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Solar days

The time we use in our daily lives is based on solar time. The fundamental unit of solar time is the solar day : the interval of time separating two passages of the Sun over the meridian, due to Earth’s rotation.

But in reality, the Earth does not carry out a rotation of 360 degrees in one solar day. The Earth also moves around the sun, by a little less than one degree per day (360/365.25 = 0.9856 degrees per day). So in 24 hours the direction of the Sun from the Earth has changed by almost a degree.

So in reality the Earth has to rotate 361 degrees for the Sun to pass over the meridian and seem to have passed 360 degrees in the sky. On average, a sidereal day is 4 minutes shorter than a solar day, due to this extra degree of rotation in a solar day.

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