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.
The earliest astronomers may have been women, who discovered that the cycle of the Moon was synchronized with their menstrual cycle. Having an external cue for this internal cycle would have allowed women to time the birth of their children and perhaps help regulate the population of their tribe or clan in an environment with limited resources.
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.
“A month is a Moonth, a Moonth is a moon.” For many traditional calendars – Hawaiian, Chinese, Hebrew, Islamic – this is still true, although our western calendar has been tweaked to fit the months into one year. The new moon is the start of the lunar month, and the time when the moon lies between the Sun and the Earth (this is called syzygy, a great Scrabble word). Imagine yourself floating out in space well above the Sun, Moon and Earth, and you will see these three bodies in a row, with the Sun-lit side of the Moon facing away from the Earth. So where is the new Moon in the sky? Directly in front the Sun!
Why doesn’t the Moon block, or eclipse, the Sun every time the new moon phase happens? It’s because the plane of the Moon’s orbit is actually inclined by about 5° relative to the plane of the Earth’s orbit (the ecliptic plane). So most of the time, the Sun, Moon and Earth are out of alignment during new moon. However, the Moon’s orbit actually rotates, or precesses, every 27.2 days. So perfect alignment at new moon occurs about once every 18 years, a period the Babylonians called a Saros cycle (a complete explanation can be found here). In fact, there are several Saros eclipse cycles because the Sun-Moon-Earth alignment doesn’t have to be perfect, so we get about two eclipses at new moon every year.
New moon is a great time for star-gazing as the Sun and the Moon will have both set in the evening. So be sure to enjoy the dark skies that accompany the new Moon!
The waxing crescent is the first phase in the new Moon to new Moon lunar cycle. You can catch it by looking toward the western sky early in the evening; there you will see a bowl-shaped sliver pointing toward the setting Sun, following it down to the horizon. The opposite side is darker but not completely dark – it is faintly lit up by sunlight reflected from the Earth’s surface, called Earthshine. The origin of Earthshine was first figured out by Leonardo Da Vinci in the 1500s; scientists now use Earthshine to track global cloud coverage and variations in the Earth’s climate. The waxing and waning crescent phases are the best time to observe Earthshine, so enjoy our spotlight on the Moon!
The waning crescent, or “old moon”, can be seen shortly before dawn, a thin sliver that rises ahead of the Sun. You have a short period to catch it; after the Sun rises, the thin crescent is hard to see in the bright glare of day. The waning crescent occurs toward the end of the new Moon to new Moon cycle, a siderial period of 27 1/2 days if you measure the Moon’s position relative to the stars, or a synodic period of 29 1/2 days if you measure relative to Sun. The difference is due to the Earth’s motion around the Sun. During a “moonth” the Earth has traveled about 1/13th of its yearly orbit (at a rate of 1.3 million miles per day). So from our point of view, the Sun has moved to a different part of the sky relative to the stars – by about 28 degrees – over the lunar cycle. Every month brings a new perspective!
Progressing from the first quarter, the Moon enters its waxing gibbous phase, on the way to full bright moon. The Moon is showing more of its sunny side to us on Earth, and is taking on an egg-like shape. That’s why Hawaiians call the gibbous phase (gibbous is derived from the latin word “gibbus”, or “hump”) the “hua” or “egg” phase.