The Equation of Time

The sun will not always be at the same azimuth, or position along the horizon, at the same clock time each day.  This is partly because the earth moves in an eliptical orbit and more quickly when it is closer to the sun than when it is further away, and partly because of the tilt of the earth's axis.  Clock time 'smooths out' these differences, averaging out the days and hours.  This means that a sundial will be 'slow' of clock times at some times of the year and fast at others.  Four times each year, the time shown on a sundial and clock time will agree.  From Christmas to April 16, the dial is 'slow', from April 16 to June 14 it is ahead, from June 14 to September 2 is it slow again, and from September 2 until Christmas it is fast again. 

When you take the position of the sun at a set clock time each day for a year, and join these positions together, you get a figure 8 shape.  This shape is called the 'analemma'. The top of the analemma corresponds with summer solstice, the bottom with winter solstice.  For an excellent illustration of the analemma by  Anthony Ayiomamitis who took photos of the sun at the same time in the morning for a whole year from the Delhi temple in Greece, visit: http://antwrp.gsfc.nasa.gov/apod/ap030320.html