The motion of the Sun across the sky is not uniform and the 'true sun' position can lag or lead the position of the sun based on what your clock/calendar would say. The above curve, called an analemma, shows how the true sun position changes throughout the year. Visit the Analemma Pages for more details. The details for why this happens are a combination of the Earth's axis tilt and the orbital motion of the Earth through space which controls the speed of the Sun across the sky as the figure below shows.
In general, the answer has to do with the observer's latitude on the Earth, the speed of the Earth in its orbit around the Sun, and the inclination of the axis of the Earth, and its orbital plane, relative to that of the solar system. If the Earth orbited in a circular orbit with constant speed, in the plane of the solar system, and with its axis tilted 90 degrees to the plane of the solar system, these dates would coincide with the Winter Solstice.
Copyright 1997 Dr. Sten Odenwald
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