The cycle of the seasons has been known since the dawn of time and human beings all over the world use them to mark moments in their personal histories. Some people are astonished to learn that the seasons are six months out of phase on opposite sides of the globe, something which amuses us in the Southern hemisphere no end. But what actually causes seasons?
Many people are dimly aware that the Earth’s orbit is not perfectly circular, so that the distance from Earth to the Sun changes over the course of a year. Naturally, they figure, when the Earth is closest to the Sun then it will get cooked, and that’s where we get Summer from. Conversely when we’re far away, we get cold. This is a neat bit of thinking, but it only works if you forget that Summer in the North always has a matching Winter in the South, and vice versa. No, the answer comes from the fact that the Earth’s axis is tilted, relative to the Sun. This one fact causes uneven heating of the surface of the Earth, which we experience as a cycle of the seasons. On two special dates, called the Equinoxes, the Earth is at a point in its orbit when the Earth’s axis is perpendicular to the plane of the Earth’s orbit. On these days, an astronaut in space floating exactly between the Earth and the Sun would be able to see both the North and the South poles. They would be on the very edge of his view of the planet, but because of the tilt of the Earth, they would not be at the top and bottom, but off by about 23°. Now if the astronaut is patient and keeps watching for several weeks, he’ll notice a change. As the Earth moves around its orbit, the angle never changes (because it’s spinning like a gyroscope, the tilt is quite stable), so one of the poles appears to swing towards him, while the other vanishes around behind the Earth. This continues until exactly three months after the equinox, when the near pole is as close to pointing at the Sun as it can get and the Earth’s axis is in line with the Sun. The two dates when we have this situation are called Solstices.
To understand how Solstices and Equinoxes affect the seasons, we need to look at how the Sun’s heat and light shines on different parts of the Earth. Think about how the Sun feels against your skin early in the morning compared to midday. At Noon, the Sun is directly overhead and shines straight down on you. The heat radiation shines through the thicknes of the atmosphere, and strikes the ground square on. So far so good. But early in the morning, or late in the afternoon, it shines at a very oblique angle. It now shines through a great deal more air than it did at noon, so a lot of its strength is robbed. As if that’s not enough, the energy is now being spread over a larger patch of ground, diluting the heat. You can demonstrate this effect by shining a torch straight down onto the ground: You get a brightly lit circular patch. Then shine it at an agle. The same amount of light is now spread over a much larger area, leaving it more dimly lit. This is part of the reason why the polar regions are much colder than the equator – their sunlight is always at a steep angle, while the equator gets a much bigger share of directly overhead sunshine.
During the two equinoxes, the Sun shines on both halves of the Earth equally. Both sides get the same share of light and heat, over the course of a 24 hour day. But what about the December solstice? On this day, the north pole is pointing far away from the Sun. The north polar region is in permanent darkness and becomes very cold. Even the sunnier parts only ever get sunlight at a steep angle, so they don’t warm up. The entire hemisphere becomes cold and experiences winter. Meanwhile, the southern hemisphere is hogging all the warmth. The south pole gets 24 hour daylight, and most of the hemisphere gets lots of direct sunlight. The south experiences a nice hot summer. We move on to the next equinox, in March. Every day, the North gets a little bit more sunlight and the temperature rises – Spring approaches. At the same time, the South gets a little less heat and starts cooling – Autumn. Then we have a northern summer and southern winter at the June solstice, followed by the September equinox. And so the cycle continues, year after year after year.
Like so much in astronomy, this is extremely simple but not obvious. Something so fundamental to life on Earth that every organism has systems for coping with it (migration, hibernation, growing seasons, shedding leaves) or even that rely on it will naturally have an arua of mystery to it, but how wonderful that we can stand back and understand the reasons and track the seasons as they change!
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