Joanne Comer, age 13, of Eaton Rapids, Michigan, for her question:
How do scientists know the exact time?
If you phoned a real expert for the exact time, he might ask, "Where are you?" Time is related to place because it takes 24 hours to swing all around the globe. A time keeping scientist is a chronologist who works in a special observatory equipped with all sorts of astronomical equipment, computers, charts and various clocks run by crystals or atoms. He might hesitate to state the exact time, because his very best estimate may be off by a millionth part of a second.
We base the passage of time on the motions of the Earth. As it spins on its axis, the celestial sphere appears to rotate overhead once every calendar day. As the Earth orbits the Sun, it brings different parts of the starry zodiac circle into view throughout the year. Hence, the sky provides an enormous celestial clock and the moving heavenly bodies mark off the passing moments.
Chronologists constantly check the celestial clock and match it to our man made system of time units the seconds and minutes, hours and days. This seems like a simple task. But it is not, because our dizzy old planet varies its speeds, wobbles and generally ignores the small units of our neat time system. This causes constant variations in the celestial clock overhead.
Expert chronologists must check these astronomical variations, day and night. In late December, they expect the sky clock to more or less match our everyday clocks. Right now, it may be about 15 minutes fast. Avery special time keeping observatory is needed to check these changes and announce the correct, or almost correct, time to the world.
It has telescopes that check the moment of noon, when the Sun crosses, or transits, the meridian line that passes overhead from pole to pole. The photographic zenith tube, the PZT, photographs a diagram of a selected star as it transits the zenith directly overhead.
These and other finicky reports are computed and relayed to the observatory's special clocks. They may be quartz clocks, run by crystals that vibrate about 360 times per second. These may vary by about one 10,000th part of a second per day. The atomic clock is accurate to within a millionth of a second per day. It is run by radio frequencies from cesium atoms. These special clocks are adjusted constantly on the basis of the astronomical data.
The observatory beeps out radio time signals around the globe. The 24 time zones use them to adjust their clocks to the correct time which is as nearly exact as all those patient chronologists can make it.
Our time units are based on the Earth's motions, but since these are so erratic, it was necessary to figure an average or mean system. The solar year begins and ends when the Sun transits the vernal equinox. The sample mean year is taken from the year 1900. It took 200 years to fissure the mean second, and there are 86,400 of them in a mean day. This is the neat system to which we adjust the erratic clock in the sky.