Earth’s eternal celestial peer
From its early days as Earth’s constant companion, to where we have come today -- the touch-down of Chandrayaan-3’s Vikram lander on its south pole -- interest in the Moon has only grown.
For aeons, the Moon has dominated the ethereality of the night sky. Long before civilisation, the Moon was one of humankind’s first clocks, guiding our ancestors with its changing phases, and with regularity of seasons, helping maintain the rhythm of life. From its early days as Earth’s constant companion, to where we have come today -- the touch-down of Chandrayaan-3’s Vikram lander on its south pole -- interest in the Moon has only grown.
The Moon is as mysterious as it is fascinating. A major focus since the earliest days of stargazing has been to observe the waxing and waning of the Moon phase, along with the changes in its apparent size in the sky, and the different shapes and shades it exhibits. The Moon has a story to tell, how it came about and its special place in the cosmos.
The Moon is the Earth’s only natural satellite. The two have been together for over four billion years. According to scientists, the Earth was formed a little before the Moon, around 4.6 billion years ago. As a terrestrial planet, it began in a disk-shaped cloud of dust and gas, rotating around a much younger Sun, which was composed of material left behind after its birth. Within this disk, gas and dust particles orbiting the Sun collided with each other, which led some particles to stick together.
In time, these dust grains transformed into large boulders, eventually forming into massive ‘planetesimals’ that were hundreds of kilometres across, and then into Mars-sized ‘protoplanets’ owing to collisions between themselves. Research by the University of Chicago reveals that the Earth grew to its final size through a major collision with another Mars-sized object, ‘Theia’. In what is termed the ‘moon-forming impact’ some 4.51 billion years ago, “this final collision was so large that apart from adding material to the new Earth, there was enough energy to vaporise some of the rock and metal from both the proto-Earth and the impacting object, forming a disc around the Earth that eventually cooled and clumped together to become the Moon”. Also called the ‘Giant-Impact Theory’, this is one of the most widely-accepted theories explaining the Moon’s formation.
NASA’s Apollo missions brought back a range of rocks and soil samples from the Moon, which provided vital clues about its infancy. “The Moon rocks showed a host of chemical and isotopic similarities between the Earth and the Moon, pointing at a shared history,” states findings from the Natural History Museum. Meanwhile, other interesting theories suggest different ways in which the Moon came into being. The ‘Capture Theory’ suggests that the Moon was a wandering celestial body that was captured by Earth’s gravity; the ‘Accretion Hypothesis’ states that the Moon was created along with the Earth at its formation; and the ‘Fission Theory’ suggests that the Earth spun so rapidly that some material broke off and began to orbit the planet.
Scientists agree that the study of the Moon will help answer questions about the universe. With a diameter of 3,474.8 km, it is the fifth largest of all moons in the solar system. It is, however, the largest moon in relation to the size of its parent planet, with a radius larger than a quarter of the Earth’s own radius. Its surface gravity is about one-sixth of Earth’s at 0.1654 g.
The Moon orbits the Earth at an average distance of 3,84,400 km, while its orbit has a sidereal period of 27.3 days. Scientists also believe that the Moon has played a critical role for life to thrive on Earth. The Moon complements the Earth with its gravitational influence, causing tides, which could have partially triggered seismic activity, in turn cooling the planet’s surface. The tides could have also pulled necessary minerals into the oceans that formed the primordial soup culminating in the origin of life, while the Moon’s gravity stabilised the Earth’s rotational tilt, making the climate here more conducive.
While it lacks a noticeable atmosphere, hydrosphere, or magnetic field, the Moon’s surface hosts many topographical features, such as mountains, valleys, craters, and maria (wide flat areas resembling seas). The landscape reflects a pale grey hue, owing to the presence of a rock called ‘anorthosite’. Despite the low reflectance of the lunar surface, its large angular diameter makes the full moon the brightest celestial object in the night sky.
In the 20th century, the Moon became a major source for space exploration, attracting at least 146 missions. Scientists say the next big frontier would be to set up a lab or permanent crew station on the lunar surface, as we get closer to understanding our heavenly origins.