SOLSTICE
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The lack of shadow is beautiful too. And it happens twice a year, for places between +23.5 and -23.5 degrees latitude. The Sun is almost never exactly overhead at noon, but usually transits a bit lower in altitude, a bit to the north or a bit to the south. We have all studied in school that the Earth's rotation axis is inclined at 23.5 degrees to the plane of its revolution around the Sun, which is why we have seasons. This also means that the Sun, in its highest point of the day, will move from 23.5 degrees south of the celestial equator to 23.5 degrees north of the equator (Uttarayan), and back again (Dakshinayan), in a year. Of course, the northern most and southern most points are the two solstices, and the crossing of the Sun across the equator are the two equinoxes.
For people living between +23.5 and -23.5 degrees latitude, the Sun's declination will be equal to their latitude twice - once during Uttarayan and once during Dakshinayan. On these two days, the Sun will be exactly overhead at noon and will not cast a shadow of an object on the ground. This Zero Shadow Day will clearly be different for different places on earth.
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You may have noticed two special lines of latitude on a globe of the world: One in the Northern Hemisphere called the Tropic of Cancer at +23.5° latitude and one in the Southern Hemisphere called the Tropic of Capricorn at − 23.5° latitude.
These are the latitudes where the Sun is directly overhead at noon once a year. In the Northern hemisphere, on the Tropic of Cancer, that is the Summer Solstice, usually June 21. In the Southern Hemisphere, on the Tropic of Capricorn, that is the Winter Solstice, usually December 21. These solstice days are the days with the most (for Summer) or fewest (for Winter) hours of sunlight during the whole year.
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