Globe sundial

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Globe sundial with which the time of day can be read on both pens and under an adjustable bracket.
The day-night border is west of South America ; it is still night over the Pacific .

A globe sundial is a special form of the sundial using an earth globe . The globe is illuminated by the sun and the time can be read from the shadow formation .

The globe is immovably mounted and oriented in such a way that its axis, like the earth's axis, points to the polar star ( north celestial pole ) (on the southern half of the earth to the south celestial pole ) and the location marked on it lies in the meridian . The sun (apparently) rotates around the globe in the same way as it does around the earth. The installation location marked on the globe is illuminated on it in the same way as the installation location itself, which is easiest to observe at sunrise or sunset. The globe is currently illuminated by the sun in the same way as the earth. One recognizes the current border between the half of the day and half of the night on the globe as the border between its illuminated and its half in shadow.

Unlike ordinary sundials there is no pointer whose shadow the time on a clock face displays . Instead, the globe sundial is usually provided with a series of pins on the equator. The time of day is given by the pen that casts the smallest shadow. If the user does not interpolate between the silhouettes of two adjacent pens , it is a digital measurement .

For another measuring method, globe sundials are equipped with a thin bracket that must be adjusted by the user of the clock. The bracket is rotated around the globe axis into the position in which it throws the smallest shadow (the sun stiffens the two side surfaces of the bracket). This position is read off as the time of day on a scale, which is usually placed on the equator. The measurement with the bracket is - as with most sundials - analog.

A globe sundial with a bow is also a world time clock . The bracket is in the reading position on the longitude of the earth where it is true noon .

The time of day can also be determined if the globe only has a scale. You read the times at the two borders of the sunny half (half of the day) and calculate the mean value.

Individual evidence

  1. René RJ Rohr: The sundial. History, theory, function. Callwey, Munich 1982, ISBN 3-7667-0610-1 , p. 152.