The analemma is the "figure-eight curve" seen on many Earth globes and maps. It indicates the difference between true sun (and hence a sundial) and the mean sun. It is a graphical presentation of the equation of time. The photograph to the left (by Steve Irvine) is of an analemma pattern in the sky above Keppel Henge. The pattern was created with images of the sun photographed every ten days, on average, during the course of one year. The photograph is a multiple exposure of 36 images on one negative. Each of the sun images was captured at 8:30 am Eastern Standard Time. The landscape image was taken in mid afternoon. The photograph demonstrates how dramatically the sun changes position in the sky during one year. This variation of the true sun relative to the mean sun is the result of two factors: the 23 1/2 tilt of the Earth's axis relative to its orbit around the sun (the ecliptic) and the variation in the rate at which the Earth orbits the sun due to Kepler's Law of Areas for an elliptical orbit.
More recently a greek photographer, Anthony Ayiomamitis, has made a number
of striking analemma images including a wonderful vertical (local noon) version
shown at
http://www.perseus.gr/Astro-Solar-Analemma-102816.htm
.
Although the Earth rotates at a (approximately) constant rate, this does not mean that the Sun will cross (transit) the meridian at the same time every day. The difference between the time of actual solar transit and the average time of solar transit is called the Equation of Time and it is illustrated on many terrestrial globes as the Analemma. Twenty four hours of Solar Time is defined to be equal to the average interval between solar transits taken over a full year.
So from Equinox to Solstice TS-MS> 0, from Solstice to Equinox TS-MS< 0
Kepler's 2nd Law (the Law of Areas) says that a planet in an elliptical orbit sweeps out equal areas in equal times. The result is that when the planet is closest to the sun (early January for Earth) it must move more rapidly in its orbit than it moves when it is farther from the sun. Since the Earth's rotation on its axis is independant of where it is in its orbit, this means that in one day the Earth will move further along its orbit near perihelion than it will near aphelion. This variation results in an added difference between the mean sun and the true sun. If the Sun's orbit were circular, the direction to the Sun would change by about 1 degree (actually 360/365.25 degress) per day thus yielding a solar 24 hour day exceeding the sidereal day by about 4 minutes. The eccentric orbit of the Earth means that when the Earth is traveling faster in its orbit near perhelion (early in January) the true Sun will transit later than on average (because the Earth will have to rotate farther because the direction will have changed more) and vice versa in early July near aphelion.
The composite of these two factors results in the equation of time. The above plot is for the year 2000. The total amplitude remains constant but the relative proportions of the four peaks changes.