Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time (sundial time) and mean solar time (clock time).

Fix a tall pole vertically in the ground; at some instant during any sunny day the shadow points exactly north or south (or disappears, if the sun is directly overhead). That instant is local apparent noon-- 1200 local apparent time. About 24 hours later the shadow will again point north/south, the sun seeming to have covered a 360-degree arc around the earth's axis. When the sun has covered exactly 15 of that 360 degrees (both angles being measured in a plane perpendicular to the earth's axis), local apparent time is 1300 exactly; after 15 more degrees it will be 1400 exactly.

The problem is that in September the sun takes less time (as measured by an accurate clock) to make an apparent revolution than it does in December; nowadays 24 "hours" of solar time can be 21 seconds less or 29 seconds more than 24 hours of clock time. As explained in the Equation of Time article, this annoyance is due to the ellipticity of the Earth's orbit and the fact that the Earth's axis is not perpendicular to the plane of its orbit.