The Reason for the Seasons

							The Reason for the
     Seasons

    For this activity, students
   should copy the underlined
   information for their notes.
    What Causes Seasons?
• Seasons are caused by the tilt of the
  Earth’s axis as it moves around the sun
  **the Earth’s axis does not move!

• It only appears to move as the Earth
  changes position as it orbits the Sun.
 The Reason for the Seasons
• Seasons happen because the Earth is
  tilted on its axis at a 23.5 º angle.

• The tilt affects the amount of sunlight—
  solar energy—the Earth receives as it
  moves around the sun.
    Why does the Earth Tilt?
• No one knows why the Earth's axis is tilted by
  23.5º.

• Some astronomers think that about 5 billion
  years ago, when the Earth was still very young,
  it was struck by a Mars-sized planet.

• If it tilted too much, the seasons would be very
  extreme—like on the planet Uranus where the
  winter lasts for 42 years in total darkness!
             Our 4 Seasons
•   Spring
•   Summer
•   Winter
•   Fall

• Areas near the equator have summer
  all year long since they get 12 hours of
  daylight every day of the year.
    Summer in the Northern
  Hemisphere—where you live
• During our summer months, the N. Hem.
  has warmer temperatures & longer days
  because it is tilted toward the sun.

• Seasons in the S. Hem. are always the
  opposite of our seasons (we live in the
  N. Hem.) due to the Earth’s tilt.
       Winter in the Northern
           Hemisphere
• During our winter months, the N. Hem. is
  tilted away from the sun, temperatures
  are lower & days are shorter.

• Seasons in the S. Hem. are always the
  opposite of our seasons due to the
  Earth’s tilt.
   Seasons and The Equator
• The equator receives generally the same
  amount of energy from the sun all year
  round.
  – Areas near the equator don’t really have seasons.
  – At the equator the Sun’s rays stay at the same angle
    all year.

• This means that the length of day temperature
  doesn't vary as much. So countries near the
  equator only have seasons based on weather
  or what fruit is in season—pineapple season,
  rainy season, etc.
     Seasons and Weather
• Different seasons bring different
  types of weather.

• This weather is caused by the
  uneven heating & cooling of the
  Earth’s atmosphere.
            Solstices

• The solstices are days when the
  Sun reaches its farthest points
  north and south of the equator.
           Winter Solstice
• The winter solstice occurs on December
  21 or 22.

• It marks the beginning of winter.

• This is the shortest day of the year in
  terms of hours of daylight.
         Summer Solstice
• The summer solstice occurs on June 20
  or 21 and marks the beginning of summer.

• This is the longest day of the year in
  terms of daylight.
             Equinoxes
• Equinoxes are days in which day & night
  are equal in length.

  – 12 hours of daylight & 12 hours of
    darkness
     Spring/Vernal Equinox
• The vernal equinox/spring equinox
  occurs in late March—around March 20 or
  21st .

• Beginning of spring in the N. Hem. &
  the beginning of fall in the S. Hem.
     Fall/Autumnal Equinox
• The autumnal equinox/fall equinox
  occurs in late September on the 21 or 22.

• Beginning of fall in the N. Hem. & the
  beginning of spring in the S. Hem.
            Earth Viewer
• http://www.fourmilab.ch/earthview/vplanet.
  html