Labs from YSI 94 :
Our Sky Clock.

This is a Teacher's Guide.

Students identify events as regular (such as the seasons, most holidays and moon phases) or as irregular (such as weather and the varying rates of heartbeat and breathing). Observations of two star patterns in the nighttime sky show some regular, repetitive events that can be used to "tell time." Earth's motion appears to make the "Big Dipper" rotate around the North Star approximately once every 24 hours. Students construct "sky clocks" using the Dippers and Polaris to discover how they can tell time by observing stars in the nighttime sky.

Objectives

• identify Earth as a rotating ball
• explain how the Sun or stars may be used to tell time

Materials Needed

Settings

Procedures

1. Discuss, with models and student volunteers, if possible:
   • Rotation
     • Cause of night & day
     • Earth's constant rate of rotation
   • Revolution
     • Length of year
   • Time
     • What happens once a second, a day, a year?
     • Develop idea that measuring time requires a something regular
     • Can we use :
       • Heartbeats? Why not? (Demonstrate by timing one minute while the class counts heartbeats. Speculate about the effect of exercise.)
       • Water clocks? Disadvantages in cold weather?
       • Sun dial? Limitations of weather and night?
2. Discuss constellations
   • Note the distinction between asterism and constellation.
   • With slide projector (or student-made projectors or other activities), identify shape of Big Bear (noting the "Big Dipper") and Little Bear ("Little Dipper")
3. We can use the stars
   • We can "tell time" if we know the position of the "Big Dipper" and the pointer stars
4. Construct star clocks
   • Distribute worksheet, scissors, paper fasteners.
   • Using star wheels, or recalling planetarium experience, trace the Little and Big Dippers and the direction to Polaris from the pointer stars on the dial.
   • Color-code the chapter ring for daytime and nighttime hours. Paint dipper stars and Polaris with glow-in-the-dark paint. Cut out the dial and chapter ring.
   • Use the paper fastener to hold the dial to the chapter ring.
   • Note clock goes counter-clockwise. Observe that Earth's surface travels West to East. (Or see if anyone notices this and comments or asks.)
5. Demonstration of use
   • Note the current month and that the star clock is square, with no particular "correct" orientation.
   • "Hang" the sky clock from appropriate month by holding it with two fingers.
   • Match position of the "pointer stars" on star clock with appearance of real sky.
   • Note time setting for specific dates and position of the Big Dipper relative to Polaris and the horizon. Demonstrate with overhead projector or better yet, nighttime sky.

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