A sundial is not sufficiently precise for navigation but it may be useful for watch keeping and similar time checks. There are many kinds of sundial. The simplest to make is an equatorial sundial (instructions are in Figure 10.3). The gnomon is set at an angle equal to your latitude. The hour lines are spaced every 15° round the face of the dial, which is at right angles to the gnomon and so in the plane of the equator. You will need:
• A piece of stiff card about 25 cm long and slightly wider than your protractor; you can also use wood, plastic, or metal providing you can mark the hours and drill a hole for the gnomon.
• A thin rod to act as a gnomon: this can be of wood, wire, knitting needle or even a drinking straw.
• A protractor to mark out the hours.
10.3 Making an Equatorial Sundial
To Make an Equatorial Sundial
1. One centimetre down from the top of your card, use the protractor to mark out the hours on both sides of the card. These are 15° apart.
2. Bend the card, or whatever, 15 centimetres down from the top of your hour dial.
3. At the centre of the hour dial, the point from where you measured the hour angles, make a hole just large enough to take the gnomon.
4. Insert the gnomon, making sure it is at 90° to the hour dial.
5. Slide the gnomon through until the angle it makes with the base is the same as your latitude. The easiest way to do this is to make AB the correct length for the angle. The length of AB is found by the formula:
AB = Cotangent of your latitude multiplied by 10.
In the northern hemisphere the gnomon points to true north and in the southern hemisphere to true south.
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