Hold your thumb up at arm's length in front of you. Imagine lines drawn from either side of your thumb to a point on the pupil of your eye. The angle formed by these two lines is a kind of measure of the width (diameter) of your thumb, known as the Angular Diameter. Hold your thumb up at arm's length again, and move it closer to your eye. What happens to the angular diameter? As you can see, the value of an objects angular diameter depends upon the distance from the observer to the object. You are going to use this same idea to measure the diameter of the moon.

MATERIALS NEEDED: Meter stick, pencil, index card, thumbtack, hole puncher

PROCEDURE1-Punch a hole in the center of an index card.

2-Thumbtack the index card on the zero end of the meterstick so that you can see along the length of the stick when you look through the hole.

3-Point the stick at the moon.

4-Slide the pencil back and forth along the meterstick, until the thickness of the pencil just covers the width of the moon. If you have short arms, you may need someone else to slide the pencil back and forth.

5-Read the distance from the pinhole to the pencil, to the nearest tenth of a centimeter.

6-Measure the diameter of the pencil, to the nearest tenth of a centimeter.

7- Using your textbook, look up the distance to the moon in kilometers.

 Using the data above, set up a proportion and solve for the diameter of the moon. You must show your proportion and how you solved it.