It consists of a staff, usually about 36 inches (91.5 centimetres) long, and a shorter crosspiece called the transom, which slides up and down the staff to measure altitudes. The earliest known description comes from Persia in the 11th century. The first western description is by Levi ben Gerson in1342 but it was 1485 before the German mathematician and navigator, Martin Behaim, took it to sea when he sailed down the east African coast with the Portuguese explorer, Diego Cao.
To make a cross staff you will need three lengths of wood: one for the staff and two for the transom as shown in Figure 13.7. A very crude cross staff can be made by lashing two pieces of wood (one long, one short) at right angles to each other.
Using a cross staff means the observer must observe the horizon and the celestial body simultaneously. This introduces ocular parallax, the scientific term for looking in two places at once. This effect can be reduced somewhat by using only one side of the transom, and placing the tip of the staff on the horizon instead of the bottom of the transom. You may find the 'long gun' cross staff easier to hold and use (see Figure 13.8) particularly at night.
13.8 Long Gun Cross Staff
Polaris is sighted along the staff and the transom then slides up or down the staff until it fills the sky between the horizon and Polaris. The tangent of the angle between the top of the transom and the eye end of the staff gives the altitude of Polaris (see Figure 13.9). The fact that it resembles a crossbow caused navigators to talk about 'shooting the sun'.
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