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The Challenge: Measure Latitude and Longitude

Measuring latitude using the sun

Measuring latitude using the sun can only be done at noon, when the sun is at its highest point in the sky. To determine when it's noon (without a watch or radio) place a stick at the southernmost end of your north-south line. Use a plumb line to make sure that the stick is vertical. When the shadow cast by the stick crosses the north-south line, it's noon.

As soon as it's noon, align the sighting nails on the quadrant's aiming beam with the sun. DO NOT USE THE SIGHT LINE TO LOOK DIRECTLY AT THE SUN.

Instead, watch the shadows formed by the nails on the ground as you tilt the aiming beam up and down. At first, the nails will cast two separate shadows, so move the end of the beam up or down so that these two shadows move closer together.

Figure 2
Two shadows on the groud - beam aimed incorrectly

When the shadows coincide, the beam is aimed exactly at the sun. Using the protractor, measure the smaller angle between the beam and the plumb line.

Figure 3
One shadow - beam aimed correctly

If the sun is directly over the Equator, this is your latitude reading.

Figure 4
The angle to measure when using the sun or North Star. Note that the horizon is always 90° to the plumb line. Don't be fooled by mountain ranges!