Marine astrolabes were about four inches in diameter with a scale marked in degrees, and heavy to make them steady. They were horribly inaccurate, partly because accuracy is a function of size but also because it took three men to use it: one person held it, another took the sight, and a third read it. When Vasco da Gama, on his way towards the Cape of Good Hope, reached St Helena Bay, he went ashore with a large wooden astrolabe because he had been unable to take a sight on the voyage south. Some complicated gadgets aimed to make the astrolabe reliable at sea. They all failed but probably kept the crew amused. Less than 100 marine astrolabes are known to have survived, probably because most were thrown overboard in frustration.
A shadow astrolabe (see Figure 13.2 and Figure 13.2b) lets you take a noon altitude without staring into the sun. All you have to do is hang it up, watch the shadow
climb towards the horizontal and note the highest reading before it starts dropping back. Do not expect much.
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