What is the duration of the lunar cycle known as the moon's phases?

The duration of the lunar cycle, which refers to the complete sequence of the moon's phases from one New Moon to the next, is approximately 29.5 days. This cycle includes the transitions from the New Moon to the Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent before returning to another New Moon.

The 29.5-day period accounts for the moon's movement around the Earth and the relative positions of the Earth, moon, and Sun that create the various visible phases of the moon. The reason this duration is not an even 30 days is due to the elliptical orbit of the moon and its alignment with the Sun and Earth which results in slight variations in the timing of the phases.

Unlike some of the other durations provided, 28 days does not account for the extra part of a day that accumulates to finally give the 29.5-day cycle. Options of 30 and 31 days are longer than the actual lunar cycle, thus missing the precise timing necessary to accurately reflect the moon's phases.