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 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.
In the center of the constellation Scorpius lies the bright red star Antares, the 16th brightest star in the night sky. Antares is what is known as a red supergiant – its surface is cool and hugely extended, 800 times larger than the Sun. Indeed, if Antares were in the Solar System, it would engulf Mercury, Venus, Earth and Mars. Stars become red supergiants at the end of their lifetimes, when they have exhausted most of their hydrogen fuel, and they on their way to a spectacular stellar death in the form of a supernova. The red supergiant stage of a star’s lifetime is relatively short – a few hundred thousand to a million years. In contrast, the light we see from this star was emitted 600 years ago, since it takes light 600 years to travel the 600 light-years of distance between us and Antares. So it is possible that Antares is no longer there. Ponder that while you’re enjoying your view of Scorpius tonight!
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!