Over the course of roughly a month, the part of the Moon that is illuminated goes through a regular cycle of phases. Starting from dark new moon phase, the Moon gradually brightens, or waxes, through waxing crescent, first quarter, waxing gibbous and finally to a bright full moon phase 14 days later. The Moon then dims, or wanes, over the next 14 days, going through waning gibbous, third quarter, waning crescent and finally dark new moon again. This cycle is due to the relative orientation of the Sun, Moon and Earth, and the dark portions of the Moon are always caused by its own shadow shielding the Sun’s rays. New moon phase happens when the Sun, the Moon and the Earth are in a line, so that the far side of the Moon we can’t see is lit up. Full moon phase happens when the Sun, the Earth and the Moon are in a line, so we see the sunlit side of the Moon. The other phases occur in between these two alignments, which are also called syzygy.
Halfway through our lunar cycle, the Moon and the Sun are now on opposite sides of the Earth, so we see a fully sun-lit full Moon. At this time, the Moon rises as the Sun sets, and the Sun rises as the Moon sets. This alignment of Sun, Earth and Moon is called syzygy (pronounced “SI-zi-gee“) and occurs during both new and full moon. This alignment magnifies the ocean tides on Earth (both the Sun and the Moon cause tides through gravitational force), and the full moon is a time when lunar eclipses can occur, about once or twice a year. Imagine standing on the Moon at this moment; you would see a “new Earth”, or the dark side of the Earth, up in the sky. Similarly, you would see a “full Earth” during the period of new moon. In many ways, full moon is a period of opposition. It is also a period of celebration, with many holidays occurring during full moon in tradiational cultures, such as the Chinese Lantern Festival, the Hebrew Passover, and the Muslim Shab-e-Bara’at. The bright night also inspires full moon parties. Have fun celebrating our closest celestial neighbor!
This Astrofact is dedicated to Mahina, our four-legged full moon.