Measuring time and calendars

Time measurement is based on the regular repeating motion of celestial bodies. A sidereal day is the time between two successive upper culminations of the vernal equinox point (23 h 56 min 4 s); a solar day is the time between two culminations of the Sun and, because the Earth also moves along its orbit, averages 24 hours. The true solar day is uneven through the year, so the idea of a mean Sun was introduced; the difference in times (the equation of time) varies through the year from about –14 to +16 minutes. Local time differs at every meridian: a longitude difference of 15° gives 1 hour, 1° gives 4 minutes. For convenience the Earth is divided into 24 zones; Uzbekistan keeps UTC+5 all year round. Modern timekeeping rests on atomic clocks: since 1967 the second has been defined by 9,192,631,770 periods of radiation of the caesium-133 atom, and UTC is kept in step with the Earth’s rotation by leap seconds for now (they are planned to be dropped by 2035).