Christmas on the 25th December roughly coincides with the winter solstice on the 21st December which since time immemorial has been a day of celebration to mark mid winter and the beginning of the long slow trek to spring. Christ was not born on 25 December and in fact nobody knows his birth date however his birth was good news as was the end of winter hence the decision to date Christ's birthday with the winter solstice.
However, no matter how important seasons are to our life and our planet, few people understand why we have seasons so here is a simple attempt to explain.
The earth orbits the sun . The earth also rotates on an axis which is 23.5 degrees off parallel with that of the sun. It takes the earth 365 days to orbit the sun.This means that at different times of the year different places on the earth are nearer to the sun but more importantly the place on earth where the sun’s rays hit the earth at a perpendicular 90 degree angle varies. This is called the sun being directly overhead or the subsolar point (SP).
This is important because at the SP solar rays are more focused and intense so they create more heat. Try shining a touch on spherical object and notice how the intensity of light striking the surface varies.
Because of the tilt in the earth’s axis the SP moves from the Tropic of Cancer in the north and Tropic of Capricorn in the south – (both at 23 degrees north/south) depending on where the earth is in its orbit. The sun does not move but the earth does and so does each point on the earth's position relative to the sun.
The sun is directly overhead at the Tropic of Cancer on 21st June which is called the summer solstice. Heating in the northern hemisphere is at a maximum on this day. After that the SP moves south until it reaches its most southerly position at the Tropic of Capricorn. This is on the 21st December, the winter solstice when heat in the northern hemisphere is at a minimum. This is mid winter in the northern hemisphere after which the SP points moves north again.
The areas between Cancer and Capricorn are called the tropics.As the sun migrates north and south it crosses the equator twice and these are known as equinoxes: the spring equinox on 21st March as the subsolar point moves north and the autumn equinox on 21st September. So astronomically speaking our seasons are : summer (21 June to 31 September), Autumn (22 September to 21 December), Winter (22nd December to 21st March) and Spring (22nd March to 21st June).
It is the movement of of these heat belts together with electro magnetic variations within the sun that are the main determinents of weather and climate on this planet.
It is therefore that simple tilt of fate in the angle of rotation on its axis that has had such an impact on evolution of this planet and life on it. To see how read next week's lesson "Heat Engine Earth".
If you are sick of talking about covid and Brexit and are interested in climate and weather or have any queries let me know and I can go in to more or less depth for you.
Brian Harris BA (Hons) MA (Dunelm), (Climatology).