Toilet Plunger Sundial

Activity:
Creating a sun dial for measuring daily time, annual time, and solar altitude.

Science Standards:
Nature of Science and Patterns of Change

Objectives:

  • The student will create a sun dial.
  • The student will calibrate the sun dial for local time.
  • The student will measure local noon.
  • The student will determine the altitude of the Sun.
  • Materials:

  • large, white poster board
  • one toilet plunger (any stick will substitute )
  • meter stick and pencil
  • scientific calculator
  • magnetic compass (for the teacher )



  • Introduction:
    Since antiquity, time has been measured using the moving shadow of a stick in the ground. This stick, called a gnomen , creates a shadow at sunrise. This shadow rotates "clockwise" until the sun sets in the west. The shadow is shortest when the Sun is directly in the South - thus defining local noon. Students will quickly discover that wristwatch time and sundial time are not the same. Sundials record local apparent solar time or where the Sun is is in the sky. The primary difference between clock time and sun dial time is because of time zones. The other difference is due to variations in the Earth's orbit around the Sun. The changing altitude of the Sun is how the duration of the year is determined.

    Directions:
    1. Construction and alignment of sun dial. Set a toilet plunger on the sidewalk or on a poster board and trace the circular bottom with chalk so that it can be returned to its original position. It works best to set the plunger on the south side of the walk or poster board. Mark the position of the very top of the shadow with an "x" or a sticker. Note the exact CLOCK time that the "x" was marked. Continue marking the top of the shadow every 30 minutes.

    2. Measure local noon. First, connect the "x"s with a curved, smooth line. The point that informed closest to the gnomen is when the Sun is directly in the south, local noon. Interpolating between the times of the known "x"s, determine the time of local noon.

    3. Calibrate the sundial. Using the known "x" times, carefully scale the arc to indicate 9, 10, 11, 12, 1, 2, and 3 pm. The tip of the shadow will point be on the west side of the paper or sidewalk during the morning. Note how the length of the shadow changes over the day. How might it be different in another season?

    4. Calculate the altitude of the Sun. The altitude of the sun is determined by finding the inverse tangent of the gnomen height divided by the shadow length.

    5. Alterations. Using masking tape, place a large "x" on a south facing window such that there is a shadow of the "x" cast on the floor. Using more masking tape, mark the shadow on the floor every hour as it moves easterly across the floor; label the time and date for each mark. The arc that is created will vary everyday, getting larger in the summer and shorter in the winter (shortest arc on Dec. 21). This "masking tape sundial" can be used to keep track of solstices and equinoxes.