How to define the phase of the moon

The lunar phase cycle repeats in about 30 days – more precisely, from 29.25 to 29.83 days. The illumination line – the terminator – is moving on the surface of the Earth’s natural satellite smoothly, but it is accepted only four positions, referring all intermediate variants to one of them. Therefore, it is believed that four lunar phases are replaced for each cycle, which are also referred to as “quarters”. Determine, in which the phases is the Moon at the moment, can be visually – there are simple mnemonic rules for this purpose.

Each new cycle begins with a new moon – at the western edge of the visible disk a very narrow illuminated sickle is visible on the first day, and with each next night its width increases. During this first phase of the cycle, as in the second after it, the moon is called growing. If you draw a vertical line to a visible sickle, you get the letter “P” – the first in the word “growing”. When the visible sickle of the natural companion grows to half of the disc in the widest part, the first phase will end and the second will begin – this occurs in approximately 7.5 days. The second phase – or the second quarter – lasts as long and by its end the entire visible disk of the Earth satellite turns out to be glowing. On the last day of the second phase, a full moon occurs, and the natural satellite justifies the title of “night light” in the most measure.

The next two quarters the moon is called “waning” or “aging”. During this period, its luminous area with each night increasingly resembles the letter “C” – the first in the word “aging.” The process occurs in reverse order – the width of the illuminated part of the disc decreases with each night, and when only half is left of it, the third phase will end and the last one begins. At the conclusion of the fourth quarter, the Moon faces the Earth with its unlit side.

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