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).

However, the indicated shapes for the Moon phases are typical of the northern latitudes only. In the southern hemisphere, the lunar disk moves in the opposite direction, both the New and the Full moon look the same as in the northern latitudes, but the shape of the First and Last quarters is the opposite: the First quarter has an arch of the illuminated part of the Moon in the shape of a capital letter C, the Last quarter corresponds to the shape of the capital letter D. Even more complex is the situation in equatorial regions. Also here the New and Full moon look the same as in other places of the Earth, but the First and Last quarters look quite different than in the northern and southern geographical regions. The Moon at the First quarter rises (let´s remind around noon) in the form of a „tunnel“, that is, with an arch in the shape of a capital letter of the Greek alphabet Omega Ω. However, it sets (around midnight) with an arch in the shape of a capital letter U. This is because when passing through the highest point of its path in the sky, the observer must turn. At the Last quarter the lunar disk rises (around midnight) with an arch in the shape of a capital letter U and it sets with an arch in the shape of a capital letter Ω.

Other phases of the Moon

In addition to the synodic month, we can define other times related to the revolution of the Moon around the Earth. The most famous of these are the sidereal, tropical, anomalistic and draconic months. The sidereal month is the period of revolution related to distant stars with a length of 27.321 661 547 days. The tropical month is related to the vernal point and equals 27.321 582 241 days, the anomalistic month is the period of revolution of the Moon at the perigee with a length of 27.554 549 878 days and the draconic month is related to the output node of the lunar track (this is the intersection of the trajectory of the centre of the lunar disc in celestial sphere with a celestial equator, in which the lunar disk gets north (that is, „above“) from the celestial equator, and its length is 27.212 220 817 days.

The phases of the Moon are a phenomenon that is very evident for every observer on Earth. They therefore became an observed phenomenon even in prehistoric times and the basis of the first type of calendar – lunar calendar, as described in the following chapter. The period of the Moon phases, the synodic month, as mentioned above, became the first longer-term measure of time, the predecessor of today‘s month. A quarter of the month, the time between the most important four phases of the Moon, that is, between the New moon and the First quarter, the First quarter and the Full moon, the Full moon and the Last quarter, and the Last quarter and the New one, became the basis of the unit for time, the week.

Synchronous rotation of the Moon

With the revolution of the Moon around the Earth, there are other phenomena that pupils can follow and understand themselves. It is a synchronous rotationof the Moon, which is a phenomenon typical of other bodies in the solar system as well. The synchronous rotationof the Moon means that the same hemisphere constantly faces the Earth, i.e. its revolving time and rotation time is identical. The synchronous rotation is caused by the tidal force between the two bodies, when the revolving time and the rotation time of the Moon are gradually synchronised (in the case of Pluto – Charon, there is even super-synchronous rotation, that is, rotation, when each of the revolving bodies keeps the same hemisphere towards the other). The second phenomenon, which is closely related to the synchronous rotation, is the libration of the Moon. Libration is the wagging of the Moon around several axes, allowing the observer on the surface of the Earth to see a little more than a half of the Moon’s surface, as described above for the phenomenon of synchronous rotation. There are more causes of the Moon’s libration; one of them is, for example, the Moon revolving around the Earth in the ellipse and a regular rotation of the Moon around its axis.

Phases of the Moon and primary education

Because the phases of the Moon are easily observable, their inclusion in the curriculum is already appropriate at a younger school age, albeit of course with regard to mental capabilities of the pupils. In fact, many calendars have pictures of the phases of the Moon. It is possible to use these images and to bring the activities related to the Moon phases closer to pupils. There are two goals of the inclusion of the phases of the Moon in the curriculum for pupils of the younger school age: first, it is a familiarisation with the various phases, including the rule for the recognition of the first and last parts, and second, the understanding of why the phases of the Moon arise. The complete understanding of the fact that changing of the shape of a lit part of a lunar disk in the sky depends on the relative position of the Earth, the Sun and the Moon is, however, rather inaccessible to most children of the younger school age due to their mental immaturity. „The transformation of the observer‘s location“ between the position of the Earth-bound observer and the position“ high above“ the Northern Pole, which is necessary for understanding, is too complex thinking operation to be managed by all the children of that age. Therefore, for understanding of the basic issues of the Moon phases, we recommend three activities: familiarization with the calendar symbols of the Moon phases in one of the table or wall calendars, modelling the Moon phases using an illuminated orange or a manufactured Moon model, and the model of the Moon revolution around the Earth with simultaneous showing of the Moon‘s synchronous rotation.