Stardust and Eternity – 3.2.1

The Moon, Moonlight and Moon phases

Most of the planets orbiting around the Sun, except Mercury and Venus, have natural satellites. The Earth’s Moon is only the fifth largest of the more than 200 known natural satellites orbiting planets of our Solar System, after Ganymede, Titan, Callisto, and Io, which orbit Jupiter and Saturn. The diameter of the Moon is about 3,500 km, a little more than 1/4 of the size of our planet; its distance is about 380,000 km. The origin of the Moon is still uncertain. It is nowadays believed that a giant impact between Earth and a Mars-sized planet (the so-called “Theia hypothesis”) occurred around 4.5 billions years ago, resulting in the ejection of debris into orbit around our planet, eventually coalescing to form the Moon. A heavy meteorite bombardment occurred soon after the solidification of the Moon’s crust, resulting in its characteristic cratered appearance. Prominent lunar “mare” basins were also formed and later filled with basaltic lava flows due to volcanic activity, creating the curious dark patterns that can easily be seen by eye during a full moon. Since our satellite rotates on its axis exactly once for every revolution it makes around the Earth – in a so-called “synchronous rotation” – we are only able to see one “face” of the Moon, while the “dark side” was only been mapped after a series of lunar orbiter missions launched after 1966 by the United States.

The Moon has no atmosphere and does not emit light but reflects the light of Sun. The “moonlight” is thus only a reflection of the sunlight and is very weak as compared to it. Due to its low “albedo”, only the 12% of the incident sunlight is reflected from the lunar surface. The moonlight is further scattered into the Earth’s atmosphere – thus increasing the brightness of the night sky and contributing to the light pollution that reduces our chance to observe dimmer stars. Moonlight’s intensity varies significantly, depending on the specific “lunar phase”. Indeed, due to its orbital sidereal period of about 27.3 days around the Earth, the visual aspect of our satellite changes continually, thus provoking the so-called lunar phases. They are caused by the reflecting Sun-illuminated portions of the Moon. Actually, the span of time between two successive full moons is a little bit longer – 29.5 days, or a “synodic month” – because at the same time the Earth also revolves around the Sun. The “new Moon” is the first lunar phase; it occurs when the Moon is between the Earth and the Sun, so that the lunar surface facing toward our planet receives no direct sunlight. Vice versa, when the Earth is between the Sun and the Moon, in its third phase, the face of the Moon is completely lit by the Sun and we experience a “full Moon”. “Half Moons” – the second and fourth lunar phases – occur when only half of the surface facing the Earth is lit by light. While the above mentioned four characteristic lunar phases last only one night, for about 7 days the Moon has a characteristic “waxing” or hump-backed shape between the first-quarter Moon and the full Moon, and a characteristic “waning” or shrinking sickle shape between the third quarter Moon and the new Moon.

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Clouds – Photo by Junior Peres Junior – Pixabay

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Images

Full Moon Photographed From Apollo 11 Spacecraft (NASA)