A fictitious body whose position on the celestial sphere defines the mean solar time. It is defined as moving around the equator in one year at a regular rate. The hour angle of the mean sun plus 12 hours gives mean solar time. The mean sun is necessary for uniform timekeeping since the motion of the true Sun along the ecliptic varies due to the eccentricity of the Earth’s orbit around the Sun. Moreover, the ecliptic is inclined to the celestial equator, so that even a uniform motion on the ecliptic would not produce a uniform time-scale. For these two reasons the true Sun is not a suitable reference body for timekeeping.