Free Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves by Bob Berman

second” into the final minute of June or December. Television stations give this job to their meteorologists, who explain that extra seconds are needed because our planet is winding down and will ultimately make each rotation, each of our days, forty days long in the far future.
    But if you’re a serious card-carrying geek you’ve surely stopped in your tracks, grabbed your calculator, and said to everyone within earshot, “Wait a minute! They add a second every year or two? Earth can’t be slowing that quickly. It just can’t!” You go tap-tap-tap on the instrument’s keys and realize that if our planet’s day was really growing a second longer every couple of years, we’d have come to a frozen halt billions of years ago. Something doesn’t add up. Something about the rotation of our world simply doesn’t make sense.
    Because the media always get this wrong, here’s the real scoop. The answer involves beauty. Poetry, even. After all, a watch set to the right time is a device synchronized with Earth’s rotation. It lets Orion and the Dog Star march to the beat of the timepieces on our wrists or, more likely these days, the überprecise digital time on our smartphones, whose signals are periodically synchronized with atomic clocks even if we don’t care about such accuracy.
    In the 1950s an important decision was made, an agreement between every nation on our whirling planet. It was, simply, that Earth’s spin rather than vibrating quartz crystals or any other timekeeping method would dictate the time. This meant we needed two parallel monitoring systems kept in sync with each other. One is our planet’s spin, constantly scrutinized by an agency in France called, not surprisingly, the International Earth Rotation Service.
    The other system requires the careful daily marking off of 86,400 seconds, each precisely defined. These official ticktocks are counted by forcing the nucleus of the cesium 133 atom to maintain a particular spin direction, which it does only when bathed in 9,192,631,770 microwave pulses per second. Any other frequency changes the cesium. So an atomic clock is simply a vacuum chamber where a fountain of gaseous cesium atoms are bathed in microwaves and the state of the cesium is continuously monitored. That’s the story. A servomechanism slightly varies the microwave frequency if required. An official second is thus 9,192,631,770 microwaves, just what’s needed to maintain cesium 133 in a fixed condition. That exact number of microwave pulses is the definition of a second.
    The official second remains constant. Earth, alas, does not. Along with spin irregularities not fully understood, observations of the stars show that our planet’s day becomes one seven hundredth of a second longer after each century has passed.
    This may seem too trifling to matter at all. Compared to the day you were born, the day you start receiving Social Security checks is one thousandth of a second longer. Sure, this adds up, but it’s way too little to require meddling with clocks every year or two. So again, why those leap seconds?
    Here’s the explanation that, guaranteed, nobody on your block knows.
    When the current system was set in place in the 1950s, astronomers had been using earth-rotation data collected over the previous three hundred years. The official length of a day was codified in 1900. But during those centuries of observation, a day’s length slowly grew. Careful analysis now shows that a day was exactly 86,400 seconds long in 1820. Before that each day was shorter. Since then it’s been longer.
    We generally labor under the illusion that 86,400 seconds make up a day. But this hasn’t been true for nearly two hundred years. A modern day is 86,400.002 seconds long. So we messed up. When the current system was put into place a half century ago, we could have then defined each second a little differently by adding a couple hundred more of those microwave beats to each official second. Who would