Tammy Hodges, age 11, of Gastonia, North Carolina, for her question:
What exactly is time?
In a general way, we all know what time is from everyday experience. We know that it never pauses and as it passes by it brings a series of changes in our lives and in everything around us. We know from experience that time moves steadily forward in one direction. The past is behind us and never returns. The future is ahead, on its way to reveal its secrets. We know from experience that the present time is but a moment between times past and times to come. These things we know, but sooner or later we begin to ponder deeper thoughts about the nature of time.
In one respect, we can say that time is a man made invention. The need for such an invention is evident when we observe the world around us. Nothing stays as it is. Each day brings a new dawn and each year brings a parade of seasons. A flower opens and its petals fall. Even the mountain peaks gradually rise and decline. Plants, animals and people grow up, grow old and complete their life spans. We live in a world of changing events and we need a suitable yardstick to measure the passing parade.
If for. some reason we discarded our time system, we would have to invent a new one. And without a doubt we would base it on the very same clockwork, the dependable motions of our whirling planet. After all, it is the earth's rotation that marks the dawning of each new day. And the earth's revolution around the sun brings us the yearly seasons. From its surface, we look up and out at the oceans of space that surround our globe on every side. Its motions turn us around to face up and out toward circles of changing celestial scenery. The sun, the moon and the distant stars appear to pass overhead in a heavenly parade.
Wherever we stand on the globe, we can depend on changing celestial scenery to keep pace with the revolving, rotating, earth. We can use this as a clock and a calendar to measure off all the multitudes of changing events that happen to everything in our everyday world, down on the earth. The heavenly parade is dependable because the motions of our spinning planet are fairly steady. This is the system we use to measure the passage of time that we need to keep track of the natural progress of events. And we are not likely to find a better method to do the job.
Our earth, of course, is a sphere shaped planet and its motions through space involve the globe as a single unit. For this reason, every spot on the surface also relates to the celestial parade passing overhead. The apparent motion of the sun completes a daily circuit around the globe. This circle is divided into 24 hour units.
Each hour is allotted 60 equal minutes and each minute has 60 equal seconds. Each second is a moment between the time that has passed and the time to come in the future.
The celestial clock also tells us that time is related to place. As the hour units appear to pass overhead, they correspond to hour units on the rotating globe. They provide us with the lines of longitude that cross the equator on their way from pole to pole. In one hour, the noonday sun crosses 15 of the 360 man made lines of longitude.