Enlarge / Without adjustments for relativity, clocks here and on the Moon would rapidly diverge. (credit: NASA)
Timing is everything these days. Our communications and GPS networks all depend on keeping careful track of the precise timing of signals—including accounting for the effects of relativity. The deeper into a gravitational well you go, the slower time moves, and we’ve reached the point where we can detect differences in altitude of a single millimeter. Time literally flows faster at the altitude where GPS satellites are than it does for clocks situated on Earth’s surface. Complicating matters further, those satellites are moving at high velocities, an effect that slows things down.
It’s relatively easy to account for that on the Earth, where we’re dealing with a single set of adjustments that can be programmed into electronics that need to keep track of these things. But plans are in place to send a large array of hardware to the Moon, which has a considerably lower gravitational field (faster clocks!), which means that objects can stay in orbit despite moving more slowly (also faster clocks!).
It would be easy to set up an equivalent system to track time on the Moon, but that would inevitably see the clocks run out of sync with those on Earth—a serious problem for things like scientific observations. So, the International Astronomical Union has a resolution that calls for a “Lunar Celestial Reference System” and “Lunar Coordinate Time” to handle things there. On Monday, two researchers at the National institute of Standards and Technology, Neil Ashby and Bijunath Patla, did the math to show how this might work.
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