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Astronomy of Seasonal Festivals: Solstices and Equinoxes: how the Earth’s tilt results in the seasons

Published by Samual Hymowitz

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The majority of festivals mark the cycle of the seasons. Theseasons are determined by the relation between the Earth and the sun, though not by the changes in distance between them (those changes have no effect). Rather, the determining factor is the tilt of the Earth’s axis.

This considerable tilt (23°26’) remains always the same as well as in the same position as the Earth turns yearly around the sun and daily on its axis.

  • hold out your right fist (representing the sun),
  • hold your left hand, flat, next to it, tilting your fingers toward it (the earth’s axis),
  • let your left hand describe a circle around your fist, keeping the tilt in the same direction.

As a result, at any particular time, each hemisphere (your fingers, your palm) is tilted towards or away from the sun. Again this has nothing to do with that hemisphere being “closer” to the sun. Rather, on the favorable side of the tilt

  • it is warmer, because the rays of the sun hit at a more direct angle.
  • the days are longer, because more of the hemisphere is exposed to the sun, and so most of the daily rotation of the earth happens in the sunlight.

There are four large astronomical events, each if which occurs at the beginning or in the middle of the season – depending on how you define “season”. For the Northern Hemisphere these are:

  • 21-22 December, the Winter Solstice. The northern hemisphere is tilted the most away from the sun. This is the shortest day in the year. At noon, the sun is at its lowest altitude above the horizon, where it seems to halt its descent – “solstice” means “the sun stands still” – and on the next day it will be higher again. (Because the oceans and atmosphere diffuse the thermal effect, the coolest day comes several weeks later.)
  • 20-21 March, the vernal or Spring Equinox. The earth’s tilt doesn’t point to the sun, so both hemispheres get the same amount of sunlight. As a result, day and night are of equal duration: “equinox” means literally “the night is the same”.
  • 21-22 June, the Summer Solstice. The hemisphere is tilted the most toward the sun. This is the year’s longest day, when the noon sun reaches its zenith. (The warmest day will arrive several weeks later.)
  • 23 September is the Autumn Equinox. Day and night are again of equal duration.

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