As the Earth rotates on its axis, the stars appear to move round the sky in the opposite direction. Since the Earth is travelling around the Sun, the Sun as viewed from Earth appears to move against the background of stars. Because of this it is possible to measure (or to define) the length of a day in two different ways. A SIDEREAL DAY is the time for the stars to move once round the sky and return to the same positions in the sky relative to the Earth. A SOLAR DAY is the corresponding time measured using the Sun rather than the stars. The Earth rotates on its axis in the same direction as it revolves around the Sun, so the solar day is longer than the sidereal day . In a year the number of sidereal days is exactly one more than that of solar days, because the Earth, in orbiting the Sun, has effectively made one less rotation relative to the Sun. The difference in the length of the two days is 1/365.24 sidereal days which is about four minutes. If we measure time by the Sun the stars will therefore be in the same positions in the sky four minutes earlier each day (or two hours earlier each month). The length of the solar day varies throughout the year because of the ellipticity of the Earth’s orbit and the tilt of its axis to the ecliptic. The position of the real Sun determines what is called APPARENT SOLAR TIME. For practical purposes it is much more convenient to use MEAN SOLAR TIME which is obtained by making all days equal in length to the average length of a solar day. The difference between apparent and mean solar time is called the equation of time and it can be as large as a quarter of an hour . A basic sundial gives apparent solar time although it is possible to mark the dial so that mean time can be used.

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