By Christopher Williams | Updated Mar 24, 2022
The Moon completes a full orbit around Earth in a sidereal period of 27.3217 days. However, the interval between identical phases—known as the synodic period or lunation—is 29.5305882 days. The extra 2.2 days arise because Earth itself is moving along its orbit around the Sun, so the Moon must travel a little farther to return to the same Earth–Sun–Moon geometry.
At any instant, half of the Moon’s surface faces the Sun and receives direct illumination, while the opposite half is in shadow. As the Moon journeys through its orbit, we observe different proportions of its lit side, giving rise to the eight standard phases: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, and waning crescent.
The terms waxing and waning describe the apparent increase or decrease in illuminated area. When the lit portion grows, the Moon is waxing; when it shrinks, it is waning. The crescent phases (less than half illuminated) and the gibbous phases (more than half illuminated) occur between the quarter and full or new phases.
When the Moon is on the far side of Earth from the Sun, we see the fully illuminated hemisphere—a full Moon. Conversely, when the Moon lies between Earth and the Sun, the illuminated side faces away from us, producing a new Moon. The intermediate positions, with the Earth, Moon, and Sun forming roughly 90‑degree angles, yield the first and third quarter phases where exactly half the surface is lit.
Occurring only at full Moon, a lunar eclipse happens when Earth’s shadow falls on the Moon. Partial eclipses are common, but total eclipses are rare and last only a few hours. During the event, the Moon may appear reddish, a phenomenon called a “Blood Moon.”
Understanding these dynamics not only demystifies the Moon’s changing face but also highlights the elegant dance of celestial mechanics that governs our sky.
