From VLBI, scientists have learned that Earth is not the most reliable timekeeper. The planet’s rotation is slowing down overall because of tidal forces between Earth and the moon. Roughly every 100 years, the day gets about 1.4 milliseconds, or 1.4 thousandths of a second, longer. Granted, that’s about 100 or 200 times faster than the blink of an eye. But if you add up that small discrepancy every day for years and years, it can make a very big difference indeed.

“At the time of the dinosaurs, Earth completed one rotation in about 23 hours,” says MacMillan, who is a member of the VLBI team at NASA Goddard. “In the year 1820, a rotation took exactly 24 hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased by about 2.5 milliseconds.”

By the 1950s, scientists had already realized that some scientific measurements and technologies demanded more precise timekeeping than Earth’s rotation could provide. So, in 1967, they officially changed the definition of a second. No longer was it based on the length of a day but on an extremely predictable measurement made of electromagnetic transitions in atoms of cesium. These “atomic clocks” based on cesium are accurate to one second in 1,400,000 years. Most people around the world rely on the time standard based on the cesium atom: Coordinated Universal Time (UTC).