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Moon Phases
 

The Moon's motion around the Earth is accompanied by a perceived change in appearance of our planet's satellite. The visible edge of the lunar disk is called the limb. The ratio of lunar disk's visible area to its total area is known as the phase of the Moon.

There are four basic phases of the lunar cycle: new Moon, first crescent, full Moon, and last crescent.

The Moon passes between the Sun and the Earth during the new Moon period. The Earth faces the dark side of the Moon, which is not illuminated by the Sun. The lunar disk sometimes seems to radiate a peculiar "ashy" light during the new Moon period. The weak luminescence of the nocturnal part of the lunar disk is in fact sunlight that is reflected onto the Moon by the Earth. The young Moon's tiny crescent appears on the western part of the sky immediately after sunset for two days after the New Moon.

The growing Moon looks like a semicircle in the West or in the Southwest immediately after the sunset for the next seven days after the new Moon is born. The Moon is situated at 90? to the East from the Sun, and is visible in the evenings and during the first half of the night.

Full Moon occurs 14 days after the new Moon is born. Full Moon stand opposite to the Sun, and the entire illuminated lunar hemisphere faces the Earth. The Moon is visible all night long: it rises at dusk and sets at dawn.

Senescent Moon appears as semicircle in the week that follows the full Moon, during its last quarter phase. Half of the illuminated and half of the non-illuminated hemispheres of the Moon face the Earth at that time. The Moon is visible in the East during the second half of the night, and before the sunrise.

The period between the two consequent similar phases of the Moon is called a synodic month. It is equal to 29.53 days. A sidereal, or regular month, i.e., the period of one regular revolution of the Moon around the Earth, is equal to 27.3 days.

This model illustrates the rotation of the Earth and the Moon around the Sun. Observe the Moon as it is seen from the Earth in the lower part of the model. Press "Run" button to initiate the model, press "Stop" button to bring the model to a halt, and press "Reset" to return the model to its initial state.

 
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