The "Atomic Age" of Time Standards

Scientists had long realized that atoms (and molecules) have resonances; each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies. These resonances are inherently stable over time and space. An atom of hydrogen or cesium here today is (so far as we know) exactly like one a million years ago or in another galaxy. Thus atoms constitute a potential "pendulum" with a reproducible rate that can form the basis for more accurate clocks.

In 1949, NIST built the first atomic clock, which was based on ammonia. However, its performance wasn't much better than the existing standards, and attention shifted almost immediately to more promising atomic-beam devices based on cesium. The first practical cesium atomic frequency standard was built at the National Physical Laboratory in England in 1955, and in collaboration with the U.S. Naval Observatory (USNO), the frequency of the cesium reference was established or measured relative to astronomical time.

The cesium atom's natural frequency was formally recognized as the new international unit of time in 1967: the second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency, replacing the old second that was defined in terms of the Earth's motions. The second quickly became the physical quantity most accurately measured by scientists.

Leap Seconds

The rotation of the Earth is slowing down. This means that a solar day (the time it takes Earth to make one complete revolution) and the time shown by atomic clocks would gradually diverge. This problem has been solved by adding "leap seconds". There have been 22 leap seconds since 1972. The last one was added on 31 December 2005, which delayed New Year's Day by one second!

The Universe in a Year

The American astronomer Carl Sagan(1934-96) first suggested a "cosmic calendar" as a way of helping people understand the history of the Universe. He put everything into a scale of calander year: the galaxies are formed over nine months and the Earth appears in September. All human history is croweded into the last five minutes of the last day of the year. Recent time has to be divded into seconds and fraction of seconds. So everything that happened over the last 475 years takes place in less than the last second of the last minute of the year.

Date/Time	Month	Event
1(Midnight)	Jan	Big Bang-Universe forms
15	Mar	First tars and galaxies forms
1	May	Milky Way galaxy forms
8	Sep	Sun forms
9	Sep	Solad System Forms
12	Sep	Earth Forms
13	Sep	Moon Forms
20	Sep	Earth's Atmosphere forms
1	Oct	Earlist known life on Earth
7	Oct	Earlist known fossils
18	Dec	First many celled life forms
19	Dec	First Fish
21	Dec	First land plants; first insects
23	Dec	First Reptiles
24	Dec	First dinosaurs
26	Dec	First Mammals
27	Dec	First Birds
28	Dec	First Flowering Plants
28	Dec	Dinosaus extinct
31 (11.55 p.m)	Dec	Modern Human appears