<P> A day, a unit of time, is approximately the period of time during which the Earth completes one rotation with respect to the Sun (solar day). In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time . The unit of measurement "day", was redefined as 86 400 SI seconds and symbolized d . In 1967, the second and so the day were redefined by atomic electron transition . A civil day is usually 86 400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time . </P> <P> In common usage, it is either an interval equal to 24 hours or daytime, the consecutive period of time during which the Sun is above the horizon . The word day may also refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns (circadian rhythms) of humans and many other species are related to Earth's solar day and the day - night cycle . </P> <P> Several definitions of this universal human concept are used according to context, need and convenience . Besides the day of 24 hours (86 400 seconds), the word day is used for several different spans of time based on the rotation of the Earth around its axis . An important one is the solar day, defined as the time it takes for the Sun to return to its culmination point (its highest point in the sky). Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year . Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours . In recent decades, the average length of a solar day on Earth has been about 86 400.002 seconds (24.000 000 6 hours) and there are about 365.242 2 solar days in one mean tropical year . </P>

1 solar day is equal to how many hours