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Where is Moon at Sunset, Sunrise

Comprehension lesson in astronomy

by
Kenneth Fuller
(copyright information 2004)

I find that many students believe that Moon always rises at sunset and sets at sunrise.  This activity is designed to build a a understanding of the relative positions of Earth, Sun, and Moon associated with the various phases of Moon as seen from Earth.  NOTE: A lunar month is about 29.5 days, one revolution around Earth takes 27.3 days.

My practice has been to have students copy onto blank paper as I draw freehand on an overhead transparency.  If you prefer, you may make a transparency of the student worksheet and have students copy on blank paper or individual copies of the worksheet.

Presentation:
After each step has been presented, make a quick check of the class to insure that each has the idea and is staying with you.

Now!  Everyone has a pencil and prepared (name etc.) paper, true?  Ok.

This line represents Earth [drawing or indicating the horizontal line in center].  As it normally appears to us, Earth is flat and I'm always in the center. [draw stick figure etc.]  Here's my house, and I have a tree in my yard.

The sky is a bowl over head [drawing circle].  The time of day is sunset [write "sunset" above the right end of "Earth"].

Remember, while you are watching Sun set, someone on the other side of Earth is watching Sun rise. [Rotate the drawing 180o and draw another stick figure.  write "sunrise" above the left end of "Earth".]

[Rotate the drawing to original position.]  If Sun is setting, and Moon is at new phase, where in the sky will Moon be?
Since at new moon Moon is in the same direction as Sun, Moon will set at the same time Sun sets. [draw circle at the right end of "Earth" and darken it completely.]  We do not see any of the lighted half of Moon from Earth.

About half a week later, what is Moon's phase?  Right, it will be waxing crescent (waxing means "getting bigger"), and it will be in the southwest sky. [draw a circle at about 45o darken all but a crescent on the side toward Sun.]  Now we see a small part of the lighted half of Moon from Earth.

About one week after new moon, what is the phase?  Right, it will be first quarter phase, and it will be high in the south, even though you see a "half moon", it is the end of the first quarter of the lunar month. [Draw a circle at the top, darken the left half.]  Now we see half the lighted half of Moon from Earth.

A week and a half after new moon, what is the phase?  Right, it is waxing gibbous, and it will be in the southeast sky at sunset. [Draw the circle and darken only a crescent.] Now we see most, but not all the lighted half of Moon from Earth.

Two weeks after new moon, the phase is?  Yes, full moon, since it is in the opposite direction from Earth as Sun, at sunset Moon will rise. [Draw circle at left end of "Earth"] Now we see all of the lighted half of Moon from Earth.

Two and a half weeks after new moon, the phase is?  Right, waning gibbous (waning means "getting smaller").  And where is it in the sky at sunset?  It's not in the sky?  Moon will not rise till some time after Sun has set.  But your opposite, on the other side of Earth, who is seeing Sun rise also sees the waning gibbous moon in the southwest.  [Rotate the diagram, draw the circle and darken a crescent on the side away from Sun.]

Three weeks after new moon, the phase is?  Yes, third quarter phase, meaning that three quarters of the lunar month have passed.  And where is Moon?  At sunrise Moon is high in the south. [draw the circle and darken the right half.]

Three and a half weeks after new moon, now the phase is?  Yes, waning crescent.  Where?  Southeast at sunrise. [draw the circle and darken all but a crescent.]

About four weeks after new moon, what phase is it?  That's right!  It is new moon again! the end of one lunar month and the beginning of the next lunar month. [Pointing to the first circle.]  And at sunrise, as Sun is rising Moon is also rising.  Even though you will not be able to see it, Moon will be in the sky all day.

diagram of Moon at sunset JPG
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