The analemma is a figure-eight shaped plot of the position of the sun in the celestial sphere at noon of each day (at 24 hour intervals) of a calendar year. The vertical coordinate of a point corresponding to a date corresponds to the declination of the sun on that date, while the horizontal coordinate indicates whether the sun is "fast" or "slow" compared to mean time.
The deviation between solar time and mean time is due to the variation in the length of the synodic day, which is due to variation in the distance between the earth and the sun. (See aphelion, perihelion)
A discussion of the derivation of this curve can be found at the following address:
