Capricorn: Dec.22-Jan.29 The Sundial Primer
created by Carl Sabanski
Capricorn: Dec.22-Jan.29

The Sundial Primer Index

Globe Sundial

Globe Sundial: (or spherical dial): a class of dial in which the "dial plate" is a globe or sphere, usually set with its axis parallel to the Earth's polar axis and often with the observer's position at the top. The gnomon is in the form of a thin semi-circular vane, which can swivel around the globe about its axis. In use, the vane is rotated until the shadow is minimised and the time read from an equiangular scale around the equator. It indicates the meridian of longitude where it is currently noon.

The globe sundial is an example of a shadow plane sundial because the gnomon is movable and must be set so that it lies in the sun's hour plane.

The sundial has hour lines equally spaced at 15 intervals around the globe. It can be used at any latitude. The noon hour line is aligned with the meridian of the dial's location and the axis through the sphere is at angle with the horizontal equal to the latitude. It will then point to the celestial pole. 

Once the sundial is set, it will indicate local apparent time. The current time is determined by rotating the vane about the polar axis until it is made as thin as possible. If the dial were an actual globe the longitude where the shadow falls would indicate where in the world it is solar noon.

The sphere can be rotated to account for the Equation of Time, longitude and Daylight Saving Time corrections. Longitude and Daylight Saving Time corrections can also be made by adjusting the hour lines.

Rather than a moveable vane, this sundial could have multiple fixed gnomons positioned on selected hour lines.

Globe Sundial

For an image complete with shadow click here.