Solar Noon

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Determine the clock time corresponding to solar noon. This is trickier than you might think, and may require several tries before you get it right. I’m happy to give a week’s extension on this experiment, but do make a first stab at it during the second week of classes, so you have a sense of the challenges and can complete the project in time.

Full details in Fabian, Ex 2.1, “Gnomons and Shadow-Casting,” pp 33-34. You need to find a spot where the sun shines at noon and where the ground is level. An outdoor patio or bench is probably ideal: sidewalks and parking lots are often built on a grade, and the sun’s light may bend as it passes through glass to reach an indoor location.

Start by guesstimating the time of solar noon by looking up the time of sunrise and sunset in your location. Set up your apparatus 10 minutes before this time, and record the position of the gnomon’s tip at 1-minute intervals.

Apparatus:

  • Solar Noon radial graph paper, ruled in cm. Download and print.
  • A gnomon with a sharp tip—a sharpened pencil or the like. If possible, bring a range of possible gnomons, choosing one that casts neither too long nor too short a shadow—no longer than the ring marked 9 and no shorter than the ring marked 5 on the sheet.
  • Clay, gum, or scotch tape to anchor the gnomon at the center of the worksheet.
  • Scotch tape to anchor the worksheet.
  • A smartphone with a compass app.
  • If possible: a carpenter’s level. Otherwise, something rectangular to serve as a right angle tool.
  • A pencil to record your measurements.

Instructions:

  1. Find a surface with good sun exposure. If available, use the level to check that the surface is level. Otherwise, do your best to check it by eye. Flat concrete benches are likely level; sidewalks generally are not.
  2. Use the compass on your phone to align the paper so that the arrow at the top points due West. Anchor the worksheet to the surface with scotch tape.
  3. Anchor the gnomon at the center of the sheet, using the level or right angle tool to ensure it is vertical.
  4. Take a photograph of your experimental apparatus in action.
  5. Record measurements at 1-minute intervals, starting about 10 minutes before your guesstimate of solar noon, and finishing 10 minutes after. For each, mark an X on the worksheet at the tip of the gnomon’s shadow along with the clock time of the measurement.
  6. If you’ve done everything right, the gnomon’s shadow should get shorter, reach a minimum, then grow longer. The mark you made at the minimum represents Solar Noon.

Calculations:

  • Reading from the mark when the gnomon’s shadow was shortest, what clock time corresponded with Solar Noon? How many minutes is Clock Noon off from Solar Noon, positive or negative, in the city where you did the experiment (specify your city in your answer)? Would you need to travel East or West in your Time Zone to be in a place where Solar Noon coincides with Clock Noon?
  • Use a ruler to draw a line from the center of the worksheet to the mark corresponding to Solar Noon. That line is True North, by contrast to the 0° mark which corresponds to Compass North. Using a protractor, or estimating from the angle measures provided on the sheet, how many degrees does Compass North deviate from True North, positive or negative, in the city where you did the experiment?
  • Calculate the ratio between the length of the shadow at Solar Noon and the length of the physical gnomon you used. From this you can calculate the latitude of the city where you live. Tangent (Latitude) = Shadow / Gnomon. You’ll need a calculator with the inverse tangent function—make sure you get an answer in degrees (DEG), not radians (RAD).

Upload your results in a comment below, attaching your a photo or scan of the completed worksheet and (in a reply to your first comment), the photo you took of your experimental apparatus.

31 responses to “Solar Noon”

  4. This experiment beautifully demonstrates how patience and precise timing yield accurate results—qualities equally valuable in strategic gaming. Just as tracking solar noon requires careful observation, successful players know that mastering timing patterns creates winning opportunities. champion zone link

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  27. Розы в облаке из живых цветочных лепестков – роскошное расточительство!

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