Why does a day last 24 hours?

What if the day isn't really 24 hours? An animation shows the difference between solar day and sidereal day

(image: Getty Images) To some it may sound strange but the Earth does not take exactly 24 hours to complete a rotation (360 °) on itself, but 23 hours, 56 minutes, 4 seconds. Actually, 24 hours is the time it takes for the Sun to return to the same position as the sky.

An animation, created by James O'Donoghue of the Japanese Space Agency Jaxa, clearly shows the difference between sidereal day (23 hours and 56 minutes) and solar day (24 hours). Here it is.

In practice, explains O’Donoghue, it's all a matter of reference points: the Sun or the other stars? Defining a day as the length of time it takes the Sun to regain the same position in the sky from the observer's point of view (to be precise we should speak of the time elapsed between two successive culminations of the Sun in the same location), we have chosen the the longest day, that of 24 hours.

For this condition to occur, since our planet orbits the Sun at an average distance of one astronomical unit and the Earth's displacement is not negligible, the Earth must rotate 360 ​​° + 1 °, taking 4 minutes longer.

The consequence of this is that the Earth does not rotate 365 on itself, but 366 in a year.

If instead if we used the sidereal day (i.e. the one that takes as a reference point the other stars in the sky, which are at such a distance that the movement of the Earth along its orbit is negligible), O'Donoghue continues, the Sun would rise about 4 minutes first every day.