Solstices and Eclipses

Solstice

Did you know that until 1975 the solstice fell on June 22 or 21, now only on June 21 and in 2012 it begins falling sometimes on June 20, that's precession for you.

Present day Druids celebrate the solstice sunrise at Stonehenge.
The Greek philosopher and librarian at Alexandria, Eratosthenes (c.250bc), measured the diameter of the Earth, making use of the sun at solstice. Yes, way back then, not only did the Greeks suspect that the earth was indeed a sphere but Eratosthenes worked out the earth's diameter. He knew that at the summer solstice, the sun's reflection was mirrored in a deep well lying on the Tropic of Cancer ( this was Syene, now lost under the waters of the Aswan Dam). On the same day the shadow of an obelisk at Alexandria, Egypt, some 500 miles due north of Syene showed the noonday sun was 7.5° from the zenith. Since the sun's rays are parallel, this means that the arc, or the difference in latitude, between Syene and Alexandria is approximately 7.5°, which is 1/48 of 360°. That is, 500 miles is 1/48th of the earth's entire circumference. Eratosthenes' figure for the earth's cicumference was 250,000 stadia, nearly 24,000 miles, which agrees within 4% with modern determinations. If Columbus had known the real distance to Cathay, he'd never have started off.

Eclipses

Like buses, eclipses come in groups; if the moon revolved around the earth exactly in the plane of the ecliptic, then every syzgy (a good word for scrabble players, it means a new or full moon) there would be an eclipse, an exactly central one. But this does not happen, only 1/6th of syzygies produce eclipses, never precisely central, some almost miss being an eclipse at all. The reason is the 5 degree inclination of the moon's orbit to the ecliptic, so the moon mostly passes above or below the sun at syzygy and the earth's shadow likewise fails to fall on the moon. Only at two places in the moon's orbit, where it passes through the sun-earth plane (the ascending and descending nodes), does the moon cause eclipses, that's why the plane is called the ecliptic. That's enough for now, good observing and avoid the street lights.

(Martin Young with aknowledgements to Guy Ottewell's Astronomical Calender dates, times and inbspiration).