5. The Cause of the Seasons

In December 2010, the UK was suffering a spell of exceptionally severe winter weather. On 18 December, a news item on the web site of my supposedly “reputable” ISP ( which shall remain nameless ), concerned with the latest update on the snow and cold weather, included the following brilliant insight:

“It may get even colder on Tuesday, with the Winter Solstice – the time when the Earth is furthest from the Sun.”

Er – what????? Is it any wonder that so many people today labour under misconceptions, when supposedly “reputable” media propagate this kind of senseless drivel?
Evidently, whoever wrote that piece actually thinks that summer and winter are caused by the Earth being closer to and further from the Sun! Sadly, he’s not alone; an incredible proportion of people apparently share that same belief.

5.1. The tilt of the Earth’s axis

I have no idea why so many people believe that! One possible contributing factor may be that people are told that the Earth’s orbit round the Sun is elliptical, not circular – and in children’s books, it’s often drawn with an exaggerated shape, which makes it look as if its distance from the Sun varies considerably. But it doesn’t. While the orbit is indeed elliptical, it’s actually so close to a circle that if it was drawn to scale, you would have to look closely to tell the difference. Its distance from the Sun only varies by about 3%; the effect of this on climate is negligible.
The Earth is in fact closest to the Sun in early January, just a fortnight after the northern Winter Solstice, and furthest away in early July. If you find that statement surprising, then you’re suffering from what has been called “Northern Hemisphere prejudice”!
The key word in that last but one sentence is “northern”; the terms “Summer Solstice” and “Winter Solstice” are misleading, as they only apply in the Northern Hemisphere. ( They were given those names, simply because the vast majority of the world’s population live in the Northern Hemisphere. ) When it’s summer in the Northern Hemisphere, it’s winter in the Southern, and vice versa. How do you explain that fact, if summer and winter are due to the Earth’s distance from the Sun? And how do you explain the varying lengths of day and night with the seasons?
The cause of the seasons has nothing at all to do with the Earth’s distance from the Sun, and everything to do with the tilt of its axis. The fact that so many people don’t know that – at least in my country - is yet another sad reflection on the decline of our “education” system! When I was at school in the 1970s, astronomy wasn’t part of the curriculum, but we learned about the cause of the seasons in geography lessons, at the age of 13.
So what do I mean by “the tilt of the Earth’s axis”? If you imagine the plane of the Earth’s orbit as being the “horizontal”, then its axis of rotation – the line through the North and South Poles – is tilted at an angle of 23.5° from the “vertical”. Therefore, the Equator is tilted at the same angle from the plane of the orbit. As the Earth travels around its orbit, it behaves like a huge gyroscope; the axis always points in the same direction, as shown in Fig. 4. ( Actually, it does change direction over a very long timescale – it traces out a circle over a period of 25800 years – but that doesn’t concern us here. )

Fig. 4

We can see that on one side of the orbit, the North Pole is tilted towards the Sun, and on the opposite side, the South Pole is tilted towards the Sun. The dates when the two poles are most tilted towards the Sun are what we call the Summer and Winter Solstices ( remember, those names are only correct in the Northern Hemisphere! ), which occur on or around 21 June and 21 December, respectively. ( The actual dates can vary by a day, due to the fact that the Earth’s orbital period isn’t a whole number of days – which is why we have leap years! ) In the Northern Hemisphere, the “longest day” occurs at the Summer Solstice, and the “shortest day” at the Winter Solstice; in the Southern Hemisphere, it’s the opposite. If you can’t already see why, it will be explained shortly.
At the Vernal or Spring Equinox on 21 March, and the Autumnal Equinox on 21 September ( again, Northern Hemisphere names ), the tilt of the axis is tangential to the direction of the Sun, so neither hemisphere receives more sunlight than the other. On these dates, everywhere on the Earth experiences equal lengths of daylight and night; in fact, the word “equinox” comes from the Latin for “equal night”.

5.2. The effects of the tilt

Now let’s look at how this axial tilt actually affects the Earth’s climate, and the lengths of day and night, throughout the year. Fig. 5 shows the Earth at the Summer Solstice, when the North Pole is most tilted towards the Sun – at the position in the orbit marked “June” in Fig. 4. In other words, when it’s midsummer in the Northern Hemisphere and midwinter in the Southern.
Drawing it as a representation of a sphere is beyond my very limited artistic ability, so I’ve drawn it as a cross-section through the Earth.

