Back during our COVID period (in 2021, I believe) when in-person group meetings were canceled and we became familiar with ZOOM, I delivered a presentation on Earth’s moon (our MOON). From that presentation, I prepared this story.
Tom Kimber
Moonglow. Moonlight. Moonshine. Moon shadow. The satellite hanging in the sky above us infuses our mood in numerous ways. Throughout human history, our Moon held sway over all cultures. From a personal perspective, the Apollo missions of the 1960s and early 1970s kindled my awareness of how applied science gives insight and knowledge of the Earth-Moon relationship.
On July 20, 1969, the world saw a lunar landing for the first time, with men from the Apollo 11 mission standing on the Moon. Neil Armstrong and Buzz Aldridge, broadcasting from the Sea of Tranquility on the Moon’s face, represented a fantastic accomplishment. Moon rocks retrieved from this mission, five more successful NASA Moon landings, and three robotic SOVIET lunar spacecraft landings have aided our knowledge of the Moon’s and Earth’s geological similarities.
Lunar rock composition mirrors Earth’s rock composition in some common minerals. Feldspar, found in the lunar crust, is foundational in the Earth’s crust. The lunar mantle contains pyroxene and olivine minerals. So too does the Earth’s crust.
Moon Origin
I am getting ahead of myself. It is proper to start at the beginning of the Earth-Moon romance, some 4,500 million years ago, shortly after the Earth formed as a proto-planet. The giant impact hypothesis, or the Big Thwack, remains the Moon’s formation’s favoured theory. It suggests that a Mars-sized planetesimal, called Theia, was jockeying with the Earth for the same narrow band of solar system real estate. No two planets can share the same orbit, a reliable rule of astrophysics. Eventually, they will collide, and the more massive world always wins.[1 ] According to the hypothesis, Theia vaporized along with a significant chunk of the Earth’s crust and mantle, blasting outwards to mix and blend into the Moon with the conditions we have come to know.
Why is this origin theory of Earth’s Moon so much different from, say, the Moons of planets Mars, Jupiter, Saturn, Uranus and Neptune? First, the Moon’s size relative to Earth’s size makes it somewhat of an anomaly. Earth’s Moon is more than a quarter of Earth’s diameter and one-eightieth of Earth’s mass. Mar’s two irregular-sized Moons, Phobos and Deimos, appear to be asteroids captured in orbit by Mar’s gravity. The other planets have satellites tiny in scale compared to the host planet’s mass. Such Moons formed from unclaimed remnants of dust and gas again captured in orbit by their host planet’s massive gravity[2].
Other theories of the Moon’s formation have long been considered possible explanations. The Co-Accretion theory proposes that the Earth’s Moon formed as the Earth formed from the same material within the nebula that created the solar system. But this theory does not explain a substantial difference discovered between Earth rocks and examined Moon rocks returned from the Apollo missions. Moon rocks contain no trace of carbon, nitrogen, sulphur and hydrogen found in the Earth rocks. So, the Moon originated from Earth-based materials and something else.
Two other Moon formation theories fell into question following detailed Moon rock examinations. The Capture Theory suggests that the already-formed Moon was captured by Earth’s gravity early in the Earth’s history. But for this theory to work, the two bodies would need to have the same average mineral composition. They do not.
The Fission theory contends that the Moon formed from a fission blob of Earth spun off its high rotational velocity. This theory fails on two counts – the lack of the Earth-based mineral elements in the Moon rocks and the theory’s inability to account in any reasonable way for the Earth-Moon angular momentum.
The Big Thwack Moon formation theory helps explain several issues that the competing theories do not. A collision between two planets might easily see the smaller world’s iron core content absorbed by the more massive object. An analysis of the Moon shows that it does not have an iron core. The Earth’s axial tilt of about 23 degrees could result from such a glancing collision. One side of the Moon and the Earth always face a tidal lock because the Earth’s angular momentum and Theia were coupled into one spinning system by this collision.[3]
All planetary objects have a limit, called the Roche limit, in which gravitational forces are too great for a satellite to form. Remnants of the rocky collisional debris would have coalesced into the Moon outside the Roche limit of 17,702 km.
As the Earth exerts a gravitational force holding the Moon in orbit, the Moon exerts a gravitational pull on the Earth, causing the oceans to form a tidal bulge. [5]
The Moon has continued moving away from the Earth ever since. Apollo missions left mirrors on the Moon’s surface, allowing laser beams from Earth to accurately provide distance measurements within a fraction of an inch. Since the 1970s, the Moon has retreated about one and a. half inches per year on average. (See this video )
You might wonder how the gap between Earth and from Moon grow to 402,336 km from its initial 22,500 km. Time and the conservation of angular momentum provide the answer.
It must still possess whatever angular momentum the Earth-Moon system had at its formation today. The tidal bulge that swept around the Earth 4.5 billion years ago every 5 hours (the time of a single rotation of the Earth around its axis) strongly influenced the Moon, continually pulling it, making it go faster and faster. The laws of planetary motion state that the quicker satellite orbits, the farther away it must be from its central planet.[6]
But, as the Moon drifts farther away, it gains angular momentum. The Earth then loses angular momentum, its rotation slowing down to conserve the Earth-Moon system’s total angular momentum. The Earth’s rotation has slowed from once every 5 hours to once every 24 hours. Conversely, the Moon has moved farther away while picking up angular momentum.
