What is the significance of the cesium-133 atom in defining the second?

[Dandy Candy]

Hi all,

I am a new member in your forum and I am very pleased to join it. This my first theard in this great forum.

My Q.:

The unit of time was originally defined as 1/86400 of the mean solar day. Because of irregularities in the Earth roatation, the definition was changed to the '' emphemeris second '', i.e., 1/31556925.9747 of the tropical year 1900. In 1967 , this definition was replaced.

the second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine of the ground state of the cesius-133 atom.

The defintion is based on the atomic clock. One of the best atomic clocks ( NIST-F1) is precise to within about 1 second in 60 million years, or 5 parts in 10^16. Commercially available atomic clocks are precise to within 3 parts in 10^12.

I didn't understand this paragraph .. it's from my textbook !

Concepts in Engineering
Second Edition
Mark Holtzapple and W. Reece
McGraw-Hill
2008Would you mind explaining it, please?

 

[Dandy Candy]

no reply ?!

:(

 

[Redbelly98]

The defintion is based on the atomic clock. One of the best atomic clocks ( NIST-F1) is precise to within about 1 second in 60 million years, or 5 parts in 10^16. Commercially available atomic clocks are precise to within 3 parts in 10^12.

I didn't understand this paragraph .. it's from my textbook !

It means that if you set the clock "exactly", and then wait 60 million years, it could be reading the wrong time. But, it will only be off by about 1 second or so, after all that time.

"5 parts in 10^16" means the time could be off by 5 seconds after having run for 10^16 seconds (that's about 300 million years).

Or it could be off by 5 nanoseconds after having run for 10^16 nanoseconds (that's 16.5 weeks). A nanosecond is a billionth of a second or 10^-9 seconds.

no reply ?!

Many of us (in USA) were sleeping when you first posted.

 

[Dandy Candy]

Redbelly98

Thanks a lot !

: )