This is the time of year when evenings are dark and there seems no end to the gloom. Never fear, our current sunset time of 4:41 p.m. is the earliest it can be – even though the Winter Solstice is more than two weeks away. In fact, by December 16th, the sun will begin to set later, continuing its trend until June.
At first I was astonished by this fact. It seemed to go against everything that I thought I knew about the Winter Solstice and the length of the daylight hours. On closer examination, there is a similar anomaly in the sunrise times which are progressively later until January 9th. OK then so what’s going on? The answer lies in the orbit of the earth and our current position. The earth doesn’t travel around the sun in a circle, in fact it’s an ellipse which is a kind of oval shape. Currently we are almost at our closest annual approach to the sun which occurs around the 3rd of January. We’re about 800,000 km closer at the moment than we will be in July. We’re also travelling faster because we’re rounding the corner at the near end of the ellipse. Because of this, it takes longer for the earth to rotate into position and the solar noon drifts later and later. In late October, solar noon is around 12:01 p.m. while today, it’s happening at 12:09 p.m. and it will move to 12:32 p.m. by February. Because of this, our sunset and sunrise times are pushed later, hence the earliest sunsets occurring well before the solstice.
Sorry, I just had to have my moment of Geekiness.
For a better explanation try this link.