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Author Topic: How do we know what time it is?  (Read 7565 times)

adrian

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How do we know what time it is?
« on: 09/08/2008 13:12:26 »
adrian  asked the Naked Scientists:

Hello Naked Scientists,

I'm Adrian, Romania, 28 years old. Thank you, I'm looking forward for every walk with my mp3 player and your shows, you keep my mind fresh and my curiosity awaken.

Question: How do we know what's the time? How is the position of our planet calculated so accurately? Is there a guy somewhere with a watch in his hand and presses its start button when our planet reaches a certain point? How does he see this point?

Regards,
Adrian

What do you think?


 

Offline syhprum

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How do we know what time it is?
« Reply #1 on: 09/08/2008 19:48:23 »
Until recent times the rotation of the Earth was used to define the value of the second, a suitable star was observed passing the crosshairs on a transit telescope examples of which are preserved at Greenwich observatory.
As you say by a guy with a watch or more likely a pendulum clock.
One of the guys who did this work was Joseph Stalin the future leader of the USSR (he was sacked for not being very good at it!)   
« Last Edit: 09/08/2008 19:52:09 by syhprum »
 

lyner

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How do we know what time it is?
« Reply #2 on: 11/08/2008 11:26:18 »
Until you look into it, it is hard to comprehend just how accurately you can make astronomical observations; fractions of a second of arc are a piece of cake. When you think of that tiny division of one degree on a School protractor and then go to 1/360 of that. . . .
There is a lot of 'averaging' and repeating of measurements to eliminate the effects of atmospheric disturbance etc..
 

Offline graham.d

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How do we know what time it is?
« Reply #3 on: 11/08/2008 13:26:28 »
Look at the Definitions and Standards section in:

http://en.wikipedia.org/wiki/Time

The time period of 1 second is defined as a standard from a Ceasium Clock and calibrated to the earths rotation on a day known as "January 0" in the year 1900 - see Ephemeris time. This is really a theoretical calculation because the earth's rotation is not constant and varies on a short term basis and also is gradually slowing down. Our astronomical clocks have to be adjusted periodically (with an added leap second) to remain approximately correct.

Ephemeris time has since been superseded, see "Revision" in:

http://en.wikipedia.org/wiki/Ephemeris_time
« Last Edit: 11/08/2008 13:32:26 by graham.d »
 

lyner

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How do we know what time it is?
« Reply #4 on: 11/08/2008 17:13:16 »
I heard, somewhere, that Astronomical time was to be used by everyone except 'Scientists'. Updating clocks to fit Universal time is a very expensive business and serves no real purpose for most applications.
As a sub-standard, the old system is on a par with the lengths of platinum which define the metre - near enough for Jazz - and most other things.
 

Offline graham.d

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How do we know what time it is?
« Reply #5 on: 11/08/2008 17:29:14 »
GPS clocks have to be kept precise and fall into a category between scientific and commercial use. I think these get periodically realigned with everyday astronomic time.
 

lyner

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How do we know what time it is?
« Reply #6 on: 11/08/2008 18:36:29 »
Oh yes - absolutely! Money no object there.
 

Offline Alan McDougall

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How do we know what time it is?
« Reply #7 on: 12/08/2008 12:59:44 »
Greetings Adrian,

I also have a facination with time.

Here is an article about the most accurate clock that might be of interest to you.

World's most accurate clock unveiled
By Roger Highfield, Science Editor
Last Updated: 6:01pm GMT 14/02/2008

 


The most accurate atomic clock of its kind is unveiled today, marking a new leap forward in efforts to synchronise telecom networks and deep-space communications, as well improve the accuracy of navigation around the world.

   
JILA's strontium atomic clock is now the world's most accurate clock based on neutral atoms
The new clock is based on a few thousand strontium atoms trapped in grids of laser light and is twice as accurate the current US time standard based on a "fountain" of caesium atoms.

This clock tops previous records for accuracy in clocks based on atoms. Although not as accurate as an experimental design based on a single mercury ion (a charged mercury atom), which is better than 1 second in 400 million years, technology to interconnect so called "neutral atom" clocks, such as the strontium one, into a "clock of clocks" promises to make them the most precise of all.

Ultra cold atoms are the "pendulums" for atomic clocks because they only absorb highly precise frequencies of light and, because a frequency is the number of oscillations per second, they can be used to measure the passing of time.

Because the strontium atoms absorb higher frequency light than earlier clocks, which rely on longer microwaves, these optical clocks have shorter and more accurate "ticks" - 430 trillion per second.

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The next-generation atomic clock is reported today in the journal Science by Dr Jun Ye and colleagues at JILA, a joint institute of the Commerce Department's National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.

The team could link the new clock with existing clocks, using a special fibre optic link, which is crucial if the new clock is to be used as a timekeeping standard, since that has to depend on polling the ticks of lots of clocks.

The JILA and NIST are home to optical clocks based on a variety of atoms, including strontium, calcium, mercury, aluminum, and ytterbium, each offering different advantages.

Dr Ye now plans to compare strontium to the mercury ion clock. "The accuracy of the strontium clock is expected to continue to advance," he tells The Daily Telegraph.

A large ensemble of neutral atom clocks will be more precise than a single trapped ion based clock, he adds. "The large ensemble of neutral atoms offers an enhanced signal strength, thus improving the clock precision."

The race to build better clocks is expected to lead to new types of gravity sensors, new tests of fundamental physical laws to increase understanding of the universe and aid efforts to build the next generation of superfast "quantum" computers.
 
 

Offline Allanon

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How do we know what time it is?
« Reply #8 on: 21/08/2008 08:16:29 »
Hello Adrian and other fans of the measurement of time.

The first thing a person needs to think of is what is time, where did it come from, how does one measure it. I recently read a book by G.J. Whitrow who explores time throughout history (title of book is "Time In History". Cognitive thought of 'time' was never measured prior to the invention of the sundial, everything prior was based upon day and night and also seasons.

What this boils down to is to measure time based upon the position of a planet be it Earth or otherwise is mundane, humans created the concept of time to understand how to measure things in their world (planting crops etc.).

If you are looking for an explanation of how humans actually calculate time I suggest you read the above book. I may be missing something but the position of the Earth at any given time has nothing to do with a humans perception of time. Please update if I have missed the point entirely.

-Allanon











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Offline Alan McDougall

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How do we know what time it is?
« Reply #9 on: 21/08/2008 16:32:46 »
Of course time is relative and subjective.  There is no absolute time and some speculate that linear time is an illusion
 

Offline Maddux

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How do we know what time it is?
« Reply #10 on: 23/08/2008 05:35:38 »
I feel the same way as Alan and Allanon, but my question would be not what is time but why is time. Why do we feel the need to "gauge" everything we do. How would life be without time? Is our aging based on our perception of time? Can I say the universrse is only 7 billion years old becasue I measure time differently--and not be wrong?
Just some questions I needed to get off my chest!
 

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How do we know what time it is?
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