Moon Geology
The Moon and the Earth are in a tidal lock. The same side of the Moon faces the Earth throughout the Moon’s rotation and orbital period, which is the same. One always sees the same face or near side of the rotating Moon from the Earth’s surface. The far side is never visible from Earth.[7]
Once the Moon formed (approximately 4.4 billion years ago) into a Magna-covered sphere, it began a long cooling process resulting in a crust on the Moon’s surface with the Magna underneath. Impact cratering and volcanism are responsible for the Moon’s characteristic shapes we see today. During the Late Heavy Bombardment period in the solar system 4 billion years ago, asteroids and meteorites struck the Moon and Earth. Without an atmosphere, the Moon’s surface has no weathering and so no erosion. With little geologic activity (volcanoes), cratering from this period remains intact today. For a long time, the scientific community considered the moon to be without existing seismic activity. This view is changing. The Lunar Reconnaissance Orbiter, launched by NASA in 2009, has taken images of the lunar surface showing it covered in small, shallow, cliff-like features: faults. The lines of these faults cut across craters considered young, less than 50 million years old. The images show that boulders on the surface have moved in recent decades, perhaps due to moonquakes. Scientists believe that these moonquakes are caused by cooling in the Moon’s interior and Earth’s gravity.[8]
The Moon’s composition is a mystery, not yet completely resolved. Its surface is more known because of the lunar explorations of the 60s and the 70s, with many rocks from various locations returned to Earth for scientific analysis. Regolith soil comprised of fine powder of breccia sedimentary and igneous rocks makes up the surface composition. Scientists estimate that the lunar regolith extends down 4 to 5 metres in most places and much deeper in the Highland areas.[9] Like the Earth, the Moon has layers. The core is composed mainly of metallic iron at the center, with smaller amounts of sulphur and nickel. This core represents 20% of the Moon’s mass. Next to the body is the mantle. Scientists believe that the cover of the Moon is primarily composed of minerals: olivine, orthopyroxene and clinopyroxene. It’s also believed to be more iron-rich than the Earth’s mantle. Then, there is the crust. The crust of the Moon is composed mainly of oxygen, silicon, magnesium, iron, calcium, and aluminum. Trace elements like titanium, uranium, thorium, potassium and hydrogen10. The Moon’s surface is divided into primarily three terrain types: maria (or seas), terrae (or highlands) and craters.
The maria constitute about 16 percent of and the terrae 84 percent of the lunar surface. Maria occupies about 30 percent of the lunar nearside, the hemisphere visible from the Earth.11 The maria are primarily circular and lie within ringed basins. These basins have mountainous rims. These rims are visible along with craters and their inner mountainous peaks through a telescope on a clear night a few days after the First Quarter phase or a night just before the Last Quarter moon phase viewed along with the Terminator, the line between the illuminated Moon surface and that surface in shadow.
Moon Phases
It takes approximately 29.5 days for the Moon to rotate once around the Earth and complete a full Moon Day. Any given location on the Moon would experience a daylight duration of little more than two Earth weeks and nighttime of the same proportion.12 During one rotation, the Moon goes through four major phases (New, First Quarter, Full, Last Quarter) and four minor phases (Waxing Crescent, Waxing Gibbous, Waning Gibbous, Waning Crescent). The time between each little step is approximately seven days. No matter where the Moon is in its orbit around the earth, half of its surface is Sunlit, with the other half in darkness. Having stated this point, I need to point out an exception at both north and south Moon poles. The Moon’s axis tilt is very slight (1.6 degrees) compared with the angle of the Earth’s axis (23.5 degrees). This puts the Moon’s axis and the poles perpendicular to the path of solar light as it reaches the Moon. The bottom of some craters near the poles has not experienced direct Sunlight in more than 2 billion years.[13 ]
Because the Moon lacks an atmosphere as we have on Earth, lunar temperatures fluctuate during the Moon’s daylight-to-nighttime transition. While illuminated by Sunlight, the Moon’s surface can reach a temperature of 127 degrees Celsius. Without Sunlight, the temperature may plummet down to -173 degrees Celsius.[14]
Where the Moon is in its orbit, relative to the Sun, determines the phase of the Moon, visible on Earth. In a new Moon phase, the Moon rises and sets with the Sun; therefore is not present at night. In this phase, the Moon is at its closest point to the Sun in its orbit and has 0% illumination on Earth. The Moon’s orbital rotation around the Earth is eastward (away from the Sun). For Northern Hemisphere observers, the Moon rises slightly later (50 minutes approximately) than Sunrise and somewhat further in the east (12 degrees) than the previous day. A thin crescent Moon seen in the West after Sunset indicates a setting Moon in the waxing crescent phase after a New Moon phase. After approximately seven days, by the First Quarter phase, the Moon has completed ¼ of its orbit. It appears half-illuminated and is 90 degrees left of the Sun. The Moon is 180 degrees left of the Sun by the complete Moon phase, rising at sunset and remaining fully illuminated through the evening and setting at sunrise. The cycle continues, with the Moon waning from full illumination to half illumination one week later, reaching the Last Quarter phase. The Moon rises in the middle of the night and sets in the middle of the day. Over the next seven days after the Last Quarter, the Moon appears in the morning sky as an increasingly waning crescent shape, with less surface illumination each night, eventually passing into a New Moon phase again.
