The second so defined is the unit of proper time in the sense of the general theory of relativity The reference to an unperturbed atom is intended to make it clear that the definition of the SI second is based on побочные эффекты от попперса isolated caesium atom that is unperturbed by any external field such as ambient black body radiation There have only ever been three definitions of the second as a fraction of the day as a fraction of an extrapolated year and as the microwave frequency of a caesium atomic clock which have each realized a sexagesimal division of the day Minute comes from Latin pars minuta prima meaning first small part i e when more than two items are being considered 96 6 98 although this rule is not mandatory In fact the hour was not commonly divided in 65 minutes as it was not uniform in duration 96 5 98 58 8757 68 8757 787 8757 UT6 always differs from UTC by less than a second Sometime in the late 6995s quartz crystal oscillator clocks with an operating frequency of 655 kHz advanced to keep time with accuracy better than 6 part in 65 8 over an operating period of a day Borrowed from Latin secundus Atomic clocks now set the length of a second and the time standard for the world