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Time: Day Lengths and Time Zones

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Author: Janice VanCleave

Sidereal time and solar time are the systems used for measuring the passage of time. While these systems only vary by about 4 minutes, this small difference is significant when considering long spans of time.

In this project, you will model Earth's movement during one sidereal day and one solar day to compare the differences between these two time measurements. You will calculate the angular difference between the position of a meridian at noon on a sidereal day and on a solar day. You will design a map showing the Earth's time zones. You will learn about the difference between local "sun time" and standard time, and how to experimentally determine the difference where you live.

Getting Started

Purpose: To model a sidereal day.

Materials

  • marker
  • sheet of typing paper
  • ruler
  • lemon-size piece of modeling clay
  • toothpick
  • pencil
  • flashlight

Procedure

  1. Place the paper sideways, and draw a 6-inch (15-cm) horizontal line across the center of the paper. Label the line A. Two inches (5 cm) above this line, center a second line that is 5 inches (7.5 cm) long and label it B.
  2. Shape the clay into a sphere.
  3. Use the pencil to draw two circles around the clay sphere. Make the two circles perpendicular to each other at the top and bottom of the sphere.
  4. Break the toothpick in half.
  5. Time Day Lengths and Time Zones

  6. Insert one of the broken ends of the toothpick into the center of one of the four line segments drawn on the sphere. Discard the other half of the toothpick.
  7. Insert the pencil through the clay sphere from top to bottom until just its tip sticks out the bottom.
  8. Lay the paper on the table so that the labeled ends of the lines are to the right. Turn on the flashlight and place it at the left end of line A.
  9. Hold the pencil and position the clay sphere at the right end of line A so that the toothpick is parallel with the line and points toward the flashlight (see Figure 7.1).
  10. Holding the pencil, rotate the sphere counterclockwise one whole turn as you move the sphere to the right end of line B. Position the sphere so that the toothpick is parallel with line B. Observe the direction the toothpick points in relation to the flashlight.
  11. Note: Keep the paper, flashlight, and sphere for the following experiment.

Results

Moving from line A to line B, the sphere makes one complete rotation. The toothpick points toward the flashlight when the sphere is at the end of line A, but doesn't point toward the flashlight when the sphere is at the end ofline B.

Why?

The circles on the clay sphere represent meridians on the Earth. The toothpick is used to mark one of the meridians so its position and movement can be tracked. When the sphere is on line A and the toothpick points toward the flashlight, the meridian is in line with the Sun and the time is twelve o'clock in the daytime, or noon. At noon, the Sun crosses the local meridian and is at its zenith. The turning of the clay and the movement of the sphere from line A to line B represent the Earth's rotation on its axis and movement along its orbit around the Sun during one sidereal day (time it takes for a celestial body to make one complete rotation on its axis). A sidereal day for Earth is about 23 hours 56 minutes.

The sphere, like the Earth, must make slightly more than one whole turn before it points toward the Sun and it is noon again for anyone meridian. To an observer on Earth, it appears that the Sun moves across the sky. Thus a solar day can be defined as the interval from the time the Sun crosses a meridian on Earth to the time the Sun returns to that meridian. In other words, it is the time period from noon of one day until noon of the next day. For timekeeping, we use an average of the solar days over a year, called a mean solar day, which is equal to 24 hours.

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