an imaginary sun moving along the celestial equator at a constant rate and completing its annual course in the same time as the sun takes to move round the ecliptic at a varying rate. It is used in the measurement of mean solar time
mean sun in American English
Astronomy
a hypothetical sun thought of as moving uniformly along the celestial equator at a speed equal to the actual sun's average speed along the ecliptic: hypothesized as a basis for keeping time, because the actual sun does not travel along the ecliptic at a uniform rate