Vernal Equinox, the First Day of Spring

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Vernal Equinox, the First Day of Spring

• By Dr. Harold Williams
• of Montgomery College Planetarium
• http//montgomerycollege.edu/Departments/planet/

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The Sky Dome
Sunrises and Sunsets
Polaris
W
On the day of an summer solstice
On the day of an winter solstice
S
On the day of an equinox
N
E
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Polaris
W
Vernal or Autumnal Equinox
S
Winter Solstice
Summer Solstice
N
E
Starting Points for the Sun
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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

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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.

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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. 0h, Dec 0o
• The Summer Solstice (about June 21) The location
  where the Sun is at its furthest north.
• Position of the Sun R.A. 6h, Dec 23.5o

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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. 12h, Dec 0o
• The Winter Solstice (about December 21) The
  location where the Sun is at its furthest south.
• Position of the Sun R.A. 18h, Dec 23.5o

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Some Resources on Seasons

• Seasons in the Wikipedia http//en.wikipedia.org/w
  iki/Seasons
• Spring Seasons in the Wikipedia
  http//en.wikipedia.org/wiki/Spring_28season29
• Tropical Year in the Wikipedia http//en.wikipedia
  .org/wiki/Tropical_year