Carolyn Hasselbring, age 11, of St. Louis, mo., for her question:
How does a sundial work?
In ancient days our ancestors put up a post and watched the shadow it cast upon the flat ground. From sunrise to sunset they watched the shadow creep around an arc, dividing morning from afternoon and pointing to the moment of midday. This shadow stick was the first clock.
A sundial is most at home in a flowery garden. It is a pedestal just high enough for you to look down on its flat top, which is the face of a clock. The dial is made of metal and marked with the hours in roman figures. The style that points a shadow toward the time of day has two arms of a metal triangle. It is set at right angles with one arm flat on the dial and the other sticking up in the air.
When you face the sun, you cast a shadow behind you. If you could stay as still as a post all day, you could watch your shadow creep around in an arc. On a sundial, the shadow of the style points to the hours as it creeps around the dial.
In the morning all shadows point westward, away from the sun in the eastern sky. In the evening, when the sun is in the west, all shadows point eastward. Morning and evening shadows are longest because the sun is low in the sky. The shortest shadows fall at noon when the sun is highest overhead.
North of the equator the sun is always south of the zenith, which is directly overhead. The long shadows of dawn point westward and veer slowly to the northwest through the morning hours, shrinking as they go. The short shadows of noon point due north. After midday they continue around the arc pointing northeast until the long shadows of evening point eastward, away from the sinking sun in the western sky.
South of the equator the sun is always north of the zenith, and the noon shadows point due south. Sundials are often marked with the north and south, east and west directions. But the hours and compass points differ in the two hemispheres. South of the equator the sixth hour of noon points to the south direction. Our sundials are set with the noon hour pointing to the north pole.
Sometimes a sundial bears the words, "I only mark the sunny hours." this is because it tells time by a shadow created by the shining sun. The first shadow clocks were sticks in the ground, and they bore no sunny messages. They were invented before our ancient ancestors had learned how to write.
Through the year the sun's path varies across the sky. It also varies as seen from different parts of the globe and this causes the shadows to differ from place to place. For this reason, an accurate sundial must take latitude into account. The angle between the pointing finger and the dial must be the same as the latitude where the sundial is to record its shadow watching.