The Week has an excellent short piece about people who want time to be synched to astronomy versus people who want time to be synched to the vibration of cesium-133.
Astronomers prefer to calibrate their telescopes, satellites, and other instruments against deep-space objects such as pulsars, which emit pulses of energy at regular intervals.
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The International Telecommunication Union decided that astronomical time could not differ from Coordinated Universal Time—which is based on atomic time—by more than 0.9 second. Because the two systems are inherently out of step, it's periodically necessary to add "leap second" to bring them into sync. Most people didn't notice, but one of those seconds was added after midnight on Dec. 31, 2005, just before 2006 began. A leap second, says Jonathan Betts of the Royal Observatory, "asks the atomic clocks to hold their breath for one second, so that the Earth can catch up." So far, the compromise has worked. But some American scientists have proposed scrapping leap seconds altogether.
