With the knowledge of the motion of the sun, the ability to find solar declination online, and how time can be used to find longitude from previous lectures in class, students must make a device to measure the altitude of the sun using a straw, protractor, and string weighted down to make horizontal.
In order to take a sighting, they move the protractor, looking at the shadow of the straw taped along the edge of the protractor. When the sun's rays are shining directly through the straw, the size of the shadow of the wall of the straw is minimized. They then look at the angle from the weighted line hanging down. The angle is the altitude of the sun, which they record, and then record the time of the observation. The time has to be precisely synchronized to some source, like the Naval Observatory master clock. They do a number of observations while the sun is rising, around the time of local noon, and then when the sun is setting. The information is then entered on a form, and a quadratic fit is performed that is used to extract the maximum altitude of the sun, and the time of local noon. Using the declination of the sun and the time of local noon from the fit, they then determine their latitude and longitude. This is called equal altitude method. This activity teaches a way of determining latitude and longitude, fits to data that go beyond a simple linear fit, and helps develop a real understanding of how mariners found their position.