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A brief history of Earth time
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Simple sundials or stone formations, which track shadows as the sun passes overhead, mark a day’s progression just as the shifting phases of the moon can log the passing of a month on Earth. Those natural timekeepers have kept humans on schedule for millennia.

But perhaps since mechanical clocks gained traction in the early 14th century, clockmakers have grown ever more persnickety about precision.

Exacting the measurement of seconds also grew more complicated in the early 1900s, thanks to Albert Einstein, the German-born physicist who rocked the scientific community with his theories of special and general relativity.
“Darn that Einstein guy — he came up with general relativity, and many strange things come out of it,” said Dr. Bruce Betts, chief scientist at The Planetary Society, a nonprofit space interest group. “One of them is that gravity slows time down.”

General relativity is complicated, but in broad terms, it’s a framework that explains how gravity affects space and time.

Imagine that our solar system is a piece of fabric suspended in the air. That fabric is space and time itself, which — under Einstein’s theories — are inextricably linked. And every celestial body within the solar system, from the sun to the planets, is like a heavy ball sitting atop the fabric. The heavier the ball, the deeper the divot it creates, warping space and time.

Even the idea of an earthly “second” is a humanmade concept that’s tricky to measure. And it was Einstein’s theory of general relativity that explained why time passes slightly more slowly at lower elevations — because gravity has a stronger effect closer to a massive object (such as our home planet).

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