Moon Libration
The Moon’s orbit around the Earth is elliptical and not circular. As well, this orbit is tilted 5 degrees to the ecliptic. Due to these conditions, the Moon’s center rises, falls, and drifts left and right through the lunar month. This growing, falling and drifting are termed Libration. Because of this motion, features of the Moon, as seen month to month, vary slightly by the amount of sunlight that is reflected.
This becomes very clear when you compare a view of the Moon on the same lunar day, for example, the First Quarter day in January, then again in February, and then in March. I suggest you use a feature downloadable from the Internet that provides you with confirmation of this point. NASA has made this available accessible through its Scientific Visualization Studio for the general public. The site is called DIAL-A-MOON. Using it, you can select any date in 2021 to view how the Moon appears. When the image appears, download it to view on your device as a tif file.
Moon Viewing
When is the best time to explore the Moon with a telescope? The answer to this question depends on what you are looking for. Are you searching for rays and lava flows from craters such as Tycho or Copernicus? Then look for the Full Moon phase to provide this viewing condition. During this phase, the Moon is opposite the Sun in the sky and does the opposite; it rises at sunset and sets at Sunrise. The Sunlight accentuates lunar geology. Look for ray systems around craters Tycho, Copernicus, and Proclus and comet-like rays emanating from Mare Fecunditatis. The terracing around Copernicus is visible through a telescope.
A good time to examine the Moon in a telescope for craters is when the Moon begins to wax (in illumination) one or two days after the First Quarter phase. You can see many holes along the terminator line, some in shadow like Plato north of Mare Imbrium and then bathed in light to the right, crater Archimedes. Following the terminator line south, you can spot crater Arzachel with prominence visible in the crater center. Then to the left (aka west) of this crater, you can make out the Lunar Straight Wall (aka Rupes Recta) with a shadow outlining this N-S aligned fault scarp feature, running S-E across Mare Nimbium, north of crater Tycho. Come back one day later and view this same feature. You will appreciate how much more visible Mare Nimbium is one day later, the Straight Wall is now just a dark slash in the landscape. Crater Copernicus, now along the terminator line, appears insight, deep and dark, with its western rim illuminated. In the far south, crater Clavius, one of the larger craters on the Moon, appears visible and bright, while to the right and north, crater Longomononus is visible but in shadow.
Three days past First Quarter will provide an excellent time to view the Golden Handle, northwest of Mare Imbrium. The Handle is the Jura Mountains around Sinus Iridium (the Bay of Rainbows). The Jura Mountains comprise the Golden Handle, along with two capes, Hereclides at the southern edge and Laplace at the northern border.
Once again, on the night of the Last Quarter, look at the Lunar Straight Wall (aka Rupes Recta )
with a shadow outlining this N-S aligned fault scarp feature, running S-E across Mare Nimbium, north of crater Tycho. Come back one day later and view this same feature. You will appreciate how much more visible Mare Nimbium is one day later, the Straight Wall is now just a dark slash in the landscape. Crater Copernicus, now along the terminator line, appears insight, deep and dark, with its western rim illuminated. In the far south, crater Clavius, one of the larger craters on the Moon, appears visible and bright, while to the right and north, crater Longomononus is visible but in shadow.
Three days past First Quarter will provide an excellent time to view the Golden Handle, northwest of Mare Imbrium. The Handle is the Jura Mountains around Sinus Iridium (the Bay of Rainbows). The Jura Mountains comprise the Golden Handle, along with two capes, Hereclides at the southern edge and Laplace at the northern border.
Once again, on the night of the Last Quarter, look at the Lunar Straight Wall (aka Rupes Recta). You cannot wait until past this date in the cycle to see this feature again.
This video is a great introduction to the Moon and its history
Footnotes
1 Hazen, Robert. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet
2 Hazen, Robert. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet
3 Hazen, Robert. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet
4 Hazen, Robert. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet
5 Hazen, Robert. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet
6 Hazen, Robert. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet
7 Lavender, Gemma. Howes, Nick. Secrets from the Far Side of the Moon. Space, Dec 2018
8 Jee, Charlotte. The Moon is A Lot More Seismically Active than We Thought. MIT Technology Review, 2019
9 Williams, Matt. What is Lunar Regolith? Universe Today, May 2015
10 Caine, Fraser. What Is The Moon Made Of? Universe Today, Nov 2008
11 Wilhelms, Don. Geological History of the Moon
12 Mancini, Mark. What are Days and Nights Like on the Moon? HowStuffWorks, Sept 2018
13 Mancini, Mark. What are Days and Nights Like on the Moon? HowStuffWorks, Sept 2018
14 Mancini, Mark. What are Days and Nights Like on the Moon? HowStuffWorks, Sept 2018
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