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History of Sundials

Why the Sun Dial keeps time

Developing and Making Sundials

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Sundial Hour Lines Lay out

Sundial "Furniture"

Declination Lines of Sundials

Portable Sundials

Interesting Sundials of the World

WHY THE SUNDIAL KEEPS TIME - Page 6

Because the sun does not actually stay on the celestial equator throughout the year, but rather moves along the ecliptic, appearing north of the equator in summer and south of it in winter, for the northern hemisphere, there is an apparent slowing up or increase in the speed of the sun, which can be determined in relation to an instrument which records a uniform speed, such as the watch. This difference in speed can be observed by comparing the sundial and watch. The sundial may record noon when the watch says ten minutes of that hour, or ten minutes after that hour. This is what your household friend, the almanac, calls `sun slow' or `sun fast'. The terms `sun slow' and `sun fast' were familiar to our fathers and grandfathers. The sun is said to be slow because it records the hours after the clock has recorded them, and is said to be fast because it records the hours before the clock does. Thus the sun will be slow part of the year and fast the remainder. The exact difference for any day can be determined, which when subtracted from the sundial reading when the sun is fast and added to it when slow will give watch time. We have mentioned the apparent motion of the sun. Do you remember when you first learned that it was the earth turning upon its axis that made the sun move? Even a child in grammar school knows that the earth makes one complete turn about its axis from the west toward the east, every day, thus causing the sun to appear to move in a reverse direction from the east toward the west.

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