Sundial - Sundials in The Southern Hemisphere

Sundials in The Southern Hemisphere

A sundial at a particular latitude in one hemisphere must be reversed for use at the opposite latitude in the other hemisphere. A vertical direct south sundial in the Northern Hemisphere becomes a vertical direct north sundial in the Southern Hemisphere. To position a horizontal sundial correctly, one has to find true North or South. The same process can be used to do both. The gnomon, set to the correct latitude, has to point to the true South in the Southern hemisphere as in the Northern Hemisphere it has to point to the true North. Also the hour numbers go in opposite directions, so on a horizontal dial they run anti-clockwise rather than clockwise.

Sundials are used much less in the Southern Hemisphere than the Northern. One reason for this is the seasonal asymmetry of the Equation of Time. (See also below.) From early November to mid-February, during the Southern Hemisphere's summer, a sundial loses about half an hour relative to a clock. This adds to the difficulty of using it as a timepiece. The change during the northern summer is only about one-third as great, and is often ignored without causing much error. Since sundials are mainly used during the summer months, they are therefore better suited to the Northern Hemisphere.