Sunday, June 19, 2022


How Long Is a Day? The question at first seems to have an easy answer, 24 hours, things are more complex than that. To know how long the day is, we first have to define it. The two most common definitions used are sidereal and sun day. As we can guess from their names, the first uses the stars as a reference, while the second the Sun.

The sidereal day is defined as the time taken by the Earth to complete a complete rotation around its axis. As a reference for calculating this time period, a distant star (or gamma point) is used that is supposed to remain motionless during rotation. 

We can visualize the sidereal day thanks to the diagram in figure. Let us consider the initial moment when the segment m and the point S are aligned with the Sun and the distant star. The sidereal day is the period of time taken by the segment m to return to its initial position. In this instant of time an observer would see the initial star pass through the meridian again.

The duration of the sidereal day is 23h 56' 4 s".

The solar day uses the Sun as a reference and is defined as the period of time taken by the Sun to return to its initial position and pass into the meridian. According to the diagram, this occurs when the m segment and the S point are again aligned with the Sun.

If the Earth were motionless in space, the sidereal day and the solar day would coincide. The Earth, however, as it rotates on itself, moves along its orbit around the Sun. Over the period of time it takes to rotate on itself, the Earth moves around the orbit by about one degree (orange angle in the figure). The difference between the solar day and the sidereal day is therefore the time taken by the rotation of the Earth to compensate for this angle and is about four minutes.

The solar day is therefore longer than the sidereal one and has a duration of 24 hours.

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