What are the phases of the moon?

The appearance of the moon changes dramatically during each month, from a crescent to a half disc, then to a full disc, then again. What is causing these so-called phases of the moon?

"The phases of the moon reflect – literally – the amount of lunar surface illuminated by the sun, as seen from Earth," said Shoshana Weider, a global scientist who has worked with NASA's Lunar Science Institute. "This amount changes over a lunar month, which lasts about 29.5 days, depending on the relative positions of the Earth, the Moon, and the Sun in the sky."

Lunar phases explained

With the exception of moments when the moon falls in the shadow of the Earth to cause a lunar eclipse, half of the moon's surface is still lit by sunlight and half is still shaded.

Sometimes the moon has its full sun side towards the Earth; at these moments we see a disc full of reflected sun rays, or a full moon. At other times, only a portion of the moonlit face indicates a reflected ray of sun – a crescent moon or a gibbous moon. Sometimes the moon only has its shaded face – what we call a new moon – and we can not see it at all.

Each lunar month begins with a new moon, followed three or four nights later by a growing crescent (thickening means growing). About seven nights after the new moon, we see half of the moon's disc – a phase called the first quarter. Then comes a growing gibbous moon, half full and full.

The full moon appears about two weeks after the new moon. Although the moon seems full for three or four days, it is actually filled only a few moments on a given day.

In the second half of the lunar month, the moon begins to decrease. First comes a descending gibbous moon, then a last quarter, then a descending crescent – then return to a new moon.

The eclipses of the sun occur only during the new moon, when the moon passes between the Earth and the sun. But as the moon turns around the Earth at a different angle than the Earth, a new moon brings a solar eclipse from time to time.

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 an eclipse only if the moon and the sun are aligned. exactly.

Moon phases in folklore and history

Human nature and the world around us have often been linked to the phases of the moon.

The term "whimsical", for example, was coined because it was thought that a strange behavior or mental illness was triggered by the evolution of the moon, Weider added, adding that the farmers once believed that fence posts were to be installed during the new moon; animals should not be weaned during a waning moon; animals slaughtered during an increasing moon give a juicier meat; and the best fishing days are between the new moon and the full moon.

Some of the world's earliest calendars, developed in the ancient Middle East by the Babylonians some 2,500 years ago, were partly based on the phases of the moon. And the phases of the moon are still used to determine many religious observations. The Islamic and Hebrew holidays, for example, are related to the dates of the lunar months, says Gordon Johnston, a NASA program officer who worked on the lunar reconnaissance orbiter. Easter, of course, falls on the first Sunday after the first spring moon.

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