What is the length of a day?
One full 360 degree rotation.
One Earth day is simply one full rotation of earth about its north-south axis.
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There are three different definitions of the length of a day and each are of different lengths.
There are three different definitions of the length of a day and each are determined by a different means.
The mean solar day is the most familiar. It is measured by clocks. It is exactly 24 hours long. As the second is a well defined standard unit a better definition is that a mean solar day is exactly 86,400 seconds long.
The solar day is the time between successive solar noons when the Sun is at its highest in the sky. It is measured by sundials. The solar day varies continuously from day to day. It can be anything between 24 hours -20 seconds and 24h hours +30 seconds. The solar day and mean solar day are the same length on four days per year.
The mean sidereal day is one complete rotation of the Earth relative to the Vernal Equinox or to the fixed stars it is 23 hours 56 minutes 4.1 seconds long.
The reason why the solar day is longer than the sidereal day is that in 24 hours the Earth has moved in its orbit and has to rotate a bit further to for the Sun to be at its highest at the same point on the Earth.