Making Astronomical Measurements

Making Astronomical Measurements

Science Bus lesson plan for 2nd- 4th graders, 11/13/03

The purpose of this lesson is to give the students an idea of how astronomers can measure the properties of planets, moons, and stars that are far away. The students will calculate the size of the moon and the distance from the earth to the moon. Start by asking the students how they think we know how big and how far away the moon and the sun are. Once you’ve heard some of their ideas, tell them that today we are going to calculatethe size of the moon, using a method developed by the Greek astronomer Aristarchos around 300 B.C.

Part 1: Size of Moon

Divide the students into groups (one group per tutor), and pass out a “Worksheet 1” and a pen to each student. Each group should also have a flashlight, a Styrofoam ball, a ruler, and a set of cardboard circles. Help the students work through the worksheet. Try to let them work on each step for a little while as a team before you help them. Encourage each student to write down their answers on their own sheet, even though they are working out the answers as a group.

The goal of question 2 is that the students figure out that if the ball is much closer to the wall than to the flashlight, and the flashlight is shining perpendicular to the wall, then the shadow the ball casts on the wall will be about the same size as the ball itself. Tell them that these conditions apply during an eclipse, so the size of the earth’s shadow on the moon is about the same size as the earth.

If the students have trouble with question 3, have them draw a circle to represent the moon, and then draw a second, partially overlapping circle to represent the earth’s shadow. They can trace the cardboard circles to make it easier. The idea here is for them to see the difference in curvature of circles of different sizes.

For question 4, the students should compare the cardboard circles to the moon and the earth’s shadow in the picture of the eclipse. Once they find circles that fit, they should measure them to determine their relative size. They should find that the earth is 3-4 times larger than the moon. (The actual value is 3.7 times, but don’t correct them if they are in the ballpark.)

Wrap-up

If the students aren’t too antsy by this point, bring the class together and have them tell you their measured values for the size of the moon. Talk a little about why there is variation in the numbers, and tell them that the variation gives them an idea of how accurate their measurement is. Ask them how the measurements could be improved.

Part 2: Movement of the earth around the sun

This part of the lesson is to make sure the students understand how the earth moves around the sun, and to let them get outside and run around a little bit. This part should also be done in groups.

Start by designating someone as the sun (maybe the tutor) and then have the other group members spread out in a circle around the sun. Have them walk (or run!) around the sun in a circle. If they have trouble making an even circle, then have the sun hold one end of a long piece of rope, and the earth hold the other end as they orbit around. It’s important that they see that the distance between the earth and the sun is constant. Tell them that the motion they are demonstrating is called orbiting. Ask them how long it takes for the earth to orbit once around the sun (1 year).

Now have them stand in place and turn around. Tell them that this is called rotation. Ask how long it takes for the earth to rotate once around (1 day). Have them all stand facing the center of the circle. Tell them that their noses are North America. Ask them, based on the way they are standing, what time is it in North America (noon). Now have them face away from the sun, and again ask what time it is (midnight).

Now designate a far-off landmark as being Polaris, the North Star. Tell them that the earth tilts towards Polaris. Have them tilt their upper bodies towards Polaris (tilt should be about 20 degrees. See if they can rotate while tilted, keeping their head pointed at Polaris. This could lead to a lot of silliness, which is ok. Let them have fun with it. See if they can put everything together and orbit, rotate, and tilt at the same time.

Now have them orbit while tilted, but leaving out the rotation for sanity’s sake. Have them stop in different parts of the circle and ask them what time of year it is in North America (summer when their heads are tilted towards the sun, winter when they are tilted away).

If there is still time left, call out different times of day and of year, and see if they can demonstrate how the earth would be positioned at that time.