Moon

The moon is Earth’s only natural satellite and is the largest of any satellite to its primary in the solar system (the moon is 0.020 Earths in volume and 0.012 Earths in mass). After the Jovian moon of Io, the moon is the second-densest satellite known. Its orbit takes it from between 362,600 kilometers from Earth at perigee to 405,400 kilometers from Earth at apogee, with an orbital period of 27.32 days, a lunar month.

The surface of the moon, based on rock samples, is composed mainly of silica (about 45 percent), alumina (15–20 percent), lime (11–16 percent), iron oxide (6–14 percent), magnesia (7–9 percent), titanium dioxide, and sodium oxide. Like the inner planets of the solar system, it is a differentiated body, meaning it has a distinct core, mantle, and crust. The inner core is solid and primarily iron, surrounded by a fluid outer core and a molten layer that is the remains of a magma ocean. The “seas” of the surface, believed in ancient times to be bodies of water, are solidified basalts with high iron concentrations, often occurring near volcanoes and volcanic domes. It was long thought that the surface was not hospitable to liquid water, which would be lost to space because there is too little atmosphere to protect it from solar radiation. While by 2020 evidence of ice had been discovered in craters in colder, darker areas of the moon and scientists had begun to suspect that water might be present in some form in sunnier parts of the surface, no distinguishing proof was reported until a confirming detection was made that year by the National Aeronautics and Space Administration (NASA)'s Stratospheric Observatory for Infrared Astronomy. Infrared cameras detected a concentration of water molecules in a southern hemisphere crater. As scientists were still unsure as to what was generating and keeping the water there, it was agreed that further studies were needed.

Relationship with Earth

The moon is in perfectly synchronous rotation with Earth, meaning that the same side of the moon is always facing Earth, a circumstance now known to be unusual in the solar system. The Earth-facing side is mottled with dark maria (seas) and deep impact craters. The far side has very few maria, consisting mostly of highlands.

In January 2017, researchers from Princeton University and the University of California-Los Angeles (who had been analyzing the mineral zircon, which was brought back to Earth from the 1971 Apollo 14 moon mission) published an article in the journal Science Advances that provided proof that the moon was at least 4.5 billion years old, which was 40 to 140 million years older than previously thought. Dating the age of the moon is important to the theory posited by scientists called the Theia Impact, for the Greek titan who was the mother of Selene (the moon goddess), that the moon was created from the accretion of debris in Earth’s orbit following an impact between Earth and a Mars-sized body known as Theia. Knowing a more specific date of impact and creation of the moon would help scientists to estimate when life was created on Earth.

The Theia Impact would have had drastic effects on Earth’s angular momentum, and Earth would also have gained mass from the destruction of Theia. Some of the debris would have become a ring around Earth that drifted away (some landing on Earth or the moon, some drifting into space) in the intervening millennia. Rocks collected during the moon landings support the theory that the moon was once part of Earth, due to similar oxygen isotope ratios and zinc isotopic composition.

The gravitational pull of Earth on the moon results in tidal forces, the most obvious effect of which is the ocean tides. As Earth rotates with the moon orbiting around it, tidal bulges rotate across the surface, elevating the sea level. The moon also gradually drains kinetic energy and angular momentum from Earth’s spin. In turn, this adds angular momentum to the moon’s orbit. The result is that Earth’s rotation is gradually slowing (increasing the length of the day by 15 microseconds a year) as the distance between Earth and the moon increases.

The moon is not fully visible from Earth at all times. Over the course of its orbit around Earth, the visible moon in Earth’s sky cycles through its phases, from the fully visible full moon to the fully occluded new moon, with the lunar terminator marking the boundary between light and dark. This cycle was the original basis for the length of a month. Today calendar months are slightly longer than lunar months so that the 365 days of the solar year can be divided into twelve units without a remainder.

Eclipses are an unusually frequent occurrence on Earth because the moon is at just the right distance from Earth to be the same apparent size in the sky as the sun. Solar eclipses always occur at new moon, when the moon is between sun and Earth and appears to block the sun. This happens about two to five times a year somewhere in the world. Roughly as frequent are lunar eclipses, in which the moon passes into the shadow of Earth.

History of Observation and Exploration

The moon is by far the brightest object in the night sky, with an apparent magnitude ranging from –2.5 to –12.9 during a full moon. It is incorporated into the mythology and folklore of virtually every ancient human culture. The moon was among the best understood celestial objects in ancient cultures; early study of the moon led to the development of the calendar.

The moon is the only celestial body other than Earth that humans have visited, and the Outer Space Treaty guarantees every nation the right to explore it for peaceful ends. Exploration of the moon was the object of the “space race” between the United States and the Soviet Union during the Cold War, as a way for each nation to demonstrate its superiority over the other. The national fervor for this justified an expenditure that resulted in a successful crewed US moon landing in 1969, just ten years after the Soviet Union landed the first uncrewed spacecraft. Twelve US astronauts landed on the moon between 1969 and 1972. The last US crewed moon mission was the Apollo 17, which took place on December 11, 1972.

By the 2010s, no program other than those of NASA had attempted a crewed mission to the moon. Both the Soviet Union and China landed uncrewed moon rovers; Japan, India, and the European Space Agency had sent uncrewed orbiters. During the 2010s, however, NASA had renewed investment in sending humans back to the moon as a key part of a larger strategy to facilitate expanded studies of the moon and space through an ongoing human presence. Enabling potential crewed missions to areas beyond the moon, specifically Mars, served as a significant end goal. In the latter half of that decade, after President Donald Trump signed a policy directive highlighting those goals, NASA announced the official Artemis program established to coordinate missions not only to land humans on the moon once more but also to set up a crewed base on its surface. Because such operations involved international partnership and cooperation, the United States and seven other countries put in place the Artemis Accords in 2020, nonbinding principles meant to ensure that civil exploration and use of outer space occurred peacefully and responsibly. As other countries became signatories over subsequent years, Artemis missions progressed, with an uncrewed lunar flight test of the Orion spacecraft and the space launch system (SLS) taking place in late 2022.

With the Artemis program underway, the United States also saw a need for establishing a standardized time zone for the moon. To that end, in 2024 the White House Office of Science and Technology Policy issued a memorandum requesting the establishment of a lunar time zone that would be named Coordinated Lunar Time (LTC). The memo directed NASA, with international input, to determine and implement an LTC to allow the conduction of more precise and organized lunar missions and, eventually, guide a more established human presence on the moon.

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