Second s
About the Second
The second is defined by taking the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom to be exactly 9,192,631,770 hertz. Before the atomic definition it was a fraction of the mean solar day and then of the tropical year 1900. Atomic clocks realise it so precisely that the second is the most accurately realised of all SI units.
Related Units
Hertz
Hz1 Hz = 1 s⁻¹
The hertz is one cycle or event per second and replaced the older expression cycles per second. It is named after...
Day
d1 d = 86,400 s (exact)
As a unit accepted for use with the SI, the day is exactly 86,400 seconds. The actual solar day varies slightly and the...
Julian year
a365.25 d = 31,557,600 s (exact)
In astronomy the Julian year is exactly 365.25 days of 86,400 seconds, or 31,557,600 seconds. It is the year used to...
Metre
mBase unit; c = 299,792,458 m/s exactly
The metre is the length of the path travelled by light in vacuum in 1/299,792,458 of a second. It was first defined by...