The sidereal day is 23 h 56 m 4.091 s and is defined as the time it takes a star to make two successive crossings of the Celestial Meridian. The solar day is defined as the time it takes the Sun to make two successive crossings of the Celestial Meridian:

If the Earth were stationary, then the Solar Day and the Sidereal Day would be precisely the same. However, we know that the Earth also revolves about the Sun. How does this affect the Solar Day?

Well, since the Earth moves roughly at a rate of (360 degrees)/(365.25 days) ~ 1 degree per day, the Earth will have moved 1 degree after 1 day. This means that in order for Sun to return to the Celestial Meridian, the Earth must turn roughly 1 more degree. That is, the Earth must turn ~1/365-th of the way around again to have the Sun reappear on the Meridian. This means that the Solar Day will be roughly 1/365-th times longer than the Sidereal Day.

If the Earth's orbital motion were steady, then this would be the entire story. Unfortunately, the Earth's orbital speed is not constant; it varies throughout the course of the year. This means that the length of the solar day also varies throughout the course of the year. From a clock standpoint, this is clearly unacceptable and so the Mean Solar Day was defined to be the average length of the true solar day. The mean solar day defines our clock day.

This is the reason that the Sun sometimes appears to be late and sometimes appears to be early. By early and late, I mean that the Sun does not cross the Celestial Meridian at the same time every day! This information is carried in the analemma.