The Reasons for Seasons

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The Reasons for Seasons

• Edward M. Murphy
• Space Science for Teachers
• 2005

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Day Night Cycle
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Rotation vs. Revolution

• Rotation is the spin of an object about its
  axis.
  
• The Earth rotates once a day (once every 24
  hours).
  
• Revolution is the orbit of one object around
  another.
  
• The Earth revolves around the Sun every 365.26
  days.

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The Zodiac

• As the Earth revolves around the sun the
  different constellations of the Zodiac appear in
  the sky
  
• What are the 12 signs of the zodiac?
• Pieces, Aries, Taurus, Gemini, Cancer, Leo,
  Virgo, Libra, Scorpius, Sagittarius, Capricorn,
  Aquarius

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Tilt of the Earths Axis

• The axis around which the Earth rotates is tilted
  by 23.5 degrees with respect to the ecliptic.

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Tilt of the Earths Axis
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Tropics
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Equinoxes and Solstices

• The Vernal (Spring) Equinox
• (about March 21)
• 1st day of spring
• Every place on earth has 12 hrs day light
• Sun is directly over equator at 12 noon

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Equinoxes and Solstices

• The Summer Solstice (about June 21)
• 1st day of summer
• Longest day of the year
• Sun is over tropic of Cancer
• Sun does not set above arctic circle

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Earth on June 21
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Equinoxes and Solstices

• The Autumnal (Fall) Equinox (about September 21)
  
  
• 1st day of Fall
• Every place on earth has 12 hrs day light
• Sun is directly over equator at 12 noon

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Equinoxes and Solstices

• The Winter Solstice
• (about December 21)
• 1st day of winter
• shortest day of the year
• Sun is over tropic of Capricorn

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Earth on December 21
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Motion on the Ecliptic
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Standing on the North Pole
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Midnight Sun
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Standing on the Tropic of Cancer
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Standing on the Equator
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Seasons Misconception

• Many people carry the misconception that the
  seasons are due to the distance of the Earth from
  the Sun. However, consider the following facts
  
• The Earths orbit around the Sun is nearly a
  perfect circle. The Earth is slightly closer to
  the Sun in January and farther from the Sun in
  July.
• Perihelion (closest to the Sun) is around January
  3 when Earth is about 91,405,436 miles from the
  Sun.
  
• Aphelion (farthest from the Sun) is around July 4
  when Earth is about 94,511,989 miles from the Sun.

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Seasons Misconception

• While it is winter in the Northern hemisphere it
  is summer in the Southern hemisphere.
  
• If the seasons were due to our distance from the
  Sun both hemispheres would have the same seasons
  at the same time.

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Orbit of the Earth
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The Seasons
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The Seasons

• Seasons are due to the tilt of the Earths axis.
  
  
• Summer
• The sunlight is spread out over less ground and
  heats the ground better.
  
• The Sun is above the horizon for a longer period
  of time.

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Solar Illumination
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Earth on June 22
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Earth on December 22
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Time
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Day Night Cycle
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Why is there Time Zones?

• Problem If the sun travels slowly across the US
  technically each city will have a different time
  
  
• Imagine starting in Charlottesville at exactly
  noon.
  
• As you travel to the west, the Sun will appear
  further east in the sky (i.e. lower and further
  from the sun).
  
• Even if you travel only a few miles west, the Sun
  gets lower in the horizon.
  
• Each city would have its own time.

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Time Zones

• With the advent of rapid travel by trains in the
  19th century, it became necessary to standardize
  the time for all cities within a certain region.
  
• In November 1883, the railroad companies divided
  the United States into four time zones.
  
• Everyone in a time zone set their clocks to the
  same standard time.

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Time Zones

• In 1884, an international conference was held in
  Washington D.C. by 26 countries.
  
• The world was divided into 24 times zones, with
  each zone being roughly 15 degrees wide in
  longitude.
  
• Divided into 24 time zones one for each hour of
  the day.
  
• Since there are 24 hours in a day, and 360/1524,
  the time in each zone differs from the time in
  adjacent zones by one hour.

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International Date Line

• Standard time gets earlier as you travel to the
  west.
  
• The International Date Line line was established
  in the middle of the Pacific Ocean.
  
• As you go from east to west, you gain a day as
  you cross the line.
  
• As you go from west to east, you lose a day as
  you cross the line.

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Time Zones
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Daylight Saving Time.. Why?

• During the late spring, summer, and early fall,
  we set out clocks ahead to have an extra hour of
  daylight at the end of the day.
  
• This change in time is called Daylight Saving
  Time.
  
• The idea of changing our clocks was first used in
  the United States during World War I to conserve
  energy. Now is used so people can make better
  use of time in the afternoons
• Since 1986, the United States has set our clocks
  one hour ahead on the first Sunday in April and
  one hour behind on the last Sunday in October.
  
• Spring ahead, fall back.

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Tilt of the Earths Axis