# Leap seconds and the rotation of the earth **URL:** \ **Category:** Factual Questions\ **Created:** [June 30, 2015, 12:48pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869 "2015-06-30T12:48:25Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![bizerta](https://avatars.discourse-cdn.com/v4/letter/b/3be4f8/32.png) [@bizerta](https://boards.straightdope.com/u/bizerta)\ **Post date:** [June 30, 2015, 12:48pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/1 "2015-06-30T12:48:25Z")
A leap second will be added to UTC at 23:59:60 tonight. The earth’s rotation is slowing down because of the tides and the moon. When (and how) did astronomers first realize this? As I understand the graph at > **[Leap second](https://en.wikipedia.org/wiki/Leap_second#/media/File:Leapsecond.ut1-utc.svg)** > > A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would r... leap seconds were added whenever there is a vertical spike in the graph. So, why is the earth’s rotation so erratic? Leap seconds occurred about every 12-18 months in the nineties, then there was a seven year gap from 1999 to 2006. If I read the graph correctly, between 1999 and 2006, the earth was spinning faster than it was between 1992 and 1999. In fact, where the line slope is positive, the earth was spinning faster than it was in the year 1900, which is the base year for the definition of one second of time. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [June 30, 2015, 12:55pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/2 "2015-06-30T12:55:48Z")
Perhaps there were more earthquakes in 1999-2006? Earthquakes can speed up the rotation of the earth if the crust “settles” and thus comes closer to the center of the earth. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [June 30, 2015, 3:54pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/3 "2015-06-30T15:54:12Z")
Ludovic, I think it’s the other way 'round. In the period from 1999 to 2006, the Earth was going _slower_ than before or after. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [June 30, 2015, 7:26pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/4 "2015-06-30T19:26:07Z")
What happens to children born during the leap-second? Do they only get to have a birthday once every ten thousand years? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [June 30, 2015, 7:34pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/5 "2015-06-30T19:34:00Z")
> [@bizerta](#): > > The earth’s rotation is slowing down because of the tides and the moon. When (and how) did astronomers first realize this? This site says: > [@](#): > > Many physicists considered the effects of tides on the Earth-Moon system. However, George Howard Darwin (Charles Darwin’s son) was the first person to work out, in a mathematical way, how the Moon’s orbit would evolve due to tidal friction, in the late 19th century. He is usually credited with the invention of the modern theory of tidal evolution. > > So that’s where the idea came from, but how was it first measured? The answer is quite complicated… > > SOURCE: [http://curious.astro.cornell.edu/about-us/37-our-solar-system/the-moon/the-moon-and-the-earth/111-is-the-moon-moving-away-from-the-earth-when-was-this-discovered-intermediate](http://curious.astro.cornell.edu/about-us/37-our-solar-system/the-moon/the-moon-and-the-earth/111-is-the-moon-moving-away-from-the-earth-when-was-this-discovered-intermediate) ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [June 30, 2015, 7:59pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/6 "2015-06-30T19:59:09Z")
> [@Senegoid](#): > > What happens to children born during the leap-second? Do they only get to have a birthday once every ten thousand years? I don’t have time to answer this. I’m too busy going around and setting all my clocks one second ahead. ---
**Author:** ![Dervorin](https://avatars.discourse-cdn.com/v4/letter/d/eb8c5e/32.png) [@Dervorin](https://boards.straightdope.com/u/Dervorin)\ **Post date:** [June 30, 2015, 8:42pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/7 "2015-06-30T20:42:32Z")
> [@Senegoid](#): > > What happens to children born during the leap-second? Do they only get to have a birthday once every ten thousand years? Someone catch that flying baby! She’s going to be born in just one second. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [June 30, 2015, 9:11pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/8 "2015-06-30T21:11:46Z")
Note that a lot of news sites are reporting that the leap second is just before midnight. But that’s only for UTC. E.g., it happens before 8pm EDT. This could mean an extra second of ads after _Wheel_ for some people on the East Coast. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [June 30, 2015, 10:01pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/9 "2015-06-30T22:01:46Z")
[Could we speed up Earth’s rotation, so that we do not need Leap Seconds?](https://what-if.xkcd.com/26/) 😃 ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [June 30, 2015, 11:24pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/10 "2015-06-30T23:24:10Z")
> [@ftg](#): > > Note that a lot of news sites are reporting that the leap second is just before midnight. But that’s only for UTC. > > E.g., it happens before 8pm EDT. This could mean an extra second of ads after _Wheel_ for some people on the East Coast. I’ll make sure to watch my clock. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [July 1, 2015, 12:09am UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/11 "2015-07-01T00:09:10Z")
> [@Sunspace](#): > > I’ll make sure to watch my clock. My computer didn’t show the extra second. ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [July 1, 2015, 1:17am UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/12 "2015-07-01T01:17:05Z")
> [@Thudlow\_Boink](#): > > I don’t have time to answer this. I’m too busy going around and setting all my clocks one second ahead. Now you’re going to be two seconds off. You need to set your clocks back one second. ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [July 1, 2015, 1:23am UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/13 "2015-07-01T01:23:20Z")
Since some of the various financial markets have gone entirely electronic since the last leap second some changes to the day’s schedule are being made. [QUOTE=TD Ameritrade] Modified Market Hours for June 30 Leap Second: Today, Tuesday, June 30, extended equity sessions and futures market openings will be modified to allow the exchanges to adapt to a Leap Second that will be added to the Coordinated Universal Time (UTC). Modified Market Hours for Tuesday, June 30, 2015: Equities: After-market equities extended-hours trading session will close 30 minutes early, at 7:30 p.m. ET. CME Futures: CME futures trading will have Pre-Open for all markets at 8:30 p.m. ET, and will open at 8:45 p.m. ET. ICE U.S. Futures: The following ICE U.S. Futures will have Pre-Open at 8:05 p.m. ET, and will open at 8:10 p.m. ET: Mini Gold, Mini Silver, Russel 2000, U.S. Dollar Index. Soft Futures will open at their regular trading time. VIX Futures: VIX Futures will have a 15-minute trading halt at 7:55 p.m. ET and will reopen at 8:10 p.m. ET. Forex: Forex will trade as normal, uninterrupted. If you have any questions, please call Client Services 24/7 at 800-669-3900. [/QUOTE] ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [July 1, 2015, 4:40pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/14 "2015-07-01T16:40:41Z")
Any reports of leap second disasters like there were last time? ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [July 1, 2015, 8:09pm UTC](https://boards.straightdope.com/t/leap-seconds-and-the-rotation-of-the-earth/723869/15 "2015-07-01T20:09:00Z")
I thought I woke up this morning a little more refreshed than usual. Cool.