The Monthly Cycle of the Moon
- Friday, June 11, 2004
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.
Recently on Homeschool
Have something to say about this article? Leave your comment via Facebook below!
Listen to Your Favorite Pastors
Add Crosswalk.com content to your siteBrowse available content