Deep Space Network
18 Characteristics of the Moon
John Spacey, January 27, 2021 updated on April 25, 2023
The Moon is an astronomical body that orbits Earth. It is the Earth's only natural satellite and is the largest moon in proportion to its planet in the solar system. The following are the basic characteristics of the Moon.
Lunar DistanceThe time-averaged distance between the center of the Earth and Moon is 385,000.6 kilometers or 239,228.3 miles. This is roughly a 1/4 million miles. The moon is 1.28 light-seconds from Earth. The distance between the Earth and Moon at any given time can be measured to within a millimeter. The Moon is slowly drifting further from Earth at a rate of 1.5 inches a year.
Apparent SizeFrom the perspective of Earth, the Moon has a similar apparent size as the Sun. This can be seen in a total solar eclipse whereby the Moon appears to almost perfectly cover the Sun.
Actual SizeThe diameter of the moon is about 1/4 that of the Earth. This is similar to the width of Australia. The mass of the Moon is approximately 1/81 that of Earth or about 1.23%.
Tidally LockThe same side of the Moon is always facing Earth. This is because it is tidally locked with the Earth meaning that the Moon rotates exactly once each time it orbits the Earth. This is the reason that the surface of the moon looks so familiar because we are always facing the same side. This does tilt a little over the lunar cycle with a total of 59 percent of the moon that is visible from Earth.
Dark SideDue to tidally locking, there are features of the moon including its far side and two poles that can't be viewed from Earth. The features of these regions were unknown to history until the 20th century when they were explored with sensors such as cameras on spacecraft and satellites. Due to the historically unknown nature of the far side of the moon, it is also colorfully known as the dark side of the moon. It was historically believed that the two sides were similar. However, explorations have revealed the two sides have remarkably different typologies and characteristics. For example, the far side of the moon has only 1% volcanic crater known as maria, whereas the near side is 31% maria. Nevertheless, the far side is more rugged with more impact craters.
Lunar DustThe Moon is covered in dust and this dust rises and falls creating what could be described a thin atmosphere of dust that extends about 100 kilometers above the surface. This is mostly space dust from comets and meteors. It is estimated that the moon is hit with about 5 tons of comet particles each day. This can remain in a remarkably stable state on the surface due to the lack of an atmosphere or flowing water. It is believed that lunar dust may be hazardous to human health. Dust is deposited in a asymmetric pattern on the surface of the Moon.
GravityGravity on the moon is 0.1654 g. If you weigh 100 pounds on Earth, you would weigh 16.54 pounds on the Moon. As such, your muscles are massively overpowered in this environment. On the Apollo missions, the astronauts wore space suits and life support systems weighing 180 pounds on Earth but were still able to jump several feet high on the surface of the Moon.
TidesGravity from the Moon is the primary force that causes a cycle of rising and falling sea levels on Earth known as tides. This is caused by the slightly stronger gravitational pull the Moon has for the side of the Earth closest to it. This causes the oceans of the Earth to stretch into a slightly elliptical form that aligns with the current position of the Moon relative to the surface of Earth. Due to the daily rotation of the Earth and the apparent 29.5 day orbit of the Moon, the Moon is in the same spot over the surface of the Earth every 24 hours and 50 minutes. The high tide at each spot occurs approximately every 12 hours and 25 minutes when the moon is directly overhead and directly underfoot.
OrbitIf you were observing the Moon from a fixed point in space, you would see that it orbits the Earth every 27.322 days. From the perspective of Earth, which is itself orbiting the Sun, the Moon appears to orbit every 29.5 days. The Moon also rotates once every 27.322 days. The Moon travels in an elliptical orbit such that the distance between the Earth and Moon varies over this cycle.
Moon PhasesThe phases of the Moon at the portions of the Moon that are sunlit as viewed from Earth. This follows a 29.53 day cycle that corresponds to one orbit of the Moon around the Earth. This varies from a New Moon that is invisible to a Full Moon that is completely visible.
OriginThe Moon formed about 4.51 billion years ago not long after the formation of Earth and appears to be made from mantle material from the Earth. The dominant theory explaining its origin is that a large Mar-sized object struck Earth expelling matter into space that eventually formed the Moon.
CultureThe moon is a prominent celestial object that historically struck people with a sense of wonder. It was common for the moon to be incorporated into culture such as religion, stories, tradition and pastimes. For example, the Japanese traditionally hold moon-viewing parties and festivals known as Tsukimi for the full moons of Autumn. This often makes use of river boats or bridges for their unobstructed views.
FolkloreA common theme of folklore and myth is that the moon brings forward dramatic events and changes to human behavior. For example, the English word lunacy meaning "crazy" is based on the historical belief that the moon, particularly a full moon, influenced sanity. Historically, in a world of torch or candlelight, the light of the full moon made the night more accessible. It seems likely that this did inspire some degree of adventure and misadventure that formed the basis of these myths.
Blood MoonA blood moon is a moon that appears to be red. This occurs during a lunar eclipse whereby the moon falls into the Earth's shadow and appears red due to the Earth's effect on blue light. On 30 June 1503, Christopher Columbus used knowledge of an upcoming lunar eclipse to manipulate indigenous leaders in Jamaica. Columbus asserted that they had made God angry by not furnishing him with provisions and that the moon would turn to blood red from his wrath. This apparently worked and the locals quickly provided Columbus with ample supplies.
StructureThe Moon has a similar structure to the Earth with a crust, mantle and core. It is believed that the core of the Moon may be solid iron surrounded by molten liquid iron.
MoonquakesA Moonquake is the lunar equivalent of an Earthquake. These were unknown until they were accidently discovered by the Apollo missions. Moonquakes are generally weak but have been recorded at up to 5.7 on the Richter scale. They can last for up to an hour due to a lack of dampening factors on the Moon. Moonquakes are often caused by the tidal forces exerted on the Moon by the Earth that are about 20 times stronger than the tidal forces going the other way. Moonquakes can also be caused by meteorite impacts. The Apollo missions placed various seismometers on the Moon that were operational on the Moon between 1969 and 1977.
Lunar WaterAs the Moon has only a very thin atmosphere, any liquid water near the surface is easily lost to space. Ice exists at the Moon's poles in craters that create permanent shadows that are always cold. Small amounts of water has also been detected in sun exposed lunar soil in amounts about 100 times less than soil from the Sahara dessert. It is currently unknown if the interior of the Moon contains usable quantities of liquid water.
Other MoonsThe planets of the Solar System are orbited by 218 moons. Jupiter alone has 79 moons. Only 19 moons of the Solar System are large gravitational rounded moons. Of these, the Moon is the 5th largest. The largest moon in the Solar System is Ganymede, a satellite of Jupiter. In terms of mass, it is about twice as large as the Moon.
NotesA blood moon can also occur when the moon is low on the horizon. This occurs for the same reason that the Sun can appear red despite emitting white light -- the atmosphere of the Earth tends to scatter short wavelengths of bluer light.Of the 19 large moons of the Solar System, all are covered in a crust of ice except the Moon and Io, a moon of Jupiter that has the lowest amount of water of any known astronomical object in the Solar System.
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