Robert Sprout, age 11, of Lancaster, Pennsylvania, for his question:

What exactly is an atomic clock?

Observatories use it to keep the precise standard time from which we set all our work a day clocks and watches. It does not tick or chime or wag a pendulum. It has no clicking gears or turning wheels. The atomic clock is a super clock run by performing atoms. And what happens is on an infinitesimal scale, much too small for human eyes to observe.

The National Bureau of Standards created the first atomic clock in 1949. The original design has been improved and more refinements are expected to make this most accurate of time keepers even more accurate. The latest model uses cesium atoms and is estimated to be accurate to one part in ten billion    more or less. The first atomic clock used simple molecules of ammonia. It is estimated to be accurate to about one part in 100 million.

These remarkable time keepers work because atoms, even the most ordinary atoms, are minuscule bundles of energy. They absorb and emit electricity and other forms of energy and vibrate somewhat like tuning forks. Molecules are packages of atoms and precisely controlled energies can make them perform in precise frequencies.

Before atomic clocks were invented, the most precise clocks were run by quartz crystals made to vibrate in precise frequencies. Such a quartz crystal emits a weak electric current. This current was converted into radio waves and used to stimulate frequencies for the first atomic clock. The vibrating energy was directed through a chamber that held a small quantity of gaseous ammonia.

Each molecule of ammonia contains an atom of nitrogen and three atoms of hydrogen. The three hydrogens form the base of a pyramid and the lone nitrogen forms the apex. A very precise performance begins when ammonia is exposed to a certain radiation. The frequency of this radiation must be exactly 23,870 megacycles. Right on the dot.

The ammonia molecule absorbs this energy    and its nitrogen atom dances a jig. One two, one two    it trips back and forth through the ring of three hydrogen atoms, precisely on time. This is the atomic time keeping used to run the first atomic clock. A quartz crystal was set to deliver the magic frequency of 23,870 megacycles. As long as this magic music stays in tune, the ammonia molecules dance their two step, precisely on time.

But even more precise models are now run by quartz crystals and frequencies of cesium atoms.

Atomic clocks are perhaps the world's most delicate instruments    and all sorts of infinitesimal events can upset them. They must be watched by super sensitive instruments, day and night. Batteries of these instruments are set up to work automatically. They check for errors and make corrections to keep those finicky atomic clocks on time.