Just a Second

Just a Second

Time and moon Moon defines me Just a Second What is a second? How is it defined?

What can happen in one second? A lot, so it maers

What is a second, what can happen in one Earth's rotaon is slowing down Why? second, and why do we need leap seconds? What are the consequences? Are seconds geng longer?

What is a leap second? Why are they needed?

Dr Steve Barre BASoc 15 Jun 2020 1 2

Time and Moon What Is a Second?

How do humans think about me? Surely that's trivial – the Earth spins on its axis once a day, where

Ulmately, we always measure me by the moon of … something. 1 day = 24 hours 1 hour = 60 minutes 1 minute = 60 seconds

For millennia the passage of 1 me has been measured by the So one second = 24 x 60 x 60 moon of astronomical objects such as the Sun moving across 1 the sky during the day … = of one day. 86400

… or the changing phases of the Moon at night. Simples!

So that's the end of the talk …? No. Let's look at how the Earth moves.

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Dr Steve Barre – 1 – BASoc 15 Jun 2020 Just a Second

Earth Orbits the Sun June Earth Orbits the Sun

Photographing the Sun at a the same me of day throughout the year produces an analemma. The Earth's orbit around the Sun is an ellipse. The North–South variaon is a result of the 23° lt of the Earth's axis. In the Northern hemisphere, the Sun appears higher in the sky in Summer and lower in the Winter. The Earth's speed varies depending on its distance The East–West variaon is a from the Sun. December result of the Earth's speed varying in its ellipcal orbit around the Sun.

5 Giuseppe Petricca 6

Earth Spins On Its Axis What Is a Second?

An analemma shows us that Since 1968 the second has had the precise definion of sundials do not accurately indicate the passage of me 9,192,631,770 because of the Earth's orbit,

not because of variaons in oscillaons of a caesium atom (or, more accurately, the microwave the rate at which the Earth radiaon corresponding to the transion between two energy levels spins on its axis. of the isotope caesium-133).

This is called the To measure the variaons in the length caesium standard. of a day (meaning the rotaon period of the Earth, not the me between sunrise and sunset) we need a clock much more accurate than a sundial.

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Dr Steve Barre – 2 – BASoc 15 Jun 2020 Just a Second

Atomic Clocks Time Maers

An atomic clock uses this caesium radiaon to determine the passage The GPS system relies on calculang the of me to a precision of beer than 1 ns per day. distances between you and a handful of satellites. Each satellite has an atomic That's equivalent to 1 s in 30 million years. clock on board.

NASA are developing a Deep Space Atomic Clock (DSAC) that is about If a clock is wrong by one milli second, the size of a toaster. the distance would be wrong by 300 km.

It will be used in If a clock is wrong by one micro second, space probes to the distance would be wrong by 300 m. improve navigaon accuracy. Accuracy maers.

GPS clocks are re-synchronised every few hours to eliminate dri.

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What Can Happen In One Second? What Can Happen In One Second?

The fastest supercomputer can do Light travels 186,000 miles or 300,000 km in one second.

200,000,000,000,000,000

Two of the fastest manmade objects calculaons per second. are the Voyager planetary probes travelling at about 16 km/s.

Even with this speed, running a simulaon of the evoluon of the Universe can take days, weeks or months of number crunching.

That's a lot of calculaons.

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Dr Steve Barre – 3 – BASoc 15 Jun 2020 Just a Second

An Internet Second The Earth Is Slowing Down

Atomic clocks tell us that the 300,000 Earth is slowing down. Texts / second Back in 1968 when 'atomic me' was insgated the Earth made 60,000 75,000 one turn (relave to the Sun) in 24 h 00 m 00 s = 86400 s. Searches / second Videos / second

So a day was exactly 24 h.

700,000 3,000,000 More than fiy years later a day

Messages / second Emails / second is now 86400.001 s.

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Theia Impact The Moon's a Drag

For the past 4.5 billion years, dal forces have slowed the Earth's rotaon

4.5 billion years ago

When the Earth recovered from the shock of the impact of Theia

its rotaon period was about 5 h.

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Dr Steve Barre – 4 – BASoc 15 Jun 2020 Just a Second

So What? So What?

So a day is not an exact number of seconds. So what? If we want our 24-hour clocks to stay synchronised with the rotaon of the Earth (so that the Sun is in the sky when our clocks say it is It's just like the problem we have with a year not being an exact dayme) then we need to add leap seconds every once in a while. number of days.

1 year = 365.2422 days ( Actually, we don't NEED to. Alternavely, we could keep clocks synchronised with the rotaon of the Earth by leng seconds get If not addressed, the calendar would dri very slowly relave to the longer as the Earth slows down. seasons. Inserng an extra day every fourth year would make the calendar year = 365.25 days. Almost right. However, sciensts would be furious!

Skipping a leap day in a century year that is not divisible by 400 makes Atomic clocks would have to be set to run at slower and slower rates. the calendar year = 365.2425 days. That's prey close. Having the definion of the second change every few minutes would be totally impraccable. )

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Length of a Day Length of a Day

milliseconds milliseconds

5 Day length – 86400 s 5 Day length – 86400 s 4 4

3 3

2 2

1 1

0 0

1970 1980 1990 2000 2010 1970 1980 1990 2000 2010

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Dr Steve Barre – 5 – BASoc 15 Jun 2020 Just a Second

Length of a Day Leap Seconds

milliseconds Leap days are inserted into the Gregorian calendar as an extra day – 29 February. 5 Day length – 86400 s 4 What about leap seconds? 3

2 That is decided by the …

1 Internaonal Earth Rotaon Service 0 How oen should a leap second be added?

1970 1980 1990 2000 2010 When should leap seconds be inserted into a day?

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Length of a Day Leap Seconds

milliseconds seconds

5 Day length – 86400 s 25 4 20

3 15

2 10

1 5

0 0

23 : 59 : 60 1970 1980 1990 2000 2010 Leap seconds introduced in 1972

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Dr Steve Barre – 6 – BASoc 15 Jun 2020 Just a Second

Leap Seconds Leap Second Smearing

When should leap seconds be inserted into a day? Local midnight? Some people really don't like leap seconds. But midnight where? Every me zone has its own midnight. Some computer systems really don't like leap seconds.

Some companies really don't like leap seconds.

For instance, Google use leap second smearing to avoid the minute before midnight having 61 seconds. They add the leap second, a few microseconds at a me, connuously throughout the day.

They do this by running their clocks 0.001% slow for a day.

If leap seconds are not inserted at the same instant, clocks around the Imagine doing that with a leap day … world will be out of synch by up to 1 second. In the world of global electronic finance, that lack of synchronisaon maers.

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Leap Seconds

Leap day smearing is a joke, but the problem of what to do about leap

seconds is serious. Just a Second

The Internaonal Tele- communicaon Union (ITU) is a UN agency that has considered whether or not me signals www.liverpool.ac.uk/~sdb/ Talks should connue to have leap seconds inserted.

In 2015 they decided … not to decide unl 2023.

Dr Steve Barre BASoc 15 Jun 2020 xkcd.com 27 28

Dr Steve Barre – 7 – BASoc 15 Jun 2020