Equatorial Sundial

The time is read by the shadow of the gnomon (the antenna piece) on the inner surface of the 1x2 jumper plates arranged in an arc at 15-degree increments. The lines between the plates indicate each hour, and the stud on the underside indicate the half hours.

The photo above was taken at 10:27 Daylight Saving Time on March 22, near 41 N lat. and 95 W long. On this date and in this location, the time shown on sundials is about 27 minutes behind that shown on local clocks. Because of the one-hour shift during daylight saving time, the sundial time is also one hour slower than clock time. Subtracting 1:27 from 10:27 gives 9:00, which is the time indicated by the gnomon shadow in the photo.

As it happens, 1xN plates placed snugly against one another on the underside of a 1x4x5 arch create the 15-degree angle needed. This is the most compact method of making a semi-circle divided into 12 equal segments.

The two blue plates in the center indicate the noon mark. It takes one solar hour for the shadow of the gnomon to travel across the width of one plate. The two outer blue plates indicate the 6:00 am and 6:00 pm hour marks.

15-degree angles can also be created using other construction techniques to create a larger dial. The base should be hinged to allow the dial to be adjusted for use at a specific latitude. The angle of the dial is simply set equal to the latitude of its location, and the gnomon is pointed either north or south, depending on which hemisphere it is in.

Building and using a sundial is an introduction to several scientific, mathematical, geographic and historical topics:

How have sundials been used over the years?
What are time zones, when did they come into use, and why?
Why do some places change their clocks by an hour twice a year?
Does a sundial show the same time as a clock? Why not?
What causes the seasons and how does a sundial reflect that?
Why must a sundial be adjusted for different locations on the globe?
What are latitude and longitude and how do they relate to timekeeping?

This model is not only educational, it's fun to build and an attractive and useful astronomical instrument.