Phases of the Moon

Moon phases are an astronomical phenomenon, which, depending on the relative position of the Sun, Earth and Moon, allows observers from the Earth to observe the differently illuminated part of the Moon‘s surface. This is a periodically cyclical phenomenon caused by a combination of two astronomical phenomena: the revolution of the Earth around the Sun, and the revolution of the Moon around the Earth, whose period is given by the synodic month with the exact value of 29.530 588 853 days (i.e. 29 d 12 h 44 min 2.9 s). The name of the synodic month is derived from the Greek word synodos (σύνοδος), meaning regular meetings, in the original definition regular meetings of bishops in order to discuss ecclesiastical matters.

Phases of the Moon

The most significant phases of the Moon divide the entire period into four equal parts. These parts are defined by the basic phases of the Moon, which for historical reasons bear the names New moon, First quarter, Full moon and Last quarter.

• A New moon is the stage when the lunar disk is over-lit by sunlight because both disks are very close to each other in the sky, practically in one place (exactly: they have the same ecliptic longitude). The lunar disk in the New moon is not observable in the sky, and because it is located practically in the same place of the sky as the solar disk, it is clear that it is in the sky at the same time as the Sun, that is, in the daytime.
  
  
• On the other hand, the Full moon, when the lunar disk is visible in the shape of the whole illuminated circle, means that the Sun and the Moon are located in a position where they are on the opposite side of the Earth in the solar system, and that we see the whole hemisphere of the Moon illuminated, is determined by the fact that we are looking at the Moon from the same direction from which the Moon is illuminated by the Sun. From the above stated, it is also apparent that the lunar disk is in the sky at the time when the Sun is not, that is, the Full moon shines throughout the night.
  
  
• The period between the New and the Full moon is divided in the middle, on one side by the First quarter, on the other side by the Last quarter. Both phases are changed by the shape of the illuminated part of the Moon, when at the First quarter it has the shape of a semicircle resembling a capital letter D, in the Last quarter it has the shape of the opposite semicircle (the arch takes the form of a capital letter C). In the solar system, in both of these cases,the line connecting the Earth and the Moon forms the right angle with the line connecting the Earth and the Moon with the Sun. In the case of the First quarter, i.e. the lit part of the D-shaped disk, the Moon already rises around noon (although it is not very visible in the sky) and it sets around midnight, it is therefore visible in the evening sky. On the contrary, at the Last quarter, the Moon rises around midnight and sets around noon, so most clearly it is visible before dawn. Mnemonics are used in the Czech Republic, Slovakia and Germany to distinguish the First and Last quarters. In the Czech Republic and Slovakia, the mnemonic is derived from the capital letters D and C (i.e. dorůstá X couvá in Czech, or dorastá X cúva in Slovakian), in German it is derived from small typewritten letters in the Schwabacher (zunehmen X abnehmen). A similar rule in French is derived from small print letters (première X dernière).