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By Tom Metcalfe
The appearance of the moon changes drastically every month, from half-moon to half-disk, then full disk – and back again. What causes these so-called moon phases?
"The phases of the moon are a reflection – quite literally – of how much of the lunar surface is illuminated by the Sun, as seen from Earth," says Shoshana Weider, a planetary scientist who collaborated with the NASA Lunar Science Institute. "This amount changes during the lunar month, which lasts about 29.5 days, depending on the relative position of the earth, the moon and the sun in the sky."
The lunar phases were explained
Except for these times, when the moon falls into the shadow of the Earth, causing a lunar eclipse, half the surface of the moon is always illuminated by sunlight, and half is always shaded.
Sometimes the moon presents its solar side towards Earth; At this time, we see a full disk of reflected sunlight or the full moon. At other times only part of the sunlit side of the moon shows us the way, so we see a fragment of reflected sunlight – a crescent moon or a gibbous moon. Sometimes the moon presents only its shadowy face – what we call a new moon – and we can not see the moon at all.
Each lunar month begins with a new moon, and after three or four nights later a waxed crescent (waxing means enlargement). About seven nights after the new moon we see half of the moon's disk – the phase called the first quarter. Then there is the waxing, lithe moon that is half-full and full.
The full moon appears about two weeks after the new moon. Although the moon looks full for three or four days, it is actually only full for a few moments on a particular day.
In the second half of the lunar month, the moon begins to weaken. First, the dying moon appears, then the last quarter, then the descending crescent – and then again the new moon.
Solar eclipses occur only during the new moon, when the moon passes between Earth and the sun. But because the moon orbits the Earth at a different angle than the Earth revolves around the Sun, the New Moon brings the solar eclipse only sporadically.
In the same way, lunar eclipses occur only during the full moon, when the Earth passes between the moon and the sun, But there is no eclipse, unless the moon and the sun are aligned exactly.
The phases of the moon in folklore and history
Human nature and the world around us were often associated with the phases of the moon.
For example, the term "crazy" was invented because it believes that strange behavior or mental illness was caused by a changing moon, adds Weider, adding that farmers once believed that fence posts should be set during the new moon; animals should not be weaned during the waning moon; animals slaughtered during the waxing of the moon give more fleshy meat; and the best fishing days are between the new and the full moon.
Some of the earliest calendars in the world, developed in the ancient Near East by the Babylonians about 2,500 years ago, were based partly on the phases of the moon. And the phases of the moon are still used to describe many religious observations. Islamic and Hebrew holidays are, for example, related to dates in the lunar months, "says Gordon Johnston, director of the NASA program who worked on the Moon Orbiter Reconans. Easter, of course, falls on the first Sunday after the first full moon of spring.
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