The Seasons

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The Seasons

• are caused by the tilt of Earth's axis (23.4)
  and not by the fact that Earth's orbit around the
  Sun is an ellipse.
• The average distance of Earth from the Sun is 93
  million miles the difference between aphelion
  (farthest away from the Sun) and perihelion
  (closest to the Sun) is 3 million miles, so that
  perihelion is about 91.4 million miles from the
  Sun.
• Earth goes through the perihelion point a few
  days after New Year's Day, just when the Northern
  Hemisphere has winter. Aphelion is passed during
  the first days of July.

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• The tilt of Earth's axis is responsible for
  four lines you find on every globe.
• When, say, the North Pole is tilted away from
  the Sun as much as possible, the farthest points
  in the North that can still be reached by the
  Sun's rays are 23.5 from the pole.
• This is the Arctic Circle. The Antarctic Circle
  is the corresponding limit 23.4 from the South
  Pole the Sun's rays cannot reach beyond this
  point when we have midsummer in the North.

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Tropic of Cancer

• parallel of latitude at 2330' north of the
  equator it is the northern boundary of the
  tropics.
• This parallel marks the farthest point north at
  which the sun can be seen directly overhead at
  noon north of the parallel the sun appears less
  than 90 from the southern horizon at any day of
  the year.
• The sun reaches its vertical position over the
  Tropic of Cancer at about June 22, the summer
  solstice in the Northern Hemisphere.
• When the Tropic of Cancer was named, the sun was
  in the constellation Cancer at the time of the
  summer solstice.

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Summer Solstice

• the sun's apparent position on the celestial
  sphere reaches its greatest distance above or
  below the celestial equator, about 23 1/2 of
  arc.
• At the time of summer solstice, about June 22,
  the sun is directly overhead at noon at the
  Tropic of Cancer.
• happens on June 21 or 22 each year, has the
  longest daylight time.

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Tropic of Capricorn

• parallel of latitude at 2330' south of the
  equator it is the southern boundary of the
  tropics.
• This parallel marks the farthest point south at
  which the sun can be seen directly overhead at
  noon south of the parallel the sun appears less
  than 90 from the northern horizon at any day of
  the year.
• The sun reaches its vertical position over the
  Tropic of Capricorn at about Dec. 22, the summer
  solstice for the Southern Hemisphere.
• The term Capricorn comes from the Latin words
  caper goat and cornu horn and is the name
  given to one of the 12 constellations in the
  zodiac.

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Winter Solstice

• the solstice on or about December 21st that
  marks the beginning of winter in the Northern
  Hemisphere.
• shortest day

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Equator

• imaginary great circle around the earth,
  everywhere equidistant from the two geographical
  poles and forming the base line from which
  latitude is reckoned.
• The equator, which measures c.24,902 mi (40,076
  km), is designated as lat. 0. It intersects N
  South America, central Africa, and Indonesia.
• The celestial equator is the projection of the
  plane of the earth's equator on the celestial
  sphere.

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Equinox

• either of two points on the celestial sphere
  where the ecliptic and the celestial equator
  intersect.
• The vernal equinox, also known as the first
  point of Aries, is the point at which the sun
  appears to cross the celestial equator from south
  to north.
• This occurs about Mar. 21, marking the beginning
  of spring in the Northern Hemisphere.
• At the autumnal equinox, about Sept. 23, the sun
  again appears to cross the celestial equator,
  this time from north to south this marks the
  beginning of autumn in the Northern Hemisphere.
• On the date of either equinox, night and day are
  of equal length (12 hr each) in all parts of the
  world
• the word equinox is often used to refer to either
  of these dates.

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Equinoxes

• the Sun is vertically above the equator, the
  day is of equal length all over Earth. This
  happens twice a year

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Arctic Circle

• is a parallel of latitude on the Earth at
  approximately 66.5 degrees north from the
  equator.
• On the day of the northern summer solstice
  (around June 22 each year), an observer on the
  Arctic Circle will see the Sun above the horizon
  for a full 24 hours.
• Observers further north than the Arctic Circle
  will see the Sun remain above the horizon for
  many days, and at the north pole, there is a
  six-month 'day' that starts on the vernal equinox
  changing to a six-month 'night' on the autumnal
  equinox.
• The 66.5 degree angle comes from the tilt of the
  Earth's rotation axis(23.5o), such that 90o -
  23.5o 66.5o.

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Antarctic Circle

• is a parallel of latitude on the Earth at
  approximately 66.5 degrees south of the equator.
• On the day of the southern summer solstice
  (around December 22 each year), an observer on
  the Antarctic Circle will see the Sun above the
  horizon for a full 24 hours.
• Observers further south than the Antarctic
  Circle will see the Sun remain above the horizon
  for many days, and at the South Pole, there is a
  six-month 'day' that starts on the autumnal
  equinox changing to a six-month 'night' on the
  vernal equinox.
• The 66.5 degree angle comes from the tilt of the
  Earth's rotation axis (23.5o), such that 90o -
  23.5o 66.5o.

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