Handbook of Nature-Study · Lesson CCXXXIII · Topic 38: The Sun and the Moon
233. How to make a sundial
Goal: Your child builds a simple sundial with its gnomon set at your latitude, aims it north, and marks the hours.
You'll need
- Stiff cardboard or foam board (or the book's painted boards, cut by an adult)
- A protractor, ruler and pencil
- Your town's latitude (from a map or atlas)
- A compass, or the noon shadow method below
- Glue or tape
- A clock
What's in the book
The lesson gives a method and materials, then step-by-step directions with diagrams for making the gnomon (cut at the angle of your latitude), drawing the dial face with its half-circles, and setting the dial up and marking the hours with the help of a watch.
The lesson, step by step
Safety first
Never look at the Sun; watch the shadow. If you use the book's wooden boards, an adult does all the sawing and drilling.
What is a gnomon?
The word gnomon is from a Greek word meaning "one who knows."
It is the triangle that stands up on a sundial and casts the shadow.
Cut the gnomon
The book cuts a square board along a slanting line:
draw the line vw making the angle at x the same as the degree of latitude where the sundial is to be placed. At Ithaca the latitude is 42°, 27' and the angle at x measures 42° 27'.
Look up your own latitude, draw that angle with the protractor on cardboard, and cut out the triangle.
Make the dial
Draw a large half-circle on a square of cardboard. Glue the gnomon upright along the middle line, with its low point at the center of the half-circle and its sloping edge rising toward the curved side. (The book's page shows the full drawing with several half-circles; older children can follow it exactly.)
Point it north
Set the dial on something level, with the gnomon's sloping edge rising toward the north. The book gives a way without a compass:
wait until noon and set the dial so that the shadow from W will fall exactly between the points A, B, and this will mean that the dial is set exactly right.
Mark the hours
Then with a good watch note the points on the arc E, K', on which the shadow falls at one, two, three, four, and five o'clock
Do it on one sunny day: mark the shadow every hour by the clock, morning and afternoon, and draw lines out from the gnomon's foot.
What we know now
A sundial shows sun time, which is not quite clock time. Clocks run on time zones, so your town's sun time may be off by up to half an hour or more; daylight saving time moves the clock an hour more in summer; and because the Earth's orbit is oval and its axis tilted, sun time runs up to about 16 minutes fast or slow through the year. If your dial is set by the clock one day, expect it to drift a little in other months.
Nature notebook
Draw your sundial and write your latitude. On a sunny day next week, compare the sundial with the clock at three different hours.
Tip
The sloping edge of the gnomon, set at your latitude and aimed north, points straight at the Pole-star. Check it on a clear night.