The tilt of the Earth's axis makes the Sun appear either north or south of the equator, and the latitude at which the Sun is overhead (i.e. 'in the zenith') at noon is called the Sun's declination. The seasonal variation of the declination has been monitored since the times of the ancient Egyptians, 5,000 years ago. The declination is furthest south on about 21 December (a solstice) when the Sun is directly overhead at 23° 27'S (the Tropic of Capricorn) at noon. The South Pole then tilts most towards the Sun, and the southern hemisphere has its longest day; south of 66° 3'S (the south Polar Circle) the Sun is still above the horizon at midnight. (Notice that 66° 33' is 90 degrees minus the angle of tilt.) Likewise, the Sun lies at the same latitude in the north (at the Tropic of Cancer) in the middle of a southern-
hemisphere winter, on about 21 June, the other solstice.
The angle of the Sun above the horizon at any moment is called the solar elevation, and its value can be shown by a diagram such as that of Figure 2.4, for any place. Its value at noon equals [90-f+d], where f is the latitude and d the declination of the Sun at that time of year. Both f and d are regarded as positive in the southern hemisphere. The annual extremes of the Sun's elevation allow us to design houses which accept the Sun's warmth in winter but shade the windows in summer (Figure 2.5).
The midday Sun is overhead on two dates each year, at latitudes between the two Tropics, which means that there are two seasons of maximum insolation in such places. The midday Sun is overhead at the equator at the equinoxes, on about 21 March and 22 September, when day and night are of equal duration, at all latitudes (Table 2.1).
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