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.
“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!
Maui astronomer Harriet Witt describes the First Quarter Moon and brings you out into space, so you can see how lunar phases work. So take sometime out of your busy day, take a deep breath, and let Harriet bring you on a journey through the night sky.
Listen here [4:24m]:
What’s the facts:
It is called the First Quarter Moon, because that is what the moon looks like to us here on Earth (1/4 of a big round ball). To us, it seems as though the stars, the planets, and the moon, change throughout time, but really, we are turning, taking a different view of the night sky as we do. Our lunar phases are relative to the Sun (our source of light), and changes as the moon follows its orbit around the Earth, and the Earth spins and follows its orbit around the Sun. It’s pretty complicated once you think about it!