Vernal Equinox, the First Day of Spring By Dr. Harold Williams of Montgomery College Planetarium

S N W E Polaris On the day of an equinox… On the day of an winter solstice… On the day of an summer solstice… The Sky DomeSunrises and Sunsets

S N W E Polaris Vernal or Autumnal Equinox Summer Solstice Winter Solstice Starting Points for the Sun

Length of Seasons Summer 93 days 15 hours 29 minutes Autumn 89 days 20 hours Winter 88 days 23 hours 54 minutes Spring 92 days 18 hours 26 minutes Sun and Earth closest, perihelion Around January 3 or 4 Sun and Earth furtherest, aphelion Around July 4

Obliquity of the ecliptic 23 ½ degrees, approximately, between day axis of rotation and year axis of revolutionary orbit about the sun Causes the seasons Seasons are not caused by the fact that the earths orbit is not a perfect circle, but its orbit is slightly elliptical. This ellipticity of the earths orbit does cause the seasons which are caused by the tilt, obliquity of the ecliptic, to be slightly unequal in length.

Equinoxes and Solstices The Vernal (Spring) Equinox (about March 21): The location where the Sun crosses the equator when going from south of the equator to north of the equator. –Position of the Sun: R.A. 0 h, Dec 0 o The Summer Solstice (about June 21): The location where the Sun is at its furthest north. –Position of the Sun: R.A. 6 h, Dec o

Equinoxes and Solstices The Autumnal (Fall) Equinox (about September 21): Where the Sun crosses the equator when going from north to south. –Position of the Sun: R.A. 12 h, Dec 0 o The Winter Solstice (about December 21): The location where the Sun is at its furthest south. –Position of the Sun: R.A. 18 h, Dec –23.5 o

Some Resources on Seasons Seasons in the Wikipedia Spring Seasons in the Wikipedia son%29 son%29 Tropical Year in the Wikipedia