Arab navigators used the Kamal (meaning Guide) which works on the same principles as the latitude hook. It was a small wooden board with a hole through its centre. A knotted line was passed through the hole. The spacing of the knots corresponded to the known latitude of different ports. The navigator put the knot for his destination between his teeth, and held the Kamal out with the bottom edge on the horizon. When Polaris touched the top of the board he was on the latitude of his destination. The Chinese also used the Kamal but with a line of a fixed length and a selection of different sized boards to measure different angles. As a Kamal is held at a constant distance from the eye it is more accurate than the latitude hook.
The version of the Kamal described here (and shown in Figure 13.4) works in degrees and allows you to measure horizontal as well as vertical angles. It is nothing
This latitude hook was made by cutting up a wire coat hanger.
The position of the crossplece was found empirically one evening and then lashed Into position. Later it was found better to lock the lashing in place with insulating tape.
On subsequent evenings checking latitude was as simple as placing the crosspiece or the horizon and looking for Polaris.
13.3 Latitude Hook
13.4 Simple Kamal
1. Hold any piece of wood with a straight edge at a hand-to-eye distance of 57 cm.
2. Line up with Merak and Alkaid and mark where they fall on the straight edge. This is a distance of 25° or 25 cm.
3. Line up Merak and Megrez and mark where Megrez falls. You now have a measurement of 0-10 cm.
4. Repeat with Merak and Dubhe to break this to a 5 cm measurement.
5. Finally, use the edge of a coin to divide into 2mm units.
This ruler measures degrees when held with a hand-to-eye distance, of 57 cm.
For finer measurements the stars in Orion's Belt are 1 ° apart.
13.5 Making a Ruler more than a shop-bought centimetre ruler with a hole drilled to take a line. A knot is tied in the line 57centimetres from the ruler.
To use it, the knot is placed between your teeth and the Kamal held straight out with the line horizontal, the Kamal vertical and zero on the horizon. The angle is then read off. One centimetre equals one degree and one millimetre 0.1 °.
If you do not have a ruler it is still possible to make a Kamal out of a length of wood and calibrate it, using stars whose separation in degrees you know (see Figure 13.5). If you require very precise measurements, the moon (and the sun) are both about half a degree across.
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