The 4 main phases of the moon are the different illuminations that our satellite presents in the course of a lunation or lunar cycle.
Depending on the location of the Moon, the Earth, and the Sun, a larger or smaller portion of the visible face of the Moon is illuminated. Although the size of the illuminated area varies continuously, the apparent moon has been classified throughout history into four lunar stages or phases:
Phases of the Moon
The New Moon, or novilunium, happens when the Moon lies between the Earth and the Sun and, as a result, we cannot see it. It exists, but the face it depicts does not get sunlight during this lunar phase.
The Moon, Earth, and Sun create a right angle in the Crescent Quarter, allowing half of the moon to be visible in the sky during its waxing period. In the northern hemisphere, the lit region looks like a capital D; in the southern hemisphere, the illuminated area looks like a C or an inverted D.
A full moon occurs when the Earth lies between the Sun and the Moon; the Moon gets the sun’s rays on its visible side, resulting in a full circle. The moon reaches its apogee at midnight during this phase.
The 4 Main Phases of the Moon
Finally, in the waning quarter, the three bodies form a right angle again, so that the other half of the Moon’s face can be seen in the sky: the left side is illuminated in the northern hemisphere (a C or an inverted D) and the right side in the southern hemisphere (a D in normal position).
How and when are the phases of the moon seen?
The Earth’s orbit intersects the Moon’s orbit at a 5o angle, such that when the Moon is between the Sun and the Earth, one of its hemispheres, the one we view, is in the dark zone and so invisible to our eyes; this is known as a new moon, or novilunium.
The lit area visible from Earth expands as the Moon continues its translational motion, passing thru the concave crescent form, until a week later it displays half of its illuminated hemisphere; this phase is the crescent.
Then it grows and takes on the shape of a convex or gibbous crescent. A week later, we see the complete lit hemisphere: this is the full moon or full moon. When this full moon approaches its closest perigee (the point in its orbit closest to Earth), it looks bigger and brighter than at previous times.
The lit surface proceeds to drop the next week, going thru the convex or gibbous waning moon form, until it is half illuminated, but this time on the other side: it is the declining quarter.
It waned further, going thru the concave fading phase, and we saw less and less of it. It returns to its original place as the new moon at the conclusion of the fourth week and then fades from view. The moon’s phases have all been finished, and a new lunation cycle has begun.
The synodic month and the sidereal month are not the same.
A complete lunar cycle lasts about 29 and a half days and is called a lunation or synodic month. However, one orbit of the Moon around the Earth (a sidereal month) lasts only 27 days and a third. How can this difference be explained?
To understand it, we must take into account that, while the Moon revolves around the Earth, the planet is not stationary and moves around the Sun and, with it, the Moon as well. As the phases of the Moon are determined by the relative positions of the three stars, our satellite has to make more than one revolution each time to be in the same relative position between the Earth and the Sun, i.e., in the same lunar phase.