Eleanor Santucci, age 13, of Concord, New Hampshire, for her question:

Are sundials useless at night?

Morning shadows are long and point towards the west   for the sun is pouring in its light from the east. Noon shadows, when the sun is nearly overhead, are short and point northwards. Afternoon shadows point eastward, getting longer as the sun slopes down to set in the west.

The sundial is a fiat table on which a lit le arm casts a shadow from the sun. The tabletop is marked in a dial and the shadow creeps from one to another of the marks as the sun travels through the day.

At night and on cloudy and rainy days, there is no sunshine to cast a shadow. The whole world is too dark and gray for shadows to show at all. So, during these dreary hours, the little sundial is of no use for telling the time. Many an old sundial informed its watchers of this with the words, "I only mark the sunny hours."