Moon, or Luna in Latin, is planet Earth’s one and only natural satellite and the brightest object in the night sky. With a diameter of 2,159.2 miles (or 3,474 km), it is roughly the size of the African continent and is the largest lunar body relative to the size of the object it orbits around. A cold, dry sphere with a surface covered in craters, rocks and dust called regolith.
The moon is estimated to have formed nearly 4.5 billion years ago, just a hundred million years after the planets of our solar system formed. There are numerous theories that explain how the moon was formed but the Giant Impact Hypothesis is by far the most widely accepted scientific explanation.
Giant Impact Hypothesis
According to the Giant Impact Hypothesis, the Moon was formed from the resulting dust and debris from a giant impact between the Earth and another celestial body the size or Mars, known as Theia. The collision created a large amount of dust and fragments that spread out into space and got caught by the Earth’s gravitational pull and ended up in an orbit around the Earth. Over 90 million years the dust and fragments collided and accumulated to a bigger and bigger celestial body that eventually became the Moon as we know it.
The Co-formation theory suggests that Moon was formed at the same time as Earth. According to this theory, gravity caused material in the early solar system to draw together at the same time when the gravity drew particles together to form earth. This theory lost its favor quite recently when the scientists discovered that Earth and Moon does not have similar density, proposing that they could not have possibly been created at the same time.
It was only in 1959 that humanity was first able to photograph the moon's far side
Another popular theory, that has lost its credibility due to recent scientific research, is known as Capture theory. The theory suggests that it was Earth’s own gravity that snagged a passing body and created the Moon. This is the case with many other moons in our Solar system and according to this theory; moon’s rocky body formed somewhere else in the solar system and was drawn to Earth’s orbit by its gravity. This theory loses its footing because celestial bodies pulled like this don’t line up with the ecliptic path of their parent body which Moon does.
The surface of Moon is different from Earth’s surface due to different atmosphere, plate tectonics and water. Nevertheless, there is no lack of beautiful mountain ranges on the surface of Moon caused by meteoroids striking its surface. The absence of water on its surface, and therefore no erosion, has helped its terrain to retain its ancient geological features. Since there are no plate tectonics, there are no volcanic eruptions or earthquakes - factors that contributed heavily to formation of much of Earth’s terrain as we see it today.
The moon was long believed not to have any atmosphere but recent studies have shown that the moon has a so called exosphere which is the uppermost layer of an atmosphere, the layer that thins out and merges with the emptiness of space. The Moons exosphere weighs in at only about 55,000 pounds (or 10 tons) consists consist mostly of hydrogen and small amounts of helium and neon gasses. There is no air which means there is no weather as we know it here on the Earth.
The moon has a nearly circular orbit around Earth’s center of mass and has what is called a bound rotation with earth. This means that the moon rotates exactly one revolution around its own axis for each revolution around the Earth. The bound rotation means that the moon always has the same side facing the Earth and a back side that we are not able to observe from Earth.
It was only in 1959 that humanity was first able to photograph the moon's far side. It was done by the Soviet Luna 3 probe and in 1968 the Apollo 8 crew became the first humans who observed the "dark side of the moon" without photo sets.
The orbital period for the Moon is 27.322 days whereas Earth’s orbital period is roughly 365 days long. As it orbits around Earth, the moon also changes its position with respect to the sun which causes the moon phases.
Close to you
Find the moonrise & moonset times for a city near you.