A Jacobs Staff, introduced in 1342. Was used to determine the latitude at sea at which people were located. Because people had to look straight at the sun every day, there were many eye complaints among the navigators.
It was a long bar with the degrees on it and a movable cross arm. Move the human eye on one side of the bar with the cross arm until the lowest part is on the horizon and the highest on the sun or a star.
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