This is a bit of a repeat, since I almost forgot about writing this week. I decided to revisit the seasons thing.
We have just begun the season of Southern Hemisphere spring. This officially starts on 1st September and runs through to 1st December. Then summer starts and runs through to 1st March, then autumn runs through until 1st June, and winter extend to 1st September and the cycle repeats.
The reason that the seasons are defined like this goes back to 1780 when an organisation called “Societas Meteorologica Palatina” defined them as above. The organisation chose those dates because the seasons pretty much aligned with those dates in terms of temperature and rainfall and so on. The coldest three months in the Northern Hemisphere tended to be December, January and February, the warmest tended to be June, July and August, and so on.
Since the seasons are reversed in the Southern Hemisphere, the southern cycle is as described above. We have Christmas on the beach and spend July wrapped up and close to any source of heat!
Astronomers do it differently. They divide the year into four seasons, but the seasons are not aligned climatically, but are defined relative to the Earth’s position in its orbit around the Sun.
Because the Earth’s axis is tilted relative to its orbit around the sun, the axis is be tilted towards the sun at one time of the year and away from it six months later. When the axis is tilted towards the sun, the sun is at its highest in the sky and more energy is received on Earth per square metre than at any other time of the year. It’s summer and warmer. When it is tilted away, the sun is at its lowest and we receive less energy than at any other time of the year. It’s winter and colder. (But read on).
On Earth, when the sun is high it is in the sky longer than when it is lower. The day is therefore longest and the night is the shortest in the yearly cycle. When the sun is midway between its highest and its lowest, the day and the night are of equal length.
The time when the sun is highest or lowest in the sky is called a “solstice“, either a winter solstice, or a summer solstice. The times when it is half way are called “equinoxes“, either an autumnal equinox or a vernal equinox, and the night and day are equal in length. These are the four main signposts of the seasons, as used by astronomers.
Strictly speaking, to say “Today is the summer solstice” or “Today is the autumnal equinox” are incorrect. Since the day and night lengths are changing all the time, the solstices and equinoxes are points in time, not whole days.
There are four lesser known and less important signposts of the seasons, they are Beltane, Lughnasadh, Samhain and Imbolc. I’ve used the Gaelic names, but they correspond, in order, to the Christian festivals of May Day, Lammas, Halloween, and St Brigid’s Day. These all fall more or less halfway between the four main seasonal signposts.
Astronomically the Winter Solstice, which occurs around 21st December in the Northern Hemisphere. Many sources identify the date of the solstice as the beginning of winter. Similarly the Summer Solstice is identified as the start of summer, and the equinoxes are identified as the start of their respective seasons.
This is odd, as the climatic seasons are usually considered to start three weeks earlier, with Northern Hemisphere winter climatically starting around the 1st December, and similarly for the other seasons. Starting the astronomical seasons on the 21st (or sometimes 22nd) of the month misses out 3 weeks or nearly a quarter of the season!
It’s also odd for another reason. The Northern Hemisphere winter solstice is when the sun is at its lowest point in its apparent position in the sky, so it is at its turning point in the cycle of the season and indeed the word “solstice” means “the point where the sun stands still”. It seems to me that this should be considered the mid point of the season, not the beginning of it.
This is obviously true for the summer solstice too, and the equinoxes, being halfway between the solstices are add the mid points of the sun’s climb or descent to the solstices. They too also should be the mid points of their seasons, not the beginning points.
If the solstices and equinoxes are the middles of their seasons, where are the start end points then? Well, they would then coincide with the Gaelic or pagan festivals of Beltane, Lughnasadh, Samhain, and Imbolc! For example Beltane is about halfway between the Northern Hemisphere spring equinox and summer solstice on 1st May.
Although Beltane is a Gaelic or pagan festival and has mostly fallen out of favour, some cultures do celebrate the festival and some of the customs persist, such as the custom of dancing around a Maypole. Beltane and the other three similar festivals coincide with important agricultural events, such as sowing seeds and gathering in of harvests, so were of interest in earlier times.
However, if the astronomical seasons starts and ends were to be moved to coincide with the Gaelic festivals they would not coincide with the climatic seasons. The reason for this is that there is a seasonal shift because of the time that the seas and land take to warm up in spring and to cool down in winter. This pushes the climatic seasons back a few weeks and the start of climatic spring in the Northern Hemisphere is pushed back to about the 1st March and the same for all the other seasons.
That’s why I think that the current idea of the astronomical seasons starting at the solstices and equinoxes is wrong! They should coincide with the Gaelic festivals instead, and then the astronomical and climatic seasons are related by the seasonal shift, instead of not being related properly at all.
Me on the net 