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Association of Bangalore Amateur Astronomers

Days of No shadows at Noon

24 Apr 2013, 01:00 UTC
Days of No shadows at Noon
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We have all heard the story on how Eratosthenes measured the circumference of the earth. Eratosthenes observed that during high noon when sun was at Zenith a well at a place called Syene did not cast any shadow on June 21st; we call this day as summer Solstice. On same day, Solstice, during high noon a shadow was cast when a vertical stick was kept in Alexandria. Based on the angle of shadow cast by the sun he calculated the circumference of the earth, he gave a value of 40233km. Sun rises in the east and sets in the west, this we notice easily as the duration is very small. The Sun does move in north-south direction too, this we normally notice as seasonal changes more than position of the sun, as the movement of the Sun in north-south direction takes months together. Sun will be directly overhead at equator on march21/22; the shadow will be at its minimum on that day during high noon. This day is called Equinox and Sun’s angle is taken as Zero degree declination. As days pass, due to earth’s axis inclination of 23.5degrees, Sun starts moving towards north from March 21st to June 21st. ...

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