When the Sun and Moon are aligned on the same side of the Earth, the moon is “new”, and the side of the Moon facing Earth is not illuminated by the Sun. As the moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress through new moon, crescent moon, first-quarter moon, gibbous moon, and full moon. The moon is then said to wane as it passes through the gibbous moon, third-quarter moon, crescent moon and back to new moon. The terms “old moon” and “new moon” are not interchangeable. The “old moon” is a waning sliver (which eventually becomes undetectable to the naked eye) until the moment it aligns with the sun and begins to wax, at which point it becomes new again.[1] Half moon is often used to mean the first- and third-quarter moons, while the term ‘quarter’ refers to the extent of the moon’s cycle around the Earth, not its shape.

When a sphere is illuminated on one hemisphere and viewed from a different angle, the portion of the illuminated area that is visible will have a two-dimensional shape defined by the intersection of an ellipse and circle (where the major axis of the ellipse coincides with a diameter of the circle). If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous (bulging outwards)[2] whereas if the half-ellipse is concave with respect to the half-circle, then the shape will be a crescent. When a crescent Moon occurs, the phenomenon of earthshine may be apparent, where the night side of the Moon faintly reflects light from the Earth.

In the Northern Hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing (moving toward a full moon). If the right side of the Moon is dark then the light part is shrinking, and the Moon is referred to as waning (past full and moving toward a new moon). Assuming that the viewer is in the northern hemisphere, the right portion of the Moon is the part that is always growing (i.e., if the right side is dark, the Moon is growing darker; if the right side is lit, the Moon is growing lighter).

In the Southern Hemisphere the Moon is observed from a perspective inverted to that of the northern hemisphere, and all of the images in this article, so that the opposite sides appear to grow (wax) and shrink (wane).

Nearer the Equator the Moon with its terminator will appear apparently horizontal during the morning and evening. Since the above descriptions of the lunar phases only apply at temperate or high latitudes and observers moving towards the Tropics from northern or southern latitudes will see the Moon rotated anti-clockwise or clockwise with respect to the images in this article. The crescent Moon can open upward or downward, with the “horns” of the crescent pointing up or down, respectively. When the Sun appears above the Moon in the sky, the crescent opens downward; when the Moon is above the Sun, the crescent opens upward. The crescent Moon is most clearly and brightly visible when the Sun is below the horizon, which implies that the Moon must be above the Sun, and the crescent must open upward. This is therefore the orientation in which the crescent Moon is most often seen from the Earth’s tropics. The waxing and waning crescents look very similar. The waxing crescent appears in the western sky in the evening, and the waning crescent in the east, in the morning.

When the Moon, as seen from Earth, is a narrow crescent, the Earth as seen from the Moon is almost fully lit by the Sun. Often, the part of the Moon that is not directly lit by the Sun is sufficiently brightly lit by light reflected from the Earth to be easily visible from Earth. This phenomenon is called “earthshine“, and is sometimes picturesquely described as “the old moon in the new moon’s arms” or, as pictured here, “the new moon in the old moon’s arms”.

Non-western cultures may use a different number of Moon phases, for example traditional Hawaiian culture has a total of 30 different Moon phases (one per day)