Observing the sun's seasonal drift.

Let's simplify the problem: we don't HAVE to get up at dawn every day to see where the sun rises on the horizon. You probably don't have a clear view of the eastern horizon anyway! Even simplified, this project takes substantial patience, so it will count as two regular projects of class credit.

So what we do is:

  • Pick a time of day that is convenient.
  • Set up a "shadow observatory".
  • At the chosen time each sunny day, mark where the sun's shadow falls on the sheet of paper and label by the date.

    • In more detail, (1) To pick a time that is convenient, first pick a window where your observatory will sit. The observatory cannot be moved over the course of the semester! The observatory can be outside. If at a window, the window cannot face north (no sun). An east window can be used only in the morning, a west window can be used only after noon - follow me? The convenient time must also fit in with your schedule - near a meal is probably best. (2) Make your observatory, something like this:


    Never look at the sun!!!! You will be tracking the shadow of some object like the tip of a pencil (gnomon) or a suspended bead as it falls on a sheet of paper. (If you make an outside observatory, maybe mark the corners where the sheet should be, and insert the paper for each observation.) (3) At the specified time, mark the location of the shadow and label it (tiny!) by date.

    Ironclad rules: (A) observations must be made within 5 minutes of your chosen time. (B) the farther away your shadow-casting object (bead or pencil) is from the paper the better. 6 inches is absolute minimum - at that scale you will be writing very tiny. 12 to 18 inches is better. (C) The observatory must not be moved (or must be replaced in exactly the same place each time). "Exactly" means less than 0.2 inches (for a 6-inch object-paper separation) movement between observations. If somebody bumps the observatory (drat!), fix it like this: during the next observation time, align it so that the sun shadow lines up on the last marked position. The sun doesn't drift far in a day or two.

    Polish: When you get ready to hand in this project, put the following finishing touches on it. Mark North-South-East-West directions on it as best you can (should be easy if you are near or inside a building, since they are always aligned N.S.E.W.). For at least one of the observations, trace the path of the shadow over plus and minus one hour from the time of observation. Measure with a ruler the distance from the paper to the object and include this number in your report. Question to answer: your observations stretch about 1/4 of a year; predict using your astronomical acumen what your sun-shadow will do the rest of the year. Question to answer: explain how you could tell a farmer when to plant, even if all the world's calendars spontaneously combusted.

    Alternative Observing Strategies: (1) Dorm room ceiling projection: put a small bit of mirror on the window sill or floor and chart the progress of the reflected spot of light on the ceiling or wall (tape or tack paper there). (2) Exploit the architecture: use the shadow cast by a building cornice or other projection on the grass. Mark the spots with labeled toothpicks or popsickle sticks. Such a large baseline will yield excellent results. Mark the final result on butcher paper. Or take a photo.

    Last modified: Thu Aug 24 14:25:43 CDT 2000