The interval of time between two successive meridional transits of the “mean sun,” an imaginary point moving with such constant angular velocity along the celestial equator as to complete one annual circuit in a time period exactly equal to that of the apparent (true) sun in its annual circuit. The mean solar day is 86 400 seconds, or 1. 0027379 sidereal day. This modification of the apparent solar day was devised as a means of smoothing the irregularities observed in apparent relative motion of sun and earth; the equation of time defines the difference between the two. Outside astronomical circles, the mean solar day is the day in common use, but since it is reckoned from noon to noon, it has been modified to the civil day for practical use.