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Speaker 1: This Day in History Class. It's a production of I

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Speaker 1: Heart Radio. Hey y'all, it's Eves. Welcome to This Day

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Speaker 1: in History Class, a podcast for people who can never

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Speaker 1: know enough about history. And it's this super special episode

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Speaker 1: today because it's the last new episode that I'll be hosting.

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Speaker 1: We will still continue to run episodes from the vault,

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Speaker 1: and you'll still hear my voice for this reason. You'll

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Speaker 1: also hear Tracy V. Wilson's voice, who previously hosted the show,

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Speaker 1: that said, it's been a joy being immersed in so

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Speaker 1: much history and getting to share it with you, And

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Speaker 1: I hope you've enjoyed it as much as I did,

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Speaker 1: and I hope that you'll all continue to return to

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Speaker 1: the show to learn about anything you may have missed,

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Speaker 1: or to give your favorite episodes a second Listen now

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Speaker 1: on with the show. Today is June thirty. The day

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Speaker 1: was June two pm and sixty nts. A leap second

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Speaker 1: was added to Coordinated Universal Time to synchronize clocks with

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Speaker 1: earth decelerating rotation. The second has been defined many different

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Speaker 1: ways over the years. At one point, it was defined

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Speaker 1: as one eighty six thousand, four hundred of the mean

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Speaker 1: solar day but a more precise measurement was needed because

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Speaker 1: the length of a day varies depending on many factors

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Speaker 1: like seasonal and daily weather variations, as well as oceanic

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Speaker 1: and atmospheric tides. By nineteen sixty seven, the second was

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Speaker 1: defined as in a quote, the duration of nine billion,

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Speaker 1: one d and ninety two million, six hundred and thirty

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Speaker 1: one thousand, seven hundred and seventy periods of their radiation,

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Speaker 1: corresponding to the transition between the two hyperfine levels of

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Speaker 1: the ground state of the caesum one thirty three atom.

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Speaker 1: This was the measurement that the International System of Units

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Speaker 1: or s I used. Since then, the wording of the

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Speaker 1: official definition has been updated slightly. A tom clocks keep

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Speaker 1: time with extreme precision. On atomic clocks, a day is

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Speaker 1: exactly eighty six thousand, four hundred s I seconds. International

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Speaker 1: atomic time is a time scale that is computed by

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Speaker 1: taking the weighted average of more than four hundred atomic

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Speaker 1: clocks around the world. It's not connected to any astronomical observations.

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Speaker 1: Universal time, on the other hand, is a time standard

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Speaker 1: that is based on Earth's rotation and astronomical observations. Coordinated

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Speaker 1: universal time or UTC, is under the umbrella of universal time,

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Speaker 1: which also includes UT zero, U T one, U T

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Speaker 1: one R, and U T two. Unlike other versions of

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Speaker 1: universal time, UTC is determined by International atomic time. Though

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Speaker 1: the practice of UTC was already being coordinated internationally, the

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Speaker 1: International Astronomical Union didn't adopt the name coordinated Universal time

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Speaker 1: until nineteen Coordinated Universal time is the primary standard by

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Speaker 1: which the world regulates time, but Earth's rotation, as measured

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Speaker 1: by UT one, is gradually slowing, so that the length

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Speaker 1: of a rotational day is about two milliseconds longer than

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Speaker 1: the eighty six thousand, four hundred seconds it was two

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Speaker 1: centuries ago. That means that there's a discrepancy between UTC

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Speaker 1: and UT one. Scientists determined that UTC would have to

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Speaker 1: be adjusted to account for the difference between the definition

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Speaker 1: of the second and Earth's rotation. This keeps UTC in

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Speaker 1: line with the apparent position of the Sun and stars.

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Speaker 1: In other words, a second would need to be added

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Speaker 1: to or removed from clocks to realign them with Earth's rotation. Occasionally,

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Speaker 1: scientists specified that UTC should not deviate more than nine

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Speaker 1: tenths of a second from UT one, So on June nine,

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Speaker 1: seventy two, the first leap second was added to UTC.

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Speaker 1: The International Earth Rotation and Reference Systems Service decides when

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Speaker 1: to add a leap second. One is typically added either

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Speaker 1: on June December thirty one. From nineteen seventy two to nine,

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Speaker 1: leap seconds were added at a rate of about one

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Speaker 1: per year. After that, they've been added less frequently. There

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Speaker 1: have been twenty seven leap seconds since nineteen seventy two.

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Speaker 1: The most recent leap second was added on December one.

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Speaker 1: Many people have called for the elimination of leap seconds

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Speaker 1: and the replacement of UTC with a new system. Leap

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Speaker 1: seconds have caused problems for some computer systems since they're

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Speaker 1: not that predictable and can't be anticipated far in advance,

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Speaker 1: and they'll need to be added more frequently as Earth's

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Speaker 1: rotation continues to slow down. Some people who support abolishing

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Speaker 1: leap seconds argue that it doesn't matter whether our perception

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Speaker 1: of time changes along with the rotation of Earth, since

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Speaker 1: that would happen over a long time anyway. I'm Eves

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Speaker 1: Jeff Coote, and hopefully you know a little more about

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Speaker 1: history today than you did yesterday. And if you have

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Speaker 1: any commerce oar suggestions. You can send them to us

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Speaker 1: at this day at i heeart media dot com. You

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Speaker 1: can also hit us up on social media where at

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Speaker 1: t D I h C podcast. Also a shout out

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Speaker 1: to the extraordinary producers of this show, Alexis and Chandler,

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Speaker 1: who y'all never get to hear but are a huge

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Speaker 1: part of why you love the show and are still

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Speaker 1: listening to it. Thanks to Ken for listening to the

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Speaker 1: show and we'll see you tomorrow. For more podcasts from

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Speaker 1: I Heart Radio, visit the iHeart Radio app, Apple Podcasts,

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Speaker 1: or wherever you listen to your favorite shows.