The basis of all civil timekeeping on the Earth is Universal time, UT. It is a measure of time which conforms, within a close approximation, to the mean daily motion of the Sun. Roughly speaking, the time is 12 : 00 noon UT when the Sun crosses the meridian at Greenwich, and 24 hours of UT is the time between two successive passages of the Sun across that meridian. However, the Sun is not really a very good timekeeper. Although the Earth spins about its polar axis at a nearly-constant rate (there are small irregular perturbations), the position of the Sun in the sky changes daily due to the orbit of the Earth about the Sun. If that orbit were perfectly circular, and if the Earth's axis were exactly perpendicular to the plane of the orbit (the ecliptic), then the daily passage of the Sun would be regular throughout the year. In reality, the Earth's orbit is slightly elliptical so that the speed of the Earth varies throughout the year, and the Earth's axis is tilted by about 23.5 degrees from the perpendicular to the plane of the ecliptic. Both these effects cause the time measured by a sundial, true solar time, to run fast during some parts of the year and slow during others by up to 16 minutes with respect to a uniform clock. The difference between the solar time and the uniform clock is called the ‘equation of time’, and you would need to know its value if you wished to tell the time accurately from a sundial.