<P> A day is a unit of time . 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 period of time during which the Earth completes one rotation with respect to the Sun is called a solar day . Several definitions of this universal human concept are used according to context , need and convenience . In 1960 , the second was redefined in terms of the orbital motion of the Earth , and was designated the SI base unit of time . The unit of measurement `` day '' , redefined in 1960 as 86 400 SI seconds and symbolized d , is not an SI unit , but is accepted for use with SI . 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 . 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 of humans and many other species are related to Earth 's solar day and the day - night cycle ( see circadian rhythms ) . </P>
how is the length of a day determined on earth
period of time during which the Earth completes one rotation with respect to the Sun