# 59. ASTRONOMICAL UNIT

SUBJECT: Science
GROUP SIZE: Small
TIME: 45 minutes
TYPE OF ACTIVITY: Student Activity Modeling
TEACHING STRATEGY: Expository Guided Discovery
CONCEPTS: Astronomical Unit Solar System Distance
SKILLS: Modeling Measuring Plotting Data
Objectives: To gain experience with the concept of Astronomical Unit (A.U.); to construct a scale model of the solar system using A.U.’s.

planets – relative sizes

Materials: 1 meter stick per group; tape measure 40 meters long (optional); 1 adding machine tape per group; pencil.
Teacher Background Information:

In the study of astronomy, astronomers use a convenient unit of measurement known as an Astronomical Unit or A.U. The A.U. is a short cut in describing distances in the solar system. The distance between the Sun and the Earth is, on the average, 149,600,000 kilometers (93 million miles.)

Astronomers call this distance between the Earth and the Sun 1 A.U. An object twice the distance away would be called 2 A.U.’s away and so on. The A.U. is really a scale showing distance. The scale is 1 A.U. = 149,600,000 kilometers.
Procedure:

1. Divide the students into groups of 4 or 5.
2. In a hallway, a gym or outdoors have each group extend their 40 meter adding machine tape.
3. Identify one end of the tape as the Sun.
4. Measure one meter from the Sun end and label that point as the Earth.
5. Use the diagram below to mark the positions of the rest of the planets and the asteroid belt as fractions or multiples of 1 A.U.
PLANET A.U.’s
MERCURY .387
VENUS .723
EARTH 1.000
MARS 1.524
asteroids 2.800
JUPITER 5.203
SATURN 9.523
URANUS 19.164
NEPTUNE 29.987
PLUTO 39.370

Extensions

Once the students have the relative distances on their tapes, try to find a place in the school where the scale can be permanently displayed. Arrange to paint the planets names and/or images in proportional sizes at the given distances.

NOTE: It is nearly impossible to get the size of the planets and the distances proportional to one another at the same time in a school building. The relative sizes in the illustration below, while proportional to one another would require that Pluto be nearly a mile from the Sun. Activity # 45 in the K-6 ‘Come Fly with Me’ manual gives an easy to use scale of sizes.

If a larger scale is desired, why not have the students compute the distances and make arrangements in your town to place a picture or a notation about the scale and the project at a building at the appropriate distance from your “Sun” at the school.

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