Although we have already passed the Summer Solstice, the Sun remains high in the sky during this month. As I write this, it's after 8:00 PM and the Sun's light is only just beginning to fade. This twilight period is amplified at higher latitdues, while seasonal differences are minimized the closer one gets to the equator.
The reason for this is the Earth's angle of rotation. If the Earth and all the other planets (except Pluto) were on the same plane as the Sun, and if the Sun rose due east and set due west everyday, there would be no seasons. Daylight and darkness would stand in perpetual balance, with 12 hours of each, year in and year out. The Winter Solstice would not have longer nights, nor the Summer Solstice longer days.
But the angle of the Earth's axis is tipped some 23 and a half degrees (23 degrees, 26 minutes and 16 seconds for all you Virgo's out there) out of the plane of the ecliptic (path of the Sun), and that means our northern and southern hemispheres alternately lean toward and away from the Sun, impacting the amount of time our favorite star stands overhead. Additional hours of Solar radiation means warmer weather, while a diminishing amount of Sunlight favors the reverse. Add to this the fact that the Earth is actually closer to the Sun for part of the year and more distant on the flip side and we have another wrinkle which must be considered in the equation.
The Southern Hemisphere is acutally closer to the Sun during their summer (our winter), and so recieves a double dip into the solar realm, but fortunately, Australia, Southern Africa and South America also have more and larger bodies of the Earth's oceans insulating them. This helps to off-set some of the excess heat by carrying it away in their cooling currents. Australia is a good example of this. Take a look at any map and you will see that its coastal towns, and the east coast in particular, carries larger population centers, while the in-land deserts, further from the oceans, are more sparsely populated.
The tropic zone is the band between the Tropic of Cancer in the north and the tropic of Capricorn in the south. Between these two boundaries the Sun is directly overhead at least once, and more often twice a year. Walled pits that are dug striaght down into the Earth (think gravity and a plumb line), cast no shadow on any of the four walls on these days. Percipitation in the tropic zones tends to be much greater that those in the northern and southern latitdue extremes. Naturally, not all this distribution is uniform and the patterns vary from the deserts of Northern Aftrica to the tropical rainforests of Asia and Central and South America. Currently, about 40% of the world's population lives in tropic zones, while that number is expected to rise to about 60% given current trends.
Of course all of this raises questions about how much exposure to the Sun, and therefore how much Vitamin D we should receive on a regular basis. In attempting to draw power from the Sun, it is important to know how to place the solar panels to maximize their potential. Plants get around this by turning their leaves and growing towards the Sun each day. They have learned how to follow these rhythms and 'ride them' to their greatest advantage.
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