The Monthly Cycle of the Moon
- Jay Ryan
- 2004 16 Jun
Like many homeschool parents, I had a public school education. And my classrooms didn't treat the subject of astronomy very well. Most of my meager astronomy lessons were concerned with "astro-facts," little nuggets of information about astronomy such as the "the Sun is a big ball of hot gas," "the Earth is 93 million miles from the Sun," and so forth. I'm sure most of you have had a similar experience, since this is the manner in which the subject is typically mishandled in our generation.
One astro-fact that I learned in school was that "the Moon orbits the Earth once each month." While this fact on its own may be terribly fascinating to some, I was never properly taught how to go outside on a clear night and apply this information to sightings of the bright, shiny Moon. In general, typical astronomy education fails in helping us correlate such astro-facts with the actual appearances of the sky, as seen from our backyards. And one of the main purposes of this Update is to fill that gap.
We can see observe the Moon's orbit around the Earth by following the cycle of the Moon's phases. Each month, the Moon appears to grow to a Full Moon and shrink back again. Many wall calendars indicate principal phases of the Moon. This is a surviving relic of the days when everyone read the Almanacks to tell time, track the seasons and follow interesting celestial events in the sky.
Many wall calendars will provide the date of the New Moon. This phase is symbolically depicted on the calendar as a black circle on a particular date. When the Moon is New, it cannot be seen from anywhere on the Earth. The Moon at this time is very close to the Sun, and lost in the Sun's glare.
At all times the Moon has a bright side and a dark side since the Sun shines on the Moon just like the Earth. Half of the Moon always has a brightly lit up "day" side and half the Moon is in the shadow of its "night" side. When the Moon is New, the entire bright side is facing toward the Sun, and the dark side is facing toward the Earth. So even if the Moon was not invisibly hidden by the Sun's glare, it would still look like the black circle shown on wall calendars.
But as the Moon moves through its orbit, it moves in a circular direction around the Earth toward the East. So in the days after the New, the Moon can be seen hanging in the western sky after sunset. And at this time, the bright side of the Moon is still mostly turned toward the Sun. But since the Moon is not completely lined up with the Sun, a little tiny edge of the bright side can be seen, facing in the direction of the sunset. This is why the Moon is in a "crescent" phase in the days after the New Moon, appearing as a bright little fingernail in the evening sky.
As the Moon continues to move toward the East, it appears to move away from the Sun. We can then see a little more of the Moon's bright side, and the crescent becomes thicker. With each passing night, the Moon's crescent looks a little thicker and a little further away from the sunset. During this period, the Moon is said to be "waxing" in phase, an old-fashioned word that means "increasing."
After about a week, the Moon is more or less "alongside" the Earth. Half of the Moon's bright side can be seen but half of the dark side is still visible. The Moon's thus appears as a half moon in the evening sky. At this time, the Moon has completed about one-quarter of its orbit. So the evening half moon is called the "First Quarter." Wall calendars will indicate this phases by name or by a little circle, with a white half and a black half. The First Quarter Moon is quite high in the evening sky and quite far from the sunset.
After the First Quarter, the Moon's circle carries it somewhat behind the Earth, revealing more than half of the bright side. The Moon looks like a half moon with an extra hump onto the dark side. For this reason, this phase of the Moon is called "gibbous," a term which comes almost directly from the Latin word "gibbus," which means "hump." The gibbous Moon can often be easily seen rising in the East in the evenings before sunset.
At mid-month, the Moon is "behind" the Earth in its orbit, in the position opposite the Sun. At this time, the entire bright side is turned toward the Earth, and so the Moon is "Full." Wall calendars show the Full Moon phase as a white circle. Since it is opposite the Sun, the Full Moon rises in the East as the Sun sets in the west. After rising, the Full Moon can be seen in the sky the entire night. If you're up a couple hours before sunrise, you can see the Full Moon setting in the west as the Sun once again rises in the east.
After the Full Moon, the Moon again begins to move out from behind the Earth. At this time, the Moon is "waning" which means it is "decreasing" in phase. The Waning Moon will draw a little closer again to the Sun with each passing day, and it will then become a little thinner in phase. Also, in the days past the Full Moon, the Waning Moon rises after the sunset. You can see it rising later and later each night, an average of 48 minutes later than the night before.
In this way, the Waning Moon is still visible in the morning sky before the sunrise. After the Full, you can see a waning gibbous Moon in the morning sky around sunrise. However, in the waning phases, the Moon's bright edge is the opposite from that seen in the waxing phases.
About a week after the Full Moon, the Moon has once again waned to a half moon visible high up in the morning sky. By this time, the Moon is three-quarters finished with its monthly cycle of phases, and this phase is thus called "Last Quarter." The Last Quarter Moon is shown on wall calendars as a circle half black and half white. But in this phase, the white and black halves are on opposite sides from how they were shown at First Quarter, to correspond with the actual appearances of the Moon.
And after Last Quarter, the month draws to an end as the waning crescent of the old Moon moves a little closer to the sunrise each morning, growing a little thinner in phase. The Moon's cycle is completed as it vanishes in the morning glare of the sunrise, and at the next New Moon another lunar month begins.
The cycle of the Moon's phases is one of the topics illustrated in "Cycles," my educational comic book, now available from the Classical Astronomy Store. Click here: http://store.classicalastronomy.com/shop/view.php?id=1
Jay Ryan is the author of "The Classical Astronomy Update," a free e-mail newsletter for helping Christian homeschool families learn more about events in the starry sky. If you would like to receive the Update, please drop Jay an e-mail at firstname.lastname@example.org. Visit the Classical Astronomy web site – www.ClassicalAstronomy.com