Fig. 5

In the diagram, the plane of the Earth’s orbit is perpendicular to that of the paper, and the direction of the Sun is to the left. The unshaded half of the circle represents the half of the Earth which is sunlit, and the shaded half the half which is in darkness. The plane of the paper is that containing the “vertical” – the division between day and night – and the axis. Imagine the Earth rotating, and any point on its surface moving through the zones of daylight and night.
We can see that in the Northern Hemisphere, the sunlight reaches the Earth’s surface at a steeper angle, and at a shallower angle in the Southern Hemisphere. To put it another way, it rises higher in the sky in the Northern Hemisphere – this is the primary reason why the climate is warmer in summer and colder in winter.
We can now also see why the length of daylight and night varies with the seasons. Look at the Equator; it’s bisected by the day-night dividing line, showing that a point on it spends equal time in daylight and darkness. In fact, any point on the Equator always has 12 hours of day and 12 of night, with the Sun rising at 6 a.m. and setting at 6 p.m., all year round. It also has the same climate all year round - apart from regional phenomena such as rainy seasons, which are due to meteorological factors, and have nothing to do with the actual seasons. Effectively, the Equator doesn’t have seasons!
But anywhere else, that isn’t the case; look at what happens as you move away from the Equator. You can see that any point in the Northern Hemisphere experiences more hours of daylight than of night, while the opposite applies in the Southern Hemisphere. The further you are from the Equator, the greater is the difference between the lengths of day and night. The dashed line shows the latitude of my home city, 53°N; at this latitude at the Summer Solstice, we have about 17 hours of daylight, and only 7 of night. At the corresponding southern latitude, it’s the opposite – though there aren’t many people there to see it!
The lines either side of the Equator are the Tropic of Cancer and Tropic of Capricorn. These are lines of latitude at 23.5° north and south, respectively – the same angle as that of the axial tilt - their significance will be explained shortly. In the region between these lines, which we call “the Tropics”, we can see that there is little difference between summer and winter, either in climate or in the lengths of day and night. At higher latitudes, the differences are distinct; the further from the Equator, the more extreme is the variation of climate between summer and winter.
More obvious is the significance of the Arctic and Antarctic Circles. These are lines of latitude which are that same 23.5° away from the North and South Poles respectively; that is, latitudes of 66.5° north and south ( 90° - 23.5° = 66.5° ). At the Solstice, as you can see in the diagram, any place north of the Arctic Circle experiences 24 hours of continuous daylight – the famous “midnight Sun” – while any place south of the Antarctic Circle has 24 hours of continuous darkness.
To see what happens six months later at the ( northern ) Winter Solstice, just imagine the sunlight coming from the right, so the left half of the circle becomes the zone of night. Everything is now reversed; the Southern Hemisphere experiences more hours of daylight than of night, with the opposite in the Northern. My home latitude now has only 7 hours of daylight and 17 of night. The Antarctic region now has 24 hours of daylight, and the Arctic 24 hours of night.
To see what happens at each of the equinoxes, imagine that the direction of the Sun is perpendicular to the plane of the paper, so that the plane of the paper itself, containing the axis, becomes the division between day and night. Now, as I hope you can see, everywhere on the Earth has equal lengths of daylight and night.
Incidentally, on the continents in the Southern Hemisphere ( with the exception of Antarctica ), the variation in climate between seasons is considerably less extreme than on those in the Northern. This is a consequence of the distribution of land and oceans; the large majority of the world’s land masses are in the Northern Hemisphere. The far greater proportion of oceans in the Southern Hemisphere has a moderating effect on the climate, simply because water warms up and cools down more slowly than land does.

5.3. A little more technical

I hope the reason for the variation in the lengths of day and night can be seen intuitively in Fig. 5. But if you want a slightly more technical explanation...
To describe the positions of objects in the sky, astronomers use a system of coordinates similar to latitude and longitude on the Earth. We imagine the sky to be the inside surface of a sphere, known as the Celestial Sphere, with the observer at its centre; at any given time, you can see half of that sphere above your horizon. The Celestial Equator is the projection of the Earth’s Equator onto the Celestial Sphere. The Ecliptic is the projection of the Earth’s orbit onto the Sphere; to put it another way, it’s the apparent path which the Sun follows around the sky, with respect to the stars and constellations, due to the Earth’s motion around it.
Not surprisingly, the Celestial Equator and the Ecliptic are tilted with respect to each other, at that same angle of 23.5°!
The celestial equivalent of latitude is called declination; it’s the angle north or south of the Celestial Equator, measured positive to the north and negative to the south. Due to that tilt once again, the Sun moves alternately north and south of the Celestial Equator during the year; it crosses the Celestial Equator ( declination 0° ) at each of the equinoxes, and reaches extremes of +23.5° at the Summer Solstice and -23.5° at the Winter Solstice.
If you plot a graph of the Sun’s declination against time, you get a sine curve, as shown in Fig. 6. If, at any one place on the Earth, you were to measure the Sun’s height above the horizon at local noon on each day of the year, you would get the same sine curve, with its maximum and minimum heights at the two solstices. The solstices are the turning points of the curve, where the Sun’s apparent north-south motion stops and reverses; the word “solstice” comes from the Latin for “Sun still”.

VE = Vernal Equinox
SS = Summer Solstice
AE = Autumnal Equinox
WS = Winter Solstice

Fig. 6

It stands to reason that the higher the Sun rises in the sky at noon, the longer it spends above the horizon between sunrise and sunset; it rises earlier and sets later. When it’s north of the Celestial Equator, the Northern Hemisphere has longer hours of daylight than night, and the Southern has longer hours of night. When it’s south of the Celestial Equator, we have the opposite. Again, the difference is greater, the further you are from the Equator.
A plot of the number of hours of daylight on each day of the year would again produce the same sine curve. We can clearly see why the times of sunrise and sunset change most quickly from day to day around the equinoxes, and most slowly around the solstices.
We can now see the significance of the Tropics, those two lines of latitude at 23.5°N and 23.5°S. Anywhere in the region between them, the Sun is directly overhead at some time of the year. At each equinox, it’s overhead on the Equator; at the Summer Solstice, it’s overhead on the Tropic of Cancer, and at the Winter Solstice, it’s overhead on the Tropic of Capricorn. At any latitude in between, the Sun is overhead on some intermediate date. Outside the Tropics, the Sun can never be directly overhead.
So we see that the entire wonderful phenomenon of the changing seasons arises from that one simple fact – the tilt of our planet’s axis!

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