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Ezra Harris
Ezra Harris

The Dark Side Of The Moon - A Complete Course I...



The phrase "dark side of the Moon" does not refer to "dark" as in the absence of light, but rather "dark" as in unknown: until humans were able to send spacecraft around the Moon, this area had never been seen.[1][2][3] In reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun. This symmetry is complicated by sunlight reflected from the Earth onto the near side (earthshine),[8] and by lunar eclipses, which occur only when the far side is already dark.




The Dark Side of the Moon - A Complete Course i...



At night under a "full Earth" the near side of the Moon receives on the order of 10 lux of illumination (about what a city sidewalk under streetlights gets; this is 34 times more light than is received on Earth under a full Moon) whereas the dark side of the Moon during the lunar night receives only about 0.001 lux of starlight.[8] Only during a full Moon (as viewed from Earth) is the whole far side of the Moon dark.


The word dark has expanded to refer also to the fact that communication with spacecraft can be blocked while the spacecraft is on the far side of the Moon, during Apollo space missions for example.[9]


It has also been proposed that the differences between the two hemispheres may have been caused by a collision with a smaller companion moon that also originated from the Theia collision.[11] In this model, the impact led to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness that may be consistent with the dimensions of the far side highlands. However, the chemical composition of the far side is inconsistent with this model.[citation needed]


In 1967, the second part of the Atlas of the Far Side of the Moon was published in Moscow,[24][25] based on data from Zond 3, with the catalog now including 4,000 newly discovered features of the lunar far side landscape.[17] In the same year, the first Complete Map of the Moon (1:5000000 scale[21]) and updated complete globe (1:10000000 scale), featuring 95 percent of the lunar surface,[21] were released in the Soviet Union.[26][27]


During a crescent moon, for example, the part of the Moon that faces Earth is mostly in shadow, and the far side of the Moon is mostly sunlit. The visible crescent is the only part of the lunar nearside that is experiencing daytime.


The Moon is always half-lit by the sun (except during a lunar eclipse). The side of the Moon facing the Sun appears bright because of reflected sunlight, and the side of the Moon facing away from the Sun is dark. Our perspective on the half-lit Moon changes as the Moon orbits Earth. When the side nearest to us is fully lit, we call this a full Moon. When the far side is fully lit and the near side is dark, we call this a new Moon. When we see other phases, we are looking at the division between lunar night (the dark part) and day (the bright part).


Moonrises and moonsets occur for the same reason as sunrises and sunsets. Earth rotates every day! This means that observers in many different parts of the world have their turn looking at the Moon throughout the day, just like we all see the same Sun over the course of 24 hours.


On Earth, our view of the illuminated part of the Moon changes each night, depending on where the Moon is in its orbit, or path, around Earth. When we have a full view of the completely illuminated side of the Moon, that phase is known as a full moon.


New moon was this Friday, Jan. 25. All this week, leading up to the first quarter phase on Feb. 1, there will be a waxing crescent adorning the southwestern sky as twilight fades into the dark of night.


Some amateur astronomers have found great sport in trying to detect the thinnest possible crescent of the moon, only hours after new moon and only minutes after the sun has set. Of course, the thin arc of the crescent is very difficult to see, with the overwhelming light of early dusk (or dawn), and with the moon so very near the flat horizon where the atmosphere is the murkiest.


While the moon is still an evening crescent, the night sky is still quite dark (except for what light pollution you may endure). Be sure to enjoy the bounty of stars spread across the winter sky. By the way, the red-orange star Betelgeuse in the upper left corner of the Orion constellation is still unusually dim this season, fainter than anyone can remember.


Like the stars and planets, the Moon doesn't stay fixed in the sky but slowly moves as the Earth rotates and as the Moon moves through its orbit about the Earth. To someone taking a casual glance at the Moon, it seems as fixed as the stars. But observation of either the Moon or the stars over a period of several hours will reveal their diurnal (daily) motion across the sky. The Moon rises and sets each day. An observer who watches the Moon over the course of many days will notice the Moon moving not only with the stars, but among them. Every month, the Moon completes one fewer pass across the sky than the stars have completed. We see this because of the Moon's orbit about the Earth. As the Moon progresses through its orbit, its rising and setting times change. Each day, the Moon rises and sets fifty minutes later than the day before.


