A unit of time equal to the period of rotation of Earth. Different sorts of day are distinguished, according to how the period of rotation is reckoned with respect to one or another direction in space. A day is normally defined as 86,400 SI (Système International) seconds (86,400 s/d = (60 s/min) × (60 min/h) × (24 h/d)), where SI seconds are measured by atomic processes.