MOON
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Courtesy of NASA
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FORMATION The most widely accepted explanation for the formation of the Moon (“The Giant Impact Hypothesis”) suggests it resulted from a collision between the Earth and a Mars-sized object, called Theia. Occurring just as the Earth was cooling about 4.5 billion years ago, this collision ejected a large mass which gradually moved into a more distant orbit around the Earth. Such giant impacts are thought to have been common as the Solar System formed.
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STRUCTURE The Moon is a very large natural satellite. It is slightly smaller than the planet Mercury but larger than any of the known dwarf planets. Our Moon is thought to be made of different layers, with an iron core surrounded by a partially molten lava mantle, and a rigid outer crust. The rocks forming the lunar crust share specific features with rocks that are found on Earth and no place else in the Solar System, as far as we know. These findings are consistent with the giant impact hypothesis. The Moon is “tidally locked” to the Earth, meaning that it spins on its axis exactly once for every orbit it makes around the Earth. Thus, the same side always faces the Earth.
The Moon has no significant atmosphere or magnetic field. It is one of the densest moons in the Solar System, second only to Io, which orbits the planet Jupiter. It has about 1/6th the gravity of Earth. Liquid water cannot exist on the Moon. However, there is strong evidence for water ice in areas that are in permanent shadow at the north and south pole. These regions are among the coldest in the entire Solar System. If these ice deposits are accessible in large enough quantities, they may support permanent habitation on the Moon at some time in the future.
Image Credit- Perry Sabey, Lethbridge Astronomy Society member
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SURFACE FEATURES The Moon’s surface shows bright, mountainous areas (“Highlands”) and large, darker plains (“Maria”). Maria were once thought to be seas but are now known to be ancient basaltic lava plains. Nearly all of the Maria appear on near side of the Moon. They are the result of volcanic activity or lava flowing into depressions left from large impacts. Though quite common in its early history, the Moon has no known active volcanoes today.
The Moon’s surface has about 300,000 impact craters larger than 1km. in diameter on the near side alone. The far side is even more heavily cratered. Most of these craters are ancient but they are very well-preserved, owing to the lack of atmosphere, weather or plate tectonics. Blanketing both the Maria and the Highlands is a thick layer of dust (“regolith”) resulting from meteor impacts that pounded the surface into ever-smaller particles. |
The gravitational attraction between the Moon and the Earth causes the oceans to bulge. (Image Credit NASA)
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LUNAR PHENOMENA AND OBSERVATION The gravitational attraction between the Earth and the Moon is the main cause of ocean tides. The oceans “bulge” outward toward the Moon, causing predictable daily tides as the Earth rotates through the bulge. The detailed picture becomes more complicated because of additional factors such as the Sun’s gravity, the land forms on Earth, the depth of the sea floor and centrifugal forces arising from the Earth’s rotation and orbital pathway.
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The Moon shines by reflected sunlight- it does not produce any light by itself. During the Moon’s monthly orbit around the Earth the angles between the Sun, the Earth and the Moon change. As a result, the part of the Moon that is sunlit (as seen from Earth) also changes, producing the lunar phases.
We see a thin crescent when the Moon is positioned very near the Sun from our perspective. Thin crescent Moons are therefore seen near the horizon shortly after sunset or just before sunrise. They are never seen directly overhead. During the thin crescent phase, one can often see the fainter “non-illuminated” portion. This is made visible because of sunlight reflecting from the Earth onto the lunar surface, called “Earthshine”. We see a full Moon two weeks later, when it is furthest from the Sun and directly “behind” the Earth. |
Earthshine at the thin crescent phase (Image Credit NASA)
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For most people, the best time to observe the Moon is before or after the full Moon phase. When the Moon is “partially lit”, the sunlight casts shadows on the lunar surface, yielding dramatic views of the mountains, valleys and craters especially along the terminator where light and shadow meet.
ECLIPSES The apparent size of the Sun and the Moon as seen from Earth is nearly the same. When the Moon lies on a straight line between the Sun and the Earth, the Moon can completely cover the disk of the Sun and cast a deep shadow onto the Earth’s surface. People within the Moon’s shadow experience a solar eclipse, which can be total, partial or annular (where the Sun appears as a “ring of fire”). Total solar eclipses are of great scientific interest, as they allow close observation of the solar corona (the super-hot region above the Sun's surface).
Total solar eclipse showing the Sun's corona (Image Credit NASA)
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 "Ring of Fire" annular solar eclipse (Image credit NASA)
Solar eclipses occur 2-5 times per year. Many observers travel long distances to witness this thrilling event. Extreme caution is needed when observing a solar eclipse because there is a great risk of permanent eye damage. Safety measures for observing solar eclipses are easily found on the internet and it is extremely important to follow them. |
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Lunar eclipses occur when the Earth lies directly between the Moon and the Sun. Unlike a solar eclipse, no safety measures are needed to view a lunar eclipse. They can be safely observed with telescopes, binoculars or the unaided eye. As the Moon enters the Earth’s shadow, it turns a deep coppery red, because the sunlight first passes through the Earth’s atmosphere. In effect, the red colour is the combined result of all the Earth’s sunsets and sunrises. The Moon has had enormous cultural significance throughout human history. Indeed, it would be difficult to identify a culture that does not attach special significance to the Moon. From calendars to cultural myths to spirituality to art and technological achievement, the Moon is intimately connected to human history. There can be no doubt it will continue to inspire our future. |
Landing site of Luna 24, the last of the Luna series. (Image Credit: NASA/Goddard/Arizona State University)
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MISSIONS The first successful missions to the Moon were the USSR’s Luna 1, 2 and 3, all in 1959. The first soft landing on the Moon was Luna 9, in 1966. During the 1960's the USA’s Lunar Orbiter Program produced detailed maps of the lunar surface in preparation for the successful crewed lunar landing by Apollo 11 on July 21, 1969. Subsequent Apollo missions left several instrument stations on the Moon and transported samples of lunar material back to Earth. Since the 1990's, many other countries including Japan, China, India and the European Space agency have launched lunar probes.
It is likely that in the near future, humans will again walk the surface of the Moon. The NASA-hosted Artemis program is scheduled to launch a crewed mission to loop around the Moon in 2024, followed by lunar landings on later missions. Several private companies are also developing mission plans. |
Installing seismic monitoring equipment on the lunar surface (Image Credit NASA)