Welcome to the shortest day of the year

Welcome to the shortest day of the year.This is the day in southern latitudes when the sun is at its lowest in the sky at noon.It is also the 2010 winter solstice, which will occur at 1.28pm today - the time when the sun reaches its northern-most distance from the equator.Today in Wollongong, the sun rises at 7am and sets at 4.54pm - meaning we will enjoy just nine hours and 54 minutes of daylight.Then why is it that sunrise continues to get later after the solstice?According to Danny Galbraith at the Geoscience Australia Earth Monitoring Group, that's a commonly asked question on the shortest day by people who try to link the winter solstice with the sunrise and sunset.Mr Galbraith said the sun does not cross the meridian (when it is highest in the sky) at precisely noon each day. The difference between clock-defined noon and the time the sun is on the meridian is called the Equation of Time. It represents the correction which must be applied to the time given by a sundial to make it agree with clock time.There are two reasons why the sun is not on the meridian at noon each day.Firstly, Earth's path around the sun is an ellipse, not a circle.The second reason is that the sun's equatorial plane and its orbital plane are inclined to one another. Both effects add together to yield the equation of time, which can amount to about 16 minutes difference between solar and mean time.The period when the equation of time is changing the fastest in the entire year is very close to the winter solstice.It changes by 10 minutes from June 16 to July 5. This means the time at which the sun crosses the meridian changes by about 10 minutes in this interval.So, the times of sunrise and sunset will change by the same amount.Mr Galbraith said that over the past decade, the winter solstice has always occurred on June 21, but he warned not to be fooled as this was just a feature of the slow drift through the dates on a 400-year cycle."The last time the solstice was not on this date was 1975, when it occurred on June 22 at 12.27am."In 2012 it will occur on June 20 at 11.09pm," he said.Simple isn't it?