The seasons

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Nuclear fusion is a controversial topic aimed at securing the future of energy supply. The technical hurdles to making it possible are high, and our possibilities are still limited, but it has been taking place in the sun for at least 4.5 billion years. The sun fuses four hydrogen nuclei each to form a helium nucleus. Calculated over the entire cross-section of the earth, this process generates an enormous 170,000 terawatts of energy, which we perceive as light and heat. The sun only contributes a limited amount of warmth to us Europeans during the winter, but it does so abundantly during the summer. What is the cause of this seasonal change?

With regard to the different temperatures associated with the changing seasons, the first suspicion that comes to mind is that the earth sometimes comes closer to the sun on its annual orbit and sometimes less so. This assumption is not entirely incorrect, as the earth’s orbit around the sun is slightly elliptical, with a point near the sun on January 3 (known as the perihelion, measuring 147.1 million kilometers), and a point far from the sun on July 4 (known as the aphelion, measuring 152.1 million kilometers). These differences in distance, however, account for only a 6% difference in the solar radiation received by the earth over the year – too little to be associated with the seasons.

The real reason for this is rather the 23.5° inclination of the earth’s axis, which brings the plane of the earth’s equator out of its perpendicular position with respect to the sun, and the resulting change in the angle of incidence at which the sun’s rays reach the earth’s surface (see figure 11). Since the earth always maintains this oblique position, it inclines different areas of its surface towards the sun during a complete orbit or stands with the inclined axis parallel to it (figure 12).

If you own a globe, whether illuminated or not, now is the time to take it out of the corner of your shelf. This globe will come in handy for the upcoming observations. If we observe the earth-sun system over a whole year from a vast distance, we can distinguish four prominent constellations on the orbit according to the observations made before: At the closest point to the sun, the earth tilts the southern hemisphere towards the sun, and the sun remains perpendicular above the 23.5th parallel south (the solstice on December 21). At a quarter of a turn, the inclination ceases to have any effect; the earth’s axis aligns with the sun, resulting in a 90° angle over the equator (the equinox on March 21). A quarter turn further, the northern hemisphere faces the sun at its furthest point from the sun, with the sun perpendicular above the 23.5th degree of northern latitude (solstice on June 21). Following the third quarter turn, we return to the central position, with the sun at 90° above the equator (equinox on September 23).

Consequently, in northern summer the rays fall much more steeply in the northern hemisphere than in winter and vice versa in southern summer in the southern hemisphere. And since according to the so-called Lambert’s law „the steeper the angle of incidence, the higher the radiation energy reaching the earth’s surface, because at a steeper angle of incidence the rays cover a smaller area„, it becomes clear why it is so hot in summer. In relation to the earth, the atmosphere’s lower filtering effect for the energy-rich short-wave (blue) spectrum at steep angles of incidence contributes to this phenomenon.

Due to the inclined earth axis, the sun apparently wanders over the relatively narrow range between 23.5° northern latitude (its northernmost point and therefore northern tropic) and 23.5° southern latitude (analogous to its southernmost point and consequently southern tropic), and each of the mentioned four positions (the following calendrical data are an arbitrary superimposed plane) marks the beginning of a season.

With the magic of the 23 ½ degrees, the most important connections in the relationship between earth and sun can be explained. They are the measures of perfect symmetry.

For us in the northern hemisphere, winter begins when the sun is at 90 degrees above the Tropic of Capricorn on December 23, giving us the shortest day and longest night. Traditionally, we call this the winter solstice. We associate the beginning of spring with the following perpendicular position of the sun over the equator on March 21. At this time, both poles are equidistant from the sun, and the day and night are of equal length. The much anticipated summer begins on June 21, when the sun reaches the northern limit of its journey, the Tropic of Cancer. The Scandinavians in particular celebrate the associated longest day (or shortest night) as the summer solstice. Autumn remains to be mentioned, and it begins when the sun on its way south is once again perpendicular above the equator on September 23, and day and night again have the same length. Thus, each of the four seasons begins or ends with either an equinox or a solstice, which is the reversal of the sun’s direction. Against this background, it is easy to explain why the tropical latitudes around the equator do not know any distinguishable seasons and why the days and nights are of always about the same length throughout the year. In relation to the tropics, there is simply not enough change in the angular relationship between the earth and the sun during the course of the year to have any influence on the climate.

These connections have determined the lives of the people for thousands of years about the dates for sowing and harvest, and for this reason, our primitive ancestors have recorded them quasi-calendrical in various stone circles.

Next The different lengths of day and night

Main Natural light

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