We like knowing what time it is. But the Earth just won’t cooperate. Its spin rate changes. The planet is rotating a tad faster than it used to. For the first time in history, world timekeepers may subtract one second from our clocks in a few years.
Clocks may have to skip a second—a “negative leap second”—around 2029.
Earth takes about 24 hours to rotate once. But many scientists say that the Earth’s rotation has generally been slowing down by tiny amounts for centuries.
Dennis McCarthy is a retired director of time for the U.S. Naval Observatory. He says the slowing is mostly caused by the effect of tides, which are caused by the gravitational pull of the Moon.
But the rate of slowing rotation varies. And in the last five decades, the Earth seems to have sped up slightly.
Geophysicist Duncan Agnew says, “It’s not a huge change in the Earth’s rotation that’s going to lead to some catastrophe or anything, but it is something notable.”
So what does this mean for timekeepers? Well, two forms of time measurement exist: universal (also called astronomical) and atomic. They often don’t match up.
Universal time is based on the rotation of the Earth on its axis. One rotation takes 24 hours—or very close to that—and we call that one day. Mechanical clocks track the time we call seconds, minutes, and hours, which make up each day. To do so, these clocks use gears and a spring. Wind the spring to store energy. As it unwinds, it turns gears that move clock hands at a fairly predictable rate. But mechanical clocks must be reset for accuracy from time to time, and wound often—usually daily.
Atomic clocks work with atoms. Some have quartz crystals inside them. Particles inside the quarts atoms vibrate back and forth at a regular rate. But changes in temperature or pressure can very slightly slow or speed up that atomic activity. Even quartz clocks need to be adjusted from time to time.
Yet another type of atomic clock works with the same idea. All atoms have a nucleus with one or more protons and neutrons surrounded by electrons. A jolt of energy in the form of microwaves can make electrons move to a higher orbit around the nucleus. Scientists count precise, predictable microwaves to measure time.
Clear as mud? Maybe you’re thinking, “Just tell me what time it is!”
That last type of atomic clock is considered most accurate. Cell phones, GPS, and even space travel depend on that kind.
The tiny time differences between universal, mechanical, and atomic measurements add up over the years. In 1972, international timekeepers decided to add a “leap second” so universal time could catch up with atomic time.
Then Earth started showing signs of speeding up. Agnew believes Earth’s hot, liquid, unpredictable core has caused the increase for the last 50 years. However, he thinks melting ice at the Earth’s poles slows the rotation.
Slow down. Speed up.
Adding leap seconds can cause hassles for computer system operators. In 2012, some computer systems mishandled a leap second. Experts say that caused problems for companies like Linux, Qantas Airlines, and others.
Tech companies such as Google and Amazon recognized the forthcoming problems and came up with their own solutions. Physicist Judah Levine says they gradually added fractions of a second over a full day.
Still, Levine doesn’t think a negative leap second will really be needed.
Levine says the overall slowing trend from tides has been around for centuries and continues, but the shorter trends in Earth’s core come and go.
“This is not a process where the past is a good prediction of the future,” Levine says. “Anyone who makes a long-term prediction on the future is on very, very shaky ground.”
My times are in your hand. — Psalm 31:15
