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 2008 Would you mind explaining it, please?