Labs from YSI 94 :
Our Sky Clock.
CARA Yerkes Summer Institute, August 1994
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.
- identify Earth as a rotating ball
- explain how the Sun or stars may be used to tell time
Paper fasteners, worksheet, crayons, scissors
Overhead: Worksheet transparencies
Slides: "Big Dipper" and "Little Dipper" ("Bear")
Important Disclaimers and
- Discuss, with models and student volunteers, if possible:
- Cause of night & day
- Earth's constant rate of rotation
- 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
- Water clocks? Disadvantages in cold weather?
- Sun dial? Limitations of weather and night?
- 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")
- We can use the stars
- We can "tell time" if we know the position of the "Big Dipper"
and the pointer stars
- 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
- 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
- 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.