial bodies. The most common method includes detecting how exoplanets’ gravitational pulls cause their stars to wobble. However, because of exomoons small size, they are quite difficult to detect
The Culture Behind the Moon
Throughout the history of mankind, the celestial body that orbits our planet has been recognised by many ancient cultures and is an important symbol even today.
It has served a practical purpose throughout the course of history, being used by ancient humans to keep track of time. Because of its consistent phases and appearance in the sky, the Moon became an easy monitor of time passing. However, after its practical purpose was fulfilled, humans gave it meaning.
There have been countless interpretations of what the Moon represents in the past. The majority of major mythologies and cultures use the Moon in their creation stories or, the Moon is involved with one of the more important deities in society. With so many examples of the Moon’s role in cultures, we will only be able to mention a select few.
In Japanese mythology, the god of the Moon is called Tsukuyomi. He is the brother of Amaterasu, the Sun goddess, and Susano, the sea god. Tsukuyomi marries Amaterasu and he is invited to a feast by Ukemochi, the goddess of food. There, Ukemochi makes the food by pulling it out of every orifice of her body. Tsukuyomi is disgusted by this and kills her. However, Amaterasu finds out and leaves Tsukuyomi, who now chases her forever – the Sun and Moon cycle.
The Mayans viewed the Moon in both a religious and scientific way. They used it to record the months of the year in the Lunar Series. The Moon’s role in mythology varies in classic and post-classic era. In the classic era, the Moon was seen as a masculine god during the full moon and called a second Sun as the Sun passed through the Underworld. In other phases, it was seen as feminine and had various names. In the post-classic era, the Moon was separated into two goddesses. The younger one represented the waning moon and the older represented the waxing moon.
In Ancient Greece, the main representation of the Moon was the goddess, Selene, sister of the Sun god, Helios. She rides a moon chariot pulled by two horses across the night sky. Her most famous story is with Endymion, who is granted eternal youth by Zeus. Because of this youth, he is put into eternal youth and in a cave. Selene falls in love with him and visits him every night, having up to 50 children with him. There are other goddesses associated with the Moon, including the well-known Artemis.
In modern society and culture, the Moon has several meanings in different circumstances. It is a key aspect of modern religions such as Islam, Christianity and Judaism. It also has meaning in astrology, tarot and zodiac signs. In gothic horror, the Moon is a symbol of madness as well as werewolves. In film noir, the Moon carries a mysterious brooding nature. In romance, the Moon is a calm and intimate figure. In fantasy, it represents the occult and magic. The Moon adopts different representations across different parts of pop culture.
