What is a Lunar Cycle - More Grades K-5 Science on Harmony Square
The lunar phase or phase of the Moon is the shape of the directly sunlit portion of the Moon as viewed from Earth. The lunar phases gradually and cyclically change over the period of a synodic month (about 29.53 days), as the orbital positions of the Moon around Earth and of Earth around the Sun shift.
The Moon's rotation is tidally locked by Earth's gravity; therefore, most of the same lunar side always faces Earth. This near side is variously sunlit, depending on the position of the Moon in its orbit. Thus, the sunlit portion of this face can vary from 0% (at new moon) to 100% (at full moon). The lunar terminator is the boundary between the illuminated and darkened hemispheres.
Each of the four "intermediate" lunar phases (see below) is around 7.4 days, but this varies slightly due to the elliptical shape of the Moon's orbit.
In western culture, the four principal phases of the Moon are new moon, first quarter, full moon, and third quarter (also known as last quarter). These are the instances when the Moon's ecliptic longitude and the Sun's ecliptic longitude differ by 0°, 90°, 180°, and 270°, respectively.[a] Each of these phases occur at slightly different times when viewed from different points on Earth. During the intervals between principal phases, the Moon's apparent shape is either crescent or gibbous. These shapes, and the periods when the Moon shows them, are called the intermediate phases and last one-quarter of a synodic month, or 7.38 days, on average. However, their durations vary slightly because the Moon's orbit is rather elliptical, so the satellite's orbital speed is not constant. The descriptor waxing is used for an intermediate phase when the Moon's apparent shape is thickening, from new to full moon, and waning when the shape is thinning.
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. 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 an illuminated hemisphere is viewed from a certain angle, the portion of the illuminated area that is visible will have a two-dimensional shape as defined by the intersection of an ellipse and circle (in which the ellipse's major axis coincides with the circle's diameter). If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous (bulging outwards), 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 dimly reflects indirect sunlight reflected from Earth.
