<STRONG><FONT color=#336699>（CNN)CNN's Tom Foreman explains why scientists are delaying the start of 2006 by one second.</FONT></STRONG>
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International experts on time have determined that a leap second must be added to the final minute of this year. Now everyone is familiar with leap years since the time it takes the Earth to orbit the sun is not actually 365 days, but more precisely, 365.242 days, we add an extra day to the end of February every four years. Of course if you do the math, you realize this means every four hundred years, we have a surplus of three days. So, when centuries change, they are not considered leap years unless they are divisible by four. Confusing? Well, leap seconds, by comparison, are easy. It's all about atomic clocks, which measure time through the natural vibration of the cesium atom. <BR><BR>The National Institute of Standards and Technology says leap seconds are necessary, because time based on the Earth's rotation is not as consistent as time based on cesium clocks which are accurate to 30 billionths of a second. So leap seconds are added now and then to keep the two in line, largely to avoid confusion in fields that require precise time coordination, technology, communications, financial transactions, scientific work, that sort of things. The leap second was introduced in 1972, and they appear slightly less often than once a year.<BR>