The moon usually takes 27 days to rotate once on its axis. So any place on the surface of the moon experiences about 13 days of sunlight, followed by 13 days of darkness. Temperatures on the Moon range from -153 C at night to 253 C during the day. For example if you were standing on the surface of the moon during sunlight hours it would be blazing hot. When the sun goes down, the temperature automatically drop 250 degrees in just a matter of moments. Furthermore, there are craters around the North and South poles of the moon which never seen the sunlight. These dark places would always be as cool as -153 C. However, there are nearby mountain peaks that are covered in continuous sunlight, and would always be hot.


Typically, one-half of the Moon will be lit up by the Sun, while the half facing away from the Sun remains dark. (The only exception occurs during a lunar eclipse, when the Earth blocks the light falling on the lit side of the Moon.) The part illuminated by the Sun is not, it should be emphasized, always the same portion of the Moon's surface! Like the Earth, the Moon turns on its axis, exposing different areas at different times. In combination with the orbital revolution of the Moon around the Earth, this phenomenon creates the phases of the Moon as seen from Earth. The phrase "Dark side of the Moon" arose before the age of artificial satellites and the back side of the Moon could not be observed. Hence, the that one side was unknown or "dark".


Some half of the Moon is always illuminated but the fraction of the illuminated part or the Moon's phases directly depend on the relative positions of the Earth, Moon, and Sun. Simply put, it's a matter of how much of the daylight side of the Moon we can see from our current viewing angle. The phase will depend on how much of the side facing toward us is illuminated at any given time. The sketch below illustrates the phases of the moon for various Earth-Moon-Sun positions (the Sun is presumed to be off of the diagram to the right):


Next to each "Moon" is a black-and-white sketch of the phase as it would be seen from Earth when the Moon is in that position. When the Moon is between the Earth and the Sun, the sunlit side of the Moon is facing completely away from us, and therefore we have the dark "New Moon". When the Moon reaches the other side of the Earth, the sunlit side will be fully toward us, and we have the "Full Moon". As the Moon moves from New to Full and the sunlit side grows increasingly large, we say the Moon is waxing; as we see less, in the decline from Full to New Moon, we say it is waning.


Midway between the Full Moon and New Moon, half of the sunlit side of the Moon is visible from the Earth. Because a half of the half illuminated Moon can be seen, this is referred to as a "quarter Moon". When the Moon is waxing and reaches this position, it's called the "first quarter Moon"; when waning, the "third quarter Moon." When less than a quarter-moon is visible, it's referred to as a "crescent Moon" - waxing crescent or waning crescent, as appropriate. When more than a quarter-moon is visible, it's referred to as a "gibbous moon", again, waxing or waning.


Up to now, we have considered the time for the Moon to complete one orbit around the Earth to be the same as the time for it to pass once through its series of phases, but this is not quite right. The Moon's phase at a particular point in its orbit changes as the Earth goes around the Sun. Once the Earth has gone halfway around the Sun, the position of the Moon for a given phase has also moved halfway around the orbit, since the Sun is on the opposite side. Thus, it takes a little longer for the Moon to go through its phases than it does for it to go through its orbit about the Earth.


Suppose a full moon marks the beginning of both the period of the orbit of the Moon about the Earth and the period of the orbit of the Earth about the Sun. At the time of full Moon, the Sun, Earth, and Moon are aligned. Once the Moon returns to that position in its orbit, the Earth has moved a little around the Sun. Now, the Moon is not aligned with the Earth and the Sun. It takes about two days before the Moon has moved back into alignment with the Earth and Sun line (synodic month). The time for the Moon to complete an orbit, called a sidereal month, is about 27 days and 8 hours. The time to move through its phases, a synodic month, is about 29 days and 12 hours.


The closing words of Pink Floyd's seminal work The Dark Side Of The Moon say, "There is no dark side of the Moon really, matter of fact, it's all dark". Truth is, by the time those words were recorded for that 1973 album, mankind had seen the dark side of the Moon some five years earlier. The crew of Apollo 8, which launched December 21, 1968, were the first to lay eyes on the far side of the Moon, and in honor of the first time that humans circled the Moon, Omega created a special Speedmaster hybrid called the Moonwatch Apollo 8 Dark Side of The Moon. 041b061a72


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