Sidewalk Chalk or Paper Sundials

Calculated for Columbus, Ohio

 

Sundials have been given a poor reputation by cheap garden varieties that are not accurate for any given date. But you can easily set up a sundial accurate to about 5 minutes during almost the entire sunlit hours!

Building a Sundial:

The following sundials were calculated for given dates in Columbus, Ohio. The black line on the sundial gives the relative length of shadows cast by the nail at particular times during the day. Sunset and sunrise correspond roughly to the last/first ten minute marker displayed.

To make your sundial, find a template for the date closest to today's date from one of 36 choices in this file (postscript, or pdf).Print it out and paste the page to a piece of cardboard. Push a small thin nail up through the center of the large dot. Make sure the nail is perpendicular to the paper. Your sundial is done!

Place your sundial on a level surface in the sun. Rotate the sundial until the shadow of the nail matches the time on your watch. Your sundial is now calibrated and should measure the correct time for the next week or so.

A Giant Chalk Sundial:

Make a giant sundial! My children found this fascinating. Set up the sundial on flat pavement and extend the lines of the sundial outward with chalk. We did this by tying an 8' long piece of thread to the nail, aligning the thread to each hour mark so that we could draw a small chalk dot on the pavement for each hour. Mark the position of the nail before removing the small paper sundial. We used a board to draw a straight line from the nail to each mark. Now stand at the apex of the lines and view the position of your shadow.

Why isn't the sun at its apex due North at 12:00 Noon ?

There are three reasons for this.

  1. Daylight Savings Time: Spring Forward Fall Back, so in summer one must add one hour.
  2. Columbus is on the edge of a time zone. The earth is divided into 24 times zones which ignoring political boarders would each be 15 degrees apart. Columbus is located at 83 degrees West latitude and thus 8 degrees from the center of an ideal time zone (e.g. 83 - 5x15 = 8 degrees, 8x60/15=33 minutes). Thus the sun reaches its peak an extra 33 minutes later.
  3. The earth's orbit about the sun is an ellipse, not a circle. Thus when the earth is closer to the sun it travels faster than when it is further away from the sun. During given times of the year this causes the sun to reach it's apex slightly early or slightly late by as much as 16 minutes. This correction is called the Analemma. For more information on this correction see: http://www.analemma.com/

Where is North ?

Actually using a standard compass is not the answer because it only points to magnetic North. To get true north from magnetic north involves a messy correction. A much better way is to use the shadow from the sun at noon. A shadow gives true North to very good accuracy. Refer to each of the sundials where the time that the sun is at is apex is written at the bottom. These times are good to a single minute (1/60 of a degree).

Here is a good way to find North on any given day at your house. Look up the sunrise and sunset times on the local weather page from the weather channel. Determine the time halfway between sunrise and sunset. At exactly this time take a plumb bob outside and observe the shadow. The direction is good to 1/60th of a degree if the shadow is cast on a perfectly level plane!

A sundial movie and other fun stuff: I have also made a sundial movie(gif)(avi) showing how the columbus sundials change as a function of the date.There are also some nice tools on the web to calculate the elevation of the sun at any time of the year. Just click here. I also include some slides I presented on this subject to my daughters sixth grade class.

A very nice sundial to build can be found at www.planetary.org.

Please send comments to Stan Durkin