Why We Have Seasons: How Daylight Length Changes

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The earth's tilt gives 24-hour daylight at the poles (photo by Derek Keats, Creative Commons licence)

The earth’s tilt gives 24-hour daylight at the poles. Photo by Derek Keats

We associate winter with cold weather and lack of daylight, summer with long balmy evenings where the sun lingers in the sky. Such variation is part of the series of cycles which affect the relative positions of the sun and the Earth – affecting not just seasonal variation but also long-term climate change.

Why we have Seasons: the Tilt of the Earth’s Axis

There are three main cycles which affect the earth’s relationship with the sun. The longest of these is eccentricity, in which the shape of the earth’s orbit varies over a period of up to 400,000 years, followed by precession, which affects the direction in which the earth’s axis tilts: this has a period of 105,000 years. With such long timescales, such variations are irrelevant to earth’s inhabitants.

Humans and animals alike are directly affected by a third factor – the tilt of the earth on its axis. The earth’s annual orbit of the sun takes approximately 365 days and 8 hours. If the earth were directly upright in its orbit, the duration of day and night would be the same at all points on the planet. In fact, the earth’s axis is tilted at an angle of 23.5 ⁰.

The Earth’s Axial Tilt: Image by Dna-webmaster

At the December solstice, the North Pole is tilted away from the sun, and the area north of 23.5 ⁰ (the Arctic Circle) receives no daylight. Meanwhile, the south polar region is tilted towards the sun and receives permanent daylight. As the earth continues its orbit, the area exposed varies (see diagram) until the position is reversed, and it is the Antarctic region which experiences total darkness and the Arctic which experiences total daylight. At the mid point of the orbit (the March and September equinoxes) all areas receive the same amount of daylight and darkness.

In fact, there is still further variation in the cycle, and the extent of tilt also varies – though over a period of 41,000 years. At its maximum, the Earth’s tilt is 24.4⁰ and at its minimum, 21.4⁰. The effect of this variation is a variation in the length of the seasons – so that, at the maximum point of tilt, the areas receiving maximum/minimum daylight at the solstices are those north or south of 24.4⁰ latitude.

Impacts and Effects of the Change in Daylight Length

The tilt of the earth doesn’t just affect the amount of daylight reaching the earth’s surface. The sun provides not only light, but heat, so the presence or absence of direct sunlight (solar insolation) affects seasonal temperatures. Such seasonal variations have far-reaching implications for life on earth – not just among humans but across the earth system.

The tilt of the earth controls the seasons (image from NOAA)

The tilt of the earth controls the seasons. Image from NOAA

Temperature and light are both significant, so plants and animals must adjust to the variations in daylight time. Relying on light for photosynthesis, plants cannot grow in the dark season, and so become dormant: this phenomenon, known as photoperiodism, varies between species as does the optimum temperature for growth but, generally speaking, the length of time during which growth takes place decreases at higher latitudes.

Absence of heat and light, and the consequent unavailability of food, is also the cause of many of the mass seasonal migrations experienced across the animal kingdom. Certain species of mammals, fish and, most particularly, birds, respond to reduced daylight and heat by seeking out more favourable environmental conditions.

The changes in the seasons can also have marked physical impacts among those who remain in situ. Most notably, some human are affected by SAD (seasonal affective disorder) which, though its mechanisms are not fully understood, is related to the seasons. According to the Oxford Dictionary of Nursing, SAD causes symptoms: “with the onset of winter, there is depression, general slowing of mind and body.”

It’s also worth noting that even these complex cycles are further complicated by the capacity of the earth’s atmosphere to ‘store’ heat, meaning that the maximum temperatures we experience ‘lag’ behind the maximum period of daylight by about a month.

Resources:

National Weather Service. “Why do we have the seasons?” Accessed 28 October 2011.

Oxford University Press. A Dictionary of Nursing, 2008. Oxford Reference Online. Accessed 28 October 2011.

University of Tennessee. “The seasons” Accessed 28 October 2011.

Wilson, R. C. L., Drury S. A., and Chapman J.L. The Great Ice Age: Climate Change and Life. Routledge/Open University 2000.

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