The moon is the natural satellite of Earth which radius is about a quarter of terrestrial. In night-time we see its disk which is differently lit with the Sun invisible at this time. Degree of illumination depends on a relative positioning of Earth, the Moon and the Sun. In total distinguish four degrees of illumination which are called "phases".
The cycle of lunar phases is repeated approximately in 30 days - more precisely, from 29.25 to 29.83 days. The line of illumination - terminator - at the same time moves on the surface of the natural satellite of Earth smoothly, but it is accepted to distinguish only four provisions, carrying all intermediate options to one of them. Therefore it is considered that for each cycle four lunar phases which are called also "quarters" are replaced. To define in which of phases there is the Moon at present, it is possible visually - for this purpose there are simple mnemonic rules.
Each new cycle begins with a new moon - at the western edge of a visible disk in the first day very narrow lit sickle is visible, and every next night its width increases. During this first phase of a cycle, as well as in following it to the second, the Moon is called growing. If to a visible sickle it is conditional to add the vertical line, letter "P" - the first in the word "growing" will turn out. When the visible sickle of the natural satellite grows to a half of a disk in the widest part, the first phase will end and the second will begin - it occurs approximately in 7.5 days. The second phase - or the second quarter - lasts as much and to its end shining is all visible disk of the satellite of Earth. In the last day of the second phase there comes the full moon, and the natural satellite in the greatest measure justifies a rank of "a night star".
The following two quarters of Long is called "decreasing" or "growing old". During this period its shining area every night reminds letter "C" more and more - the first in the word "aging". Process happens upside-down - width of the lit part of a disk decreases every night and when from it there is a half, the third phase will end and the last will begin. Upon completion of the fourth quarter of Long it is turned to Earth by the dark party.