Astronomy

Why Planet Earth has Seasons



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Planet Earth takes 365 (and a quarter) days to complete its orbit round the Sun, a day being defined as the time it takes to revolve once on its axis, which is the imaginary line that connects the two poles by going straight through the middle. During that journey, every place in the temperate zones of the planet (i.e. between the Tropics and the Polar zones) will experience the four distinct seasons of spring, summer, autumn (fall) and winter. Why?

For reasons that are beyond the scope of this article, planet Earth is on a tilt. In other words, the angle between the plane of its orbit and its axis is not at 90 degrees. In fact, the axis is inclined at 23.5 degrees. It is this tilt that creates the seasons. 

It is easy to picture Earth at a point in its orbit where the Northern Hemisphere is tilted towards the Sun as far as it can go. This is the state of affairs on 22nd June every year, which is known as the summer solstice. Six months later Earth will be on the opposite side of the Sun, but, because the tilt is constant, it is the Southern Hemisphere that will be inclined towards the Sun.

It is sometimes thought, mistakenly, that the reason for the seasons is that the distance from the Sun to the part of Earth that is tilted towards it is greater than when the tilt favours the opposite hemisphere. The closer to the Sun, it is assumed, the higher the temperature.

However, this is not the factor that counts. What matters is the fact that the Sun’s rays strike the surface at a more direct angle when a hemisphere is tilted towards the Sun than when tilted away from it. When the Sun’s radiation strikes the surface at a more oblique angle it is spread over a larger area and therefore has less effect.  Another aspect of this obliqueness is that the Sun’s rays have to travel through a greater distance of the Earth’s atmosphere before reaching the surface, so a greater proportion of the radiation energy is absorbed by the air before the ground is reached. For these reasons, summers (when the radiation is more direct) are hotter than winters.

Conventionally, the distance from the Equator (the imaginary line running round the planet at its widest point) to each pole is divided into 90 degrees, such that the line 23.5 degrees either side of the Equator is the point at which the Sun will appear directly overhead at noon on the summer solstice. These lines are termed the Tropics, with the Tropic of Cancer in the Northern Hemisphere and the Tropic of Capricorn in the Southern Hemisphere. The further north or south one goes, the highest point that the Sun reaches in the sky will appear to be gradually lower, so that at the poles the Sun will never reach higher in the sky than 23.5 degrees.

Apart from the angle of the Sun’s rays, the other main consequence of the Earth’s tilt is that the length of time during which the Sun is visible varies considerably at different times of the year. In the Northern Hemisphere, the daily rotation of the planet means that, when the tilt towards the Sun is greater, the time during which the Sun shines on it is longer. Indeed, within the Arctic Circle there will be days during the summer when the Sun never sets and days during the winter when it never rises. At the North Pole itself, there are six months of daylight followed by six months of darkness.

However, on two occasions on its journey round the Sun, the Earth crosses the point at which the celestial equator intersects the ecliptic. The celestial equator can be defined as Earth’s equator extended indefinitely out into space. The ecliptic is the apparent path taken by the Sun due to the Earth’s tilt. When the two coincide, at about 21st March and 22nd September each year, we get the Vernal (i.e. Spring) and Autumn Equinoxes, respectively. The word “Equinox” means “equal night” because, on those two dates only, every place on the planet gets exactly twelve hours of daylight and twelve hours of night.

What this all means is that, when the Northern Hemisphere is tilted towards the Sun, the latter has more opportunity to heat the surface of that hemisphere, both because the intensity of the radiation is greater due to the angle at which the rays strike the ground, and because there is more time during which the Sun is visible. When the tilt favours the opposite hemisphere, these effects are reversed and the amount of heat reaching and retained by the surface is greatly reduced.

The seasonal effects are seen and felt very clearly, with temperatures being higher in summer and lower in winter, and plant and animal life responding accordingly. When summer is experienced in the Northern Hemisphere it is winter in the Southern Hemisphere and vice versa.

The contrast between the seasons is more noticeable the further one goes away from the Equator and towards the poles. This is because of the greater difference between the lengths of day-time and night-time, although the more oblique angle of solar radiation experienced in the more extreme latitudes (north and south) means that summers are never going to be as warm as they are nearer the Equator. They will also be shorter, but winters will be longer. The intermediate seasons of spring and autumn (fall) are also relatively short as one approaches the Arctic and Antarctic Circles.

In the temperate zones in which much of the world’s population lives, summers are warm and long enough for crops and animals to thrive, and the winters relatively mild and survivable. The seasons of spring and autumn also occupy several months during which the changes from summer to winter and back again can take place, thus allowing time for plant and animal life to adjust.

The passage of Planet Earth around the Sun is not circular but elliptical, which means that it is closer to the Sun at some times than at others, the variation in distance being around 3% from one extreme to the other. The planet reaches the closest point, or “perihelion” at around 4th January and its most distant point (the “aphelion”) around 5th July. The Northern Hemisphere therefore experiences winter when Earth is closest to the Sun, whereas the Southern Hemisphere winter is when the planet is furthest away. Given that one might expect more energy to reach Earth when it is closest to the Sun and less when it is further away, the result should be that the effects of winter are mitigated in the Northern Hemisphere but exaggerated in the Southern Hemisphere.

However, this does not turn out to be the case, and the seasonal contrast is no greater in the South than the North. The reason for this is that the proportion of the surface that is ocean is much greater south of the Equator than north of it, and this has a greater influence on climate than the 3% difference in distance. However, if Planet Earth were to have the much greater orbital eccentricity of Mars, say, the effect on seasonal lengths and contrast would be considerably greater.

Sources:

Abell, G. (et al) Exploration of the Universe. 5th ed. Saunders, 1987

Kaufmann, W. Universe. 2nd ed. Freeman, 1987




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