The Winter Solstice

In looking for a nice link to send someone explaining the concept of the Solstice, I came across this excellent series of images from the Wikipedia article on the subject. They were created by Wikipedia user Tau’olunga and show a small island at different latitudes on the Earth. The celestial sphere is shown, along with the track of the Sun during both the Summer and Winter solstices.

If you’re new to the idea of the celestial sphere then you have to imagine the sky depicted as a really big sphere, with us sitting inside it. The sky stays stationary as the Earth rotates inside it. So the North pole of the sky, Polaris, lines up with the Earth’s North Pole and so on. If you are sitting at the North Pole then Polaris appears directly overhead all the time. If you’re sat at the equator then Polaris is at the horizon and the celestial equator rolls more or less overhead.

Over the course of a year, the Sun will appear to oscillate just above and below the celestial equator (by about 23 degrees), because of the tilt of the Earth’s axis (see above). However, on any given day it will seem to stay in same place, circling the sky following an approximate line of latitude. This is what is depicted below. The path of the Sun at each solstice is drawn and the celestial sphere. In each case the longest path is the Summer solstice and the shortest is the Winter solstice. As you move from the equator to the pole, the difference between the paths drawn by the Sun gets larger until near the pole (at about 70 degrees latitude) there is no Winter track because the Sun is below the horizon all the time. You are now inside the arctic/antarctic circle.

There are a couple of interesting points raised by these images. Note that at the equator, the Sun does not always go directly overhead. In fact this only happens at the equinoxes (the time in the middle of the two plotted tracks). In the 50 degree latitude diagram, there are little faint circles depicting the Sun below the horizon – these indicate that it is still contributing to some sort of twilight – which above 50 degrees lasts all night during the Summer solstice! This is why astronomy can be so difficult in midsummer here in the UK, for example, but not in Southern France.

The diagram showing the solstice at the pole lets you imagine the Sun spiralling closer and closer to the horizon as the year goes by. Six months from perpetual midday comes perpetual midnight, each day the Sun creeping lower and lower before skirting the horizon and then disappearing at the equinox.

Something I had never though of, but which is pointed out in the Wikipedia article, is that until 20 degrees latitude, the Sun is either in the North or the South depending on the time of the year. This is counterintuitive to most of us who are used to the Sun always being in the South (for the Northern Hemisphere) or the North (for the Southern Hemisphere). For example, in the UK, south-facing views are sunny ones. However below 20 degrees latitude in the Northern Hemisphere, the Sun can arc through the sky in the North, not the South, because of its oscillating position around the celestial equator. This becomes obvious in these diagrams but I had never thought about it before.

All of this is as much geometry as it is astronomy but I think it is interesting. These excellent diagrams do a great job of giving you feel for the why the seasons are so different and why the solstice happens (they also have added detail in the types of tree shown).

The 2009 Winter Solstice occurs at 17:47 on December 21st (the exact publication time of his blog post), the 2010 Summer solstice will be June 21st at 11:28.