19.3

• Earth's rotation


  Causes day and night
• Earth's revolution


  Causes a year
• Earth's axial tilt


  Causes seasons
• Earth's motion


  1. Spin on its axis, producing night and day

  2. Moves about the sun in an elliptical orbit, taking 365 1/4 days to complete

  3. Spin axis is tilted with respect to orbital plane, causing seasons
• Season


  A quarter of the year with unique weather pattern characteristics

  
• The seasons are four different times during the year with different types of weather
• Seasons


  Determined by the amount of sunlight being received

  
• Equinox


  The day on which the duration of day and night is equal

  
• Solstice


  An event on which the unequal duration between day and night is greatest

  
• Seasonal transitions


  • Affected by length of daytime, noon altitude, length of the shadow of a vertical object
  • Explained using the celestial sphere

  
• Celestial sphere


  An imaginary sphere centered on Earth upon which all celestial bodies can be projected

  
• The celestial poles and the celestial equator lie on the celestial sphere with respect to Earth's poles and equator

  
• The Sun moves in an ecliptic path in the celestial sphere, and seasons happen at the four quarters of this ecliptic

  
• Solstice comes from a Latin word meaning "Sun stand still"

  
• Summer Solstice


  Happens every June 21 or 22 where Sun is at its highest altitude and longest path, marking the start of summer with longer days and shorter nights
• During summer solstice, the North Pole points toward Sun and the N. Hemisphere receives more sunlight

  
• Summer is the hottest time of the year since it is in this period when Earth's surface receives the greatest insolation
• Winter Solstice


  Happens during December 22 or 23 where Sun is at its lowest altitude and shortest path, marking the start of winter with shorter days and longer nights
• During winter solstice, the North Pole points away from Sun, and the noon altitude is lowest in the Northern Hemisphere
• During winter, Sun follows a shorter path and Earth's surface receives the least insolation throughout the year
• In the Philippines, the solstices are experienced through a subtle difference in the length of day and night

  
• Spring Equinox


  Marks the beginning of spring, also called the vernal equinox, happens on March 21 after the winter season
• During the spring equinox, plants start to 'spring' back to life, aided by the mild temperature and the abundance of meltwater from winter ice, and some animals come out of hibernation
• Fall Equinox


  Marks the beginning of autumn, also called the autumnal equinox, occurs on September 23 after summer

  
• During the fall equinox, the leaves of non-evergreen plants turn red and orange and eventually 'fall' as plants' preparation for the winter
• In the Philippines, the meteorological and ecological effects of the equinoxes are not felt, only the equal length of day and night are observed
• The shifting in the direction has been earlier noticed by ancient people, and the Stonehenge on Salisbury, England was probably built to mark the seasonal rising and setting points of Sun