Science Sunday: The Reason for the Season
As I’m writing this it’s late evening, and I have been spending a few days at home. Funny thing is, even though I’m going to bed soon, the Sun isn’t setting. Actually, it won’t set for at least another month. During that time, it will circle the sky, dip a bit up and down, but never go under the horizon. So, how does that work?
The axial tilt, or obliquity, of the Earth is defined as the angle between the rotational axis and the orbital axis. Presently, the axial tilt is at about 23.5 degrees, a number that varies slightly over a 41 000 year cycle. Other astronomical bodies have different tilts, where some vary a lot over time. The stability of the Earth’s axial tilt is largely because of the presence of our Moon. Without it, the Earth would most likely tip drunkenly back and forth between different degrees of obliquity. This would not happen overnight, but it has been theorized that a greatly varying tilt would not be positive for the evolution of higher species. This is because a great change in tilt would cause great climate changes.
Subsequently, it is also this tilt that gives the Earth its seasons. If the Earth had been completely upright, then the length of day and night would never change, and different times of the year would look suspiciously alike. There would only be different climate zones depending on latitude. Instead most places have varied seasons, disregarding a thin belt around the equator. A common misunderstanding is that the seasons are influenced heavily by the distance between the Earth and the Sun. In reality, the Earth is closest to the Sun in January, at 147 million kilometres, and furthest away in June, at 153 million kilometres. This causes a small difference in the heat received by the Sun, but the effect is much smaller than that caused by axial tilt.
To put it simply, the difference between seasons is mainly the average daytime temperature. The average daytime temperature is affected by how long the Sun is over the horizon, and also by how long it is at its peak elevation. In other words, winters become cold because the Sun doesn’t get high enough to heat the surface efficiently*. As mentioned, if the Earth was completely upright, then every day would have the same length, and the same average temperature. This is because every point of the Earth – still depending on the latitude – would receive the same amount of sunlight, and for the same duration, every day of the year. Tilting the Earth changes this status quo, so that different places receive a different amount of sunlight during the year.
The North and South Pole experience the most extreme effects of this, as the Sun is continously over the horizon for one half of the year, and under the horizon for the other half. These are of course not the same halves, as the poles have summer and winter at oppsite times of the year. The North Pole is in essence pointing away from the Sun for the six months that it is completely dark.
This effect wanes as you get closer to the equator. Still, at the southernmost point of the Arctic Circle – at about 66.5 degrees latitude, there will be one day where the Sun doesn’t set, and one day with complete darkness. The effect is still strong enough over most of the Earth to be noticeable, with winter typically being darker than summer. For that, axial tilt is the reason for the season.
*This also explains why evenings are colder than noon; during the evening, the angle between the sun and the horizon is smaller, so that the surface is heated less.