Earthquake, Earth rotation and the atomic clock

by Treloar Bower, Curator of Education

Image courtesy of NASA

NASA and the Jet Propulsion Lab in California have reported that the Chilean earthquake on Saturday, February 27, that measured 8.8 on the Richter scale, was powerful enough to disrupt the earth’s rotation that day, causing the planet to spin a fraction of a second faster. Read about it here: http://news.bbc.co.uk/2/hi/americas/8547955.stm

How do we measure something like the length of a day, and how does that time change? Astronomical time (which is based on the rotation of the earth compared to distant celestial objects) is not constant, and factors such as movement in the atmosphere, the oceans or the earth’s mass (such as the shifting of tectonic plates during an earthquake) can speed or slow the earth’s rotation. Atomic time, however, is constant, measured by the “frequency of the microwave spectral line emitted by atoms of the metallic element cesium.” Basically, atoms of cesium emit energy at a never-changing rate, creating a standard unit for the measurement of time.

You might see where I’m going with this: if we measure time with a tool that is constant, but astronomical time itself is not (as in, February 27 was actually shy of a full 24-hour day; or the one-year revolution of the earth around the sun is really 365¼ days but we only account for 365 days on our calendars), at some point our clocks and our sunlight will not match. So the engineers at the atomic clock station here in Fort Collins (which broadcasts time and frequency information from the National Institute of Standards and Technology’s atomic clock 24 hours per day, 7 days per week to millions of listeners worldwide) have a solution to this problem. Just like every four years we add a leap day to the month of February, occasionally a leap second must be added or subtracted to the atomic clock to make up for the variation in the earth’s rotation that causes shortened or lengthened days.

I emailed our good friend Matt Deutch, an engineer with the WWV atomic clock station, and asked if the Chilean earthquake meant that they needed a leap second on the atomic clock. Essentially, the change in the earth’s rotation this time did not amount to a full second, so no adjustments are required now. However, the next time a full second of rotational change is accumulated, this jolt will be part of that accumulation. The interesting thing is that, as Matt wrote to me, “the earth’s rotation is gradually slowing down so I would think these earthquakes (author’s note: Matt’s plural “earthquakes” is referring to both the Chilean earthquake and the 2004 Indian Ocean earthquake of 9.1 on the Richter scale that not only caused the massive December 26 tsunami but also impacted the earth’s rotation) are just postponing when a leap second needs to occur, but just by a small amount.”

Thanks, Matt!

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