There are four phases of the Moon: new moon, first quarter, full moon, and last quarter. These are the different ways in which we see the Moon as a consequence of its translational movement around the Earth.
The Moon is the only natural satellite of the Earth, that is, it is a celestial body that orbits or revolves around our planet. Despite not having its own light, the Moon reflects the light it receives from the Sun.
Depending on the position of the Moon with respect to the Sun and the Earth, we can see the different lunar phases:
the new moon: It is not visible on Earth, since it is between the Sun and the Earth.
the crescent room: Shows an illuminated half of the Moon, which is in the first quarter of its path.
Full moon: Appears fully illuminated at the point in the lunar orbit furthest from the Sun.
the waning quarter: the other half of the moon is illuminated, and it is a quarter of completing the lunar cycle.
1. New Moon
It is the phase in which the lunar cycle begins, also called new moon. It is when we cannot see the Moon because the illuminated part is on the opposite side to what we see on Earth.
When the Moon is between the Sun and the Earth, we do not see it because none of the Sun’s rays that illuminate it are reflected back to us.
As the Moon moves in its orbit away from the Sun, an illuminated part of the Moon begins to be seen, known as the Sun. Crescent moon.
The crescent Moon in the southern hemisphere grows from left to right, while it grows from right to left in the Northern hemisphere.
2. First quarter
It is when we can see half of the Moon illuminated by the Sun. The line connecting the Moon and the Earth makes an angle of 90 degrees with the line connecting the Earth and the Sun. At this point, the Moon has traveled a quarter of its orbit. This phase can be seen between noon to midnight.
After the crescent quarter, as the Moon continues its journey, it brightens beyond the middle of the lunar circle, giving it a humpback or “hump” appearance. Astronomers call this transition crescent gibbous moon.
3. Full moon
It is when we see the Moon fully illuminated as a circle of light. At this point, the Moon is halfway through its orbit and is at the far end of its orbit, opposite the Sun.
This phase can be observed between sunset and sunrise.
The days following the full Moon, it moves towards the Sun, decreasing the portion of the Moon that we can see. It reacquires its hunchbacked form, but it receives the name of waning gibbous moon.
4. Last quarter
It is when we see half of the lunar circle illuminated after the full Moon. At this point it lacks a quarter of its journey to complete its orbit. Appears at midnight and hides at noon.
As the Moon gets closer to the Sun again, the illuminated side that we can see is reduced. This is what we know as waning moon.
moon cycle
The lunar cycle begins when the Moon is not visible from Earth and appears “off” (new Moon). Then, it begins to light up to half (Crescent Moon). The Moon continues to illuminate until it completes the circle (full Moon). From here, the illumination begins to decrease until it reaches half (last quarter) and continues to decrease until it is no longer visible from Earth. (New Moon).
Half of the Moon is always illuminated by the Sun (except when a lunar eclipse occurs). As the Moon orbits the Earth, the line between the Earth, the Moon, and the Sun changes angle. This explains the different phases that we observe from Earth.
The Moon makes one complete revolution around the Earth in 27.3 days. However, the period of time that elapses between two equal phases is called the synodic lunar month or lunation and is equal to 29 days, 12 hours and 44 minutes.
Location of the Moon with respect to the Earth and the Sun in each phase.
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References
Kriner, A. (2004). The phases of the Moon, how and when to teach them? Science & Education 10:11-120
Marán, SP (2013). Astronomy for Dummies. Banshee Digital Publisher.