Time-Second

1. How has the definition of how long a second is changed over time? Early definitions of the second were based on the apparent motion of the sun around the earth. The solar day was divided into 24 hours, each of which contained 60 minutes of 60 seconds each, so the second was 1⁄86 400 of the mean solar day. However, 19th- and 20th century astronomical observations revealed that this average time is lengthening, and thus the sun/earth motion is no longer considered a suitable basis for definition. With the advent of atomic clocks, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. []

~ Samuel 2.What is a leap second?

A leap second is a second added to Coordinated Universal Time to make it agree with astronomical time to within 0.9 seconds. UTC is an atomic time scale, based on the performance of atomic clocks. Astronomical time is based on the rate of rotation of the Earth. Since atomic clocks are more stable than the rate at which the Earth rotates, leap seconds are needed to keep the two time scales in agreement.

The first leap second occurred on June 30, 1972. There have been a total of 18 leap seconds to this date. This means that leap seconds occur at a rate of slightly less than one per year. Although it is possible to have a negative leap second (a second removed from UTC), so far, all leap seconds have been positive (a second has been added to UTC). Based on what we know about the Earth's rotation, it is unlikely that we will have a negative leap second in the foreseeable future. [] [|the link] (Tianyi)

3.How is the actual time defined? (TAI), International Atomic Time, is the s a high-precision atomic coordinate time standard. It is calculated by the average time of over 200 atomic clocks. These atomic clocks are usually designed to find the number of cycles of radiation corresponding to the transition between two energy levels of the caesium-133 atom, where every 9192631770 cycles is a second. As of 1 January 2011, the TAI is exactly 34 seconds ahead of UTC, which is what we use. http://en.wikipedia.org/wiki/Atomic_clocks http://en.wikipedia.org/wiki/International_Atomic_Time ~Yi Jia

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5. Subdivisions of the hour probably have their origin in ancient Babylon. The Babylonians developed a number system called Sexagesimal around 4,000 years ago. Sexagesimal is based on 60. The system that we are more familiar with is the decimal system, which is based on 10. The Babylonians found that, in astronomy, Sexagesimal was far superior to the other systems, and it was later adopted to subdivide the hour into 60 minutes, and the minute into 60 seconds. Through the ancient Greek Empire, this system gained acceptance in the Mediterranean cultures and later spread throughout the world. It is still in use today, not only in time but also in angular measurements. || ~Jarret
 * Why must 60-second make 1 minute and 60 minutes make 1 hour, not the other way round?
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