Waxing Moon vs. Waning Moon

moon-waxing-waning1Questions:

1. In the picture, which moon phase is waxing, A or B?

2. In the picture, which moon phase is waning, A or B?

Answer:

From Earth, the Moon looks like a round disk in the sky. At times this disk appears to have part of it cut away or is overlapped by something that is dark.

The edge or line where the lighted part of the disk ends is called the terminator.

Moon A

What do you expect the Moon A in the picture to do during the next few days? Will the lighted part get bigger (wax) or get smaller (wane)?

The diagram shows the lighted part of Moon A decreasing from November 8 to November 12. This means the Moon is waning.

Look at the terminator for the Moon on November 8. Which way does the terminator move? I visualize the terminator being attached to a dark cover that is being pulled toward the left across the face of the Moon.

Moon B

What do you expect the Moon B in the picture above to do during the next few days? Will the lighted part get bigger (wax) or get smaller (wane)?

Look at the terminator for the Moon on day 1. Which way does the terminator move from day 1 to day 7? Again, I visualize the terminator being attached to a cover, bit this time the cover it light and is being pulled toward the left across the face of the Moon.

While this is all imaginary ideas, the one true fact is that the Moon’s terminator always progresses from right to left across the Moon Disk in the sky.

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