Since the dawn of civilisation mankind has sought to understand and explain the seasons. The development of life on earth is fundamentally linked to seasonal change, from trees shedding their leaves to herds of antelope migrating across the plains of Africa. Early astronomers were aware that at certain times of year the sun was lower in the sky than at other times. This coincided with changes in weather and temperature which would dictate when crops should be sown and harvested – information that was vital to their survival.
Without the benefits of modern science, their only explanation for these changes was supernatural. Every country has its mythological stories to explain the seasons. The Greeks believed that for six months of the year Hades, god of the underworld, would take Persephone to his kingdom as his queen. While she was there, her mother Demeter, goddess of the harvest, would mourn her parting and the world would descend into winter.
As the years have passed, and our understanding of the solar system and the earth has improved, scientists have been able to explain the seasons without resorting to stories. However, there are still many aspects of the process which are misunderstood by the population at large. There are three primary driving forces behind seasonal change on earth. The most important is the earth’s axial tilt. Of secondary importance are the varying distance of the earth from the sun, and the pattern of oceans and continents on the earth’s surface.
Most of the bodies orbiting the sun do so on the same plane, called the ecliptic. Pluto is the only large object which seriously deviates from this flat disc of material. However, most of the planets don’t spin in the same direction as they orbit the sun. This is known as axial tilt or obliquity. Examples of the two extremes are Jupiter and Uranus. Jupiter’s rotation is only three degrees different from its orbit. This means its equator is always pointing at the sun. In contrast, Uranus has an axial tilt of 97 degrees. This means that depending on the time in the Uranian year, the sun could be directly overhead at the equator, or either of the poles, or anywhere between!
The earth sits between these two extremes – its axial tilt is currently 23.4 degrees. The earth’s tilt was probably caused by a collision with another large body early in the formation of the solar system – possibly the same collision that gave us the moon. This tilt means that over the course of the year, different latitudes become tilted towards the sun by varying amounts. Every point between 23.4 degrees north and 23.4 degrees south will experience the sun directly overhead at some point in the year. In the northern hemisphere’s summer, northern latitudes are tilted towards the sun. This gives them more hours of daylight as a larger proportion of each line of latitude is illuminated by the sun. At the extremities, beyond the Arctic circle, there is at least one day a year when the sun never sets. Additionally, because the northern hemisphere is tilted towards the sun, the sun is more directly overhead, and sunlight strikes the surface of the earth less obliquely, increasing its power. More powerful sunlight and longer days both equate to warmer weather.
So while the northern hemisphere basks in long days with more powerful sunlight, the southern hemisphere has to wait for six months to experience the same. On June 21st each year, the earth is a position such that the northern hemisphere is the most tilted towards the sun that it can be. The sun will move directly over the Tropic of Cancer and every location in the north of the tropic will experience its longest day of the year. Correspondingly, on December 21st, the northern hemisphere experiences its shortest day.
So why are these two days not the warmest and coldest days of the year in the north? Another phenomenon has be taken into account – that of thermal lag. Although the aforementioned dates are when the sun provides the most and least power of the year, it takes time for the oceans and continents to build up the heat which will result in what we observe as the seasons. The average warmest and coldest days of the year can follow the shortest and longest days by five weeks, but regional variation means no specific days can be named. Typically, the first few days of August are considered mid-summer, and mid-winter is a couple of weeks into January, in the northern hemisphere.
Seasons are further affected by the distance of the earth from the sun. Around January 4th, the earth is as close to the sun as it gets, a position called periapsis. Around July 3rd, the sun is at its most distant. These changes work to oppose the seasonal variations caused by axial tilt in the northern hemisphere – but reinforce the variations in the southern hemisphere. In theory, this means that northern summers and winters are milder, while southern ones are more extreme. However, water acts as a good heat buffer – it takes longer to warm up than land, and retains its heat for longer. Because much more of the southern hemisphere’s surface is ocean, the temperature variations are moderated, and the southern hemisphere is not as badly affected by the seasons as might be expected. An exception to this model are the polar temperatures. The south pole winter is significantly colder than the north pole winter, because the south pole is continental, while the north pole is surrounded by water.
The four seasons, as they are currently defined, are an artifact of the Asian and European domination of early scientific study. Scientists, and particularly ecologists, now recognise that this historical system is quite inappropriate for many of the world’s ecosystems. Consequently, some areas recognise as many as six, or as few as two seasons. Notably, tropical regions which don’t experience a cold winter, tend to have simply a wet season and a dry season.
In the end, a thermal definition of the seasons makes more sense than an astronomical one. It is, after all, the weather on the surface of the earth that affects daily life, and not the position of the sun. Seasons are mainly recognised by the weather patterns that we associate with them. This means that although we can approximate when the seasons will be, actually recognising their beginnings and ends can only be done retrospectively, based on observed data such as temperature and rainfall.
Sources:
http://www.nasa.gov/audience/foreducators/postsecondary/features/F_Planet_Seasons.html
http://www.bbc.co.uk/dna/h2g2/A526673
