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--------------------------------- 2369 - Earth’s Leap Second Slow Downs
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- Have you been feeling a bit sluggish lately. Well, you are not alone. The whole world has gotten more sluggish. Scientists blame it mostly on the Moon but some on global warming.
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- The problem is the Moon’s gravity pull on the Earth is not the same everywhere. On the side of the Earth closest to the Moon the gravity is strongest, weakest on the opposite side farthest from the Moon. So the Moon pulls the oceans up on the closest side and lets the oceans sink outward on the farthest side. This is the tidal affect that happens twice each day.
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- The oceans sloshing back and forth twice a day with the tides is slowing the Earth’s rotation. The friction of the ocean waters moving back and forth is using energy and robbing rotational energy. The result is the Earth’s rotation is slowing down.
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- The year 2005 we had to add one whole second to the last day of the year. This is the 23rd time we have added a second since 1972. The problem here is that when to add the second is not predictable.
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- Scientists have 200 atomic clocks all over the world and they all have to add a second at the same time, midnight Greenwich Mean Time, or 3:59:59 California time on New Year’s Eve.
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- Atomic Clocks are accurate to within a tenth of a billionth of a second a day, or 36.5 billionth of a second a year. The Earth’s rotation is only accurate to within one second a year, or 1,000,000,000 billionths of a second a year.
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- If we want to have atomic clocks agree with Earth’s rotation than they have to be adjusted every year or so. It is like changing to Daylight Savings, there is a chance of a mistake. Some clock somewhere will be on the wrong time.
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- Each year during June scientists decide if the that year gets an additional second. They have to wait until within six months because that degree of accuracy is not predictable in advance of that.
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- This is where global warming comes in. It is the melting of the glaciers that is causing the Earth’s rotation to speed up. So, the glaciers are countering the effect of a slower rotation, and predicting the melting time for glaciers is not well understood. It is the fact that the huge water supply in the frozen ice is changing elevation when it melts. We will come back to this explanation later.
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- Now, one second does not sound like much of a sluggish problem but it is a disaster to have a one second problem in the Global Positioning System. If timing was not perfect in phasing in the North American power grid the result could be a blackout for the entire hemisphere.
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- People are writing software that is going to be used decades in the future and some of the code is time dependent. Software writers need to know what time it is going to be in the future.
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- A day is about .002 seconds shorter today than it was during the San Francisco earthquake in 1906. Astronomers have solar eclipses and comets recorded back 4,000 years. In order to account for the eyewitness locations the Earth had to have been rotating faster then than it is today. If you go back to 150,000,000 years ago the dinosaurs only had a 22 hour day.
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- Historically time has always based on Earth’s rotation. In 1900 the second was defined as 1/86,000 the length of an Average Day, indicated by the setting Sun. By 1949 atomic clocks were invented and the second was based on the transition time of electrons between two energy levels in Cesium atoms. There are exactly 9,192,631,770 of these transitions each second.
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- That is an accuracy of 2 nanoseconds per day, or 2 * 10^-9 seconds per day (2.3 parts per 10^16). Since then atomic clocks using Hydrogen atoms have been made even more accurate. Both types of clocks are used to keep accurate times in 55 locations around the world.
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- There are 100 of these clocks in the U.S. Naval Observatory in Washington, D.C. and Schriever Air Force Base in Colorado Springs, Colorado. Newer clocks are accurate to
1 part in 10^16. Even newer ones based on laser cooled atoms and ions are accurate to 1 part in 10^18.
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- We are starting to approach problems with Einstein’s general theory of relativity. Albert’s equations show that clocks run faster at higher elevations, where gravity is less. So now the elevation of these clocks must be specified to with one meter accuracy around the world.
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- Soon the elevation accuracy will need to be specified to within a centimeter. So now we have the problem of defining elevation from what reference point? We can not use sea level that moves up and down all the time. These atomic clocks are becoming like pendulum clocks used in the open sea in the 1400s. Small ships bobbing around the world’s oceans each keeping his own time.
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- How do we even know these clocks are all the same time? Astronomy to the rescue. Astronomers have a network of radio telescopes around the world precisely locating 600 quasars, beacons of radio energy from distant galaxies.
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- By tracking the position of these quasars astronomers can precisely measure the rotation of Earth. This system can detect the smallest slowdown in rotation over a period of a week. So astronomers are using this method to determine in June if we should add a leap second.
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- It is the unpredictability of when to add this extra second that is the real problem. Why don’t we have just one clock somewhere in the world with it’s elevation well known and set all the other clocks against it?
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- Politics is the biggest hurdle here. Then there is the problem that everyone does not set their clocks the same way. Some go blank for a second. Some read the 60th second twice, or some stay at zero for 2 seconds. The chance for error here is a real concern.
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- Why don’t we just bundle all the leap seconds up and do the leap once a decade? And, give everyone plenty of advanced warning. Or, why not wait until we are 3600 seconds off and move all the clocks back an entire hour? Just like a worldwide Daylight Savings? The hour shift won’t happen for another 400 years, around 2406.
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- In the meantime Earth is just going to have to continue to become more sluggish. And, we humans are just going to have to agree on what time we want to keep. The clocks themselves have become more accurate than our definitions of what time is. As Albert so rightly said time is relative.
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- May 16, 2019. 639
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--------------------- Thursday, May 16, 2019 -------------------------
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