Waxing Your Knowledge About the Moon
The Solar System >
It's been my experience in teaching and sharing astronomy with people that one of the most common everyday occurrences - the changing face of the Moon --- is also one of the least understood.
The best way to show the Moon's phases is for you to demonstrate it for yourself. You might even make this a family thing, or use it in a homeschooling situation. You'll need all of one golfball, a strong direct light source like a flashlight, and a darkened room.
The strong light source represents the sun. The golfball represents the Moon. Your head is the third heavenly body here --- the Earth.
Hold the golfball at arm's length. Hold it in the same direction as the strong light source, a direction which we'll refer to as the traditional "12 o'clock" position. Notice that it isn't too easy to see the ball due to the blinding light of our "sun." This is what is called New Moon, and is the time when solar eclipses can occur.
Slowly orbit the tiny ball to the left (counterclockwise) to about the 11 o'clock position. You should see just a sliver of a crescent Moon. We always see this phase just after the sun has set.
Continue now to the 9 o'clock position. See how half the ball is lit up, and the other half is in shadow? It has taken a week to go a quarter way around the Earth, to reach this phase called First Quarter.
Notice also that the divots in the golfball are strikingly detailed. This is the phase which, when looking at the Moon through a telescope, is the most fascinating. The sun casts long shadows along the day/night line that cuts through the Moon and vividly reveals the stark details of lunar valleys and craters.
Moving along to the 8 o'clock and 7 o'clock positions notice that the golfball-moon is growing in brightness --- it is a "waxing" Moon. The lit part of the Moon appeared to the ancients as a hump; the Latin for hump is gibbous. Thus, in astrospeak, we are seeing a waxing gibbous. And that is the Moon we have at the moment. Look for it this evening!
We now come to the 6 o'clock position, two weeks through our orbit, to a point on the opposite side from the sun. You should see a fully lit golfball-moon, a "Full Moon." Notice also that the dramatic shadows are gone. So it is with the very bright, but very flat, Full Moon that we'll be seeing a couple days from now.
Going on through the rest of the trip from the 5 to 4 to 3 o'clock position we see the lit side getting smaller and smaller from our point of view. We are now in the "waning" phases of the Moon.
At the 3 o'clock position --- three weeks and three quarters of the way around now --- we have the Third Quarter Moon. Few of us see this phase because it doesn't even rise above the horizon until after midnight.
We finish our orbit with a very "old" Moon, nearly back at the 12 o'clock location again, with a tiny sliver seen only before sunrise and just days before it gets back to New Moon.
This whole show takes about 29.5 days, which is essentially a month. Can you now guess where the word "month" comes from? "Month" is a word derived, to no one's surprise, from the word "moon," and reflects the time taken by our lunar companion to complete an entire roundtrip flight.
Until next time, clear skies!
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2003.06. 7 01:28 PM | Comments (0)
