Sundial and nocturlabium
Inv V03091, V28990

Long before Christiaan Huygens invented the pendulum clock in 1656, it was possible to measure time accurately. The earth rotates around an imaginary axis with the result that the stars apparently describe circles in the vault of heaven over twenty-four hours. The only star that can be seen during the day is the sun, which outshines all the others. The height of the sun is a direct measure of the local time. With sundials the height of the sun can be read from the direction of the shadow of an indicator. Most sundials have an indicator positioned parallel to the earth's axis: the polos. Sometimes, however, it is vertical and then is called a gnomon. With the nocturlabium the time could also be measured in darkness. The user adjusted the date and looked through the central aperture at the only star not rotating: the Pole Star. Then he pointed the indicator of the nocturlabium along the two outermost stars of the Great Bear. Their positions determine the time shown on the instrument by the same indicator.