Synodic day

A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.

In the case of a tidally locked planet, the same side always faces its parent star, and its synodic day is infinite.

[4] The mean length, however, is 24 hours (with fluctuations on the order of milliseconds), and is the basis of solar time.

As viewed from Earth during the year, the Sun appears to slowly drift along an imaginary path coplanar with Earth's orbit, known as the ecliptic, on a spherical background of seemingly fixed stars.

Combined with a nodal precession, this allows them to always pass over a location on Earth's surface at the same mean solar time.