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Apparent Movement of the Sun

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Author: Christopher Crockett

Every day, the Sun rises in the east, rides across the sky, and sets in the west. Once upon a time, people thought gods like Apollo carried the Sun on a chariot. But as it turns out, the Sun doesn’t move at all—we’re the ones doing all the moving.

The sun’s motion is apparent, caused entirely by the movement of the Earth. Our planet both spins on its axis and orbits the Sun. These two motions combine together to create the Sun’s apparent motion. But because the Earth’s motions aren’t as steady as we like to think, relying on the Sun’s apparent motion to keep track of time leads to all sorts of problems. For one, noon wouldn’t happen at the same time every day!

Our clocks don’t follow the Sun—they only do so approximately. The time on your watch is based off something called mean solar time, which is basically what time it would be if the Sun and Earth were a little more reliable. In this project, you’ll get to see just how far off our clocks are from the Sun.

Problem: Design a simple sundial.

You’ll use this to keep track of how “solar time” (based on the apparent movement of the sun) changes with respect to “clock time” over the course of a year.

Materials

  • Wide board (20” x 4” x 8” or larger)
  • Hammer and nail
  • Watch
  • Marker/pen
  • Compass

Procedure

  1. Draw a line down the center of the board. Label one end “North”, the other “South”.
  2. About 2” from the south end, hammer a nail partway into the line. Make sure the nail is perpendicular to the board.
  3. About an hour before noon, set the board down outside on a flat surface (don’t worry about getting north and south lined up right just yet). Over the next couple of hours, keep track of the length of the nail’s shadow and record the time when it is shortest. (You may need to adjust for daylight savings time—all times should be in “standard” time, so don’t forget to subtract an hour if you observe daylight savings!) The time you identify is “solar noon.”
  4. The next day, at solar noon, position the board so that the nail’s shadow lies along the line between “North” and “South”. The north end will be pointing towards “true north”—the direction towards the North Pole. This is slightly different from “magnetic north”—the direction a compass needle will point. We’ll use this difference to get the board lined up correctly from now on. Use a magnetic compass to record how much the needle deviates from true north (you can just make another mark on the board).
  5. Once a week, place the board down in the same place roughly an hour before noon. Use the compass and the marks you made to align the board with true north.
  6. Record the date and (standard) time that the nail’s shadow lines up with the line on the board (use a table like the one below). This will tell you the local time for solar noon.
  7. Repeat steps 5 and 6 for as many weeks as you like. Ideally, you need a few months worth of data to track how solar noon changes over the year. If you’re feeling ambitious, make a plot tracking the difference between solar and standard noon as a function of date.

Solar time vs. standard time data

Date

 

Clock time at solar noon

Difference between solar noon and standard noon; ahead (+), behind (-)

Jan 1

12:03 P.M.

-3

May 11

11:56 A.M.

+4

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