Benchmarks: June 30, 1972: Timekeepers add first "leap second" to clocks

On June 30, 1972, at exactly 11:59 p.m. and 60 seconds, timekeepers did something most people only dream about: They added more time to the day.
 

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Gain some, lose some

Scientists now know that Earth’s rotation is very slowly decelerating, such that the length of the rotational day is about two milliseconds longer than the 86,400 seconds it was nearly two centuries ago. But that doesn’t mean Earth’s rotation is always slowing down. The tidal force of the moon acts to slow Earth’s rotation by taking away some of the planet’s energy. But other factors, such as the expansion and contraction of the atmosphere with the seasons, the churning of material within Earth’s core, and the rebounding of Earth’s surface as the weight of glaciers is removed, can sometimes act to speed up Earth’s rotation by redistributing its mass, thereby altering Earth’s moment of inertia and thus its speed of rotation.

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Brian Fisher Johnson

Johnson is a contributer to EARTH Magazine.

Wednesday, June 30, 2010 - 06:00

Brian Fisher Johnson

Johnson is a contributer to EARTH Magazine.

Wednesday, June 30, 2010 - 06:00