FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Moon Dance

Just a couple Saturdays ago, on the 19th, you may have witnessed the sublime site of Venus nearly touching a young crescent Moon in the western skies after sunset.

It was a gem of an experience. But if you went out the next evening to see it again, you may have noticed that things were not exactly the same, that the Moon was actually a little higher in the sky and had a slightly fuller crescent.

What's all this then? What you experienced is the movement of the Moon around our planet, movement that gives us those phases.

We are all familiar with the Crescent Moon and Full Moon and Quarter Moon (often called Half Moon). But few of us actually sit down and wonder why these things are.

We all learned in grade school that the Moon goes around the Earth. Many of us mistake its movement through the night sky, side by side with the stars, as the Moon orbiting around the Earth. But that movement is an illusion; it's just our planet spinning below the sky, the same fata morgana that "moves" the stars and the sun through the heavens.

The Moon really moves the other way, from west to east. But it moves slowly, very slowly, taking a whole month to complete one circuit. Test it! Go out tonight and notice exactly where the Moon is at a certain time. Go out tomorrow night at the same time and see that it has moved about a handspan eastward.

Now this journey is why we have all those phases of the Moon. Here's how. Imagine the Moon in the part of the sky right near the Sun. The lit side of course is facing the Sun, its dark side is facing us. We cannot see the moon because of this. We call it New Moon because a new cycle has begun.

Now let's fast forward two weeks, halfway through its cycle when the moon is on the opposite side of the sky, opposite the sun. Now it is fully lit from our point of view. It is a Full Moon.

The phases we are accustomed to result in the lighting of the Moon as it goes from New to Full and back again.

For example: In the next days after New, as the Moon passes by the Sun, it is at such an angle from us that we can just begin to see its lit side, but just barely. That is the crescent Moon, a thin one to be sure.

As the Moon moves around us it appears to be getting more fully lit. It is "growing" or, as we say, waxing.

After a week of a waxing crescent, it is now half lit from our point of view and directly above at sunset. Some call this half Moon. It is a quarter of the way around us, so astronomers call it First Quarter Moon.

It has lost now its crescent look. For the next week, until Full Moon, it will appear more football-like. This is called a gibbous Moon - a waxing gibbous to be more exact.

The Moon will continue to "fill out" as it goes to the other side of the Earth.

Once it is full, growing time is over. Now it is swinging around back towards the sun again, and its lighted part will appear to be getting smaller - it is now a waning moon.

After two weeks of waning from Full to Gibbous to Third Quarter to Crescent, the Moon will begin the cycle all over again.

Did you notice that during the full circuit that the lit part was facing the setting sun as it was waxing, and that it faced the soon-to-be-rising sun as it was waning?

None of these facts were lost to, well, probably every people group that has gone before us. Knowing the Moon and its phases was a great timekeeper and calendar source for a thousand generations of people.

Maybe for you, too! With just a little practice you can start predicting when what phases will be present. After a while you can put that watch away, you'll be able to estimate the time of the night just by looking at the Moon, just like our ancestors did.

Until next time, clear skies!
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me