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Time of the Seasons

VIEWS: 1 PAGES: 20

  • pg 1
									        Astronomical Coordinates
               Summary
• Relative to Observer
   – Local Horizon
   – Local Meridian
   – Local Zenith
• Altitude-Azimuth Coordinates
   – Pro: Easy to record for
     instantaneous position
   – Con: Objects change angles
     (coordinates) throughout the
     night
   – Con: not easy to compare
     measurements made by different
     observers

                                      1
Ancient Alt.-Az. Observatory
              • Chankillo, Peru
              • 2300 years old
              • Marks positions of
                sunrise throughout the
                year




                                         2
              Last Class Summary:
            Astronomical Coordinates
• Fixed to rotating Earth
    – Celestial Equator
    – Right Ascension: Measure from Vernal
        (March) Equinox
    – Declination: measure from Celestial
        Equator
Celestial Coordinates
    – Pro: stars have fixed coordinates
    – Pro: easy to compare measurements made
        by different observers
    – Note: Sun changes celestial coordinates
        throught the year as it follows the ecliptic
        through the Zodiac
    – Note: Require a clock at each observatory
        for coordinate system reference.

                                                       3
Time of the Seasons
           • We have seasons because of the
             tilt of the Earth’s axis.
           • The Ecliptic appears tilted 23.5°
             from the Celestial Equator because
             Earth’s axis is tilted 23.5° from the
             vertical to the plane of its orbit
           • Because of this tilt the Sun appears
             half the year North of the Equator
             and half the year South of the
             Equator
           • The seasons are defined based on
             the location of the Sun compared to
             the Celestial Equator



                                           4
Summer
   On the first day of Summer:
       – The Sun is at its farthest
          distance North of the Celestial
          Equator (23.5° N declination).
          The is known as the Summer
          Solstice.
       – The Northern Hemisphere is
          tilted towards the Sun & we
          experience the longest day of
          the year (about June 21).
       – The Southern Hemisphere is
          tilted away from the Sun. This is
          the beginning of Winter and the
          shortest day of the year there.
       – Sun rise and sets at its most
          northerly
                                      5
6
     Why it’s hotter in the Summer and colder in the Winter




At the summer solstice, the North gets more direct sunlight
and more hours of sunlight. At the same time it is winter in the South
(less direct sunlight, fewer hours of daylight).                         7
In the winter months the Sun is lower in the sky for
   fewer hours and the sunlight is spread over a larger
   area.                                                  8
 “But I thought the reason it was
hotter in the summer was because
the Earth was closer to the Sun in
           the Summer.”


       • If this were true then why is the Earth
         closest to the Sun in January and
         farthest from the Sun in July? and
       • Why does the Southern Hemisphere
         have seasons that are opposite that in
         the Northern Hemisphere?
                                               9
Winter
   On the first day of Winter:
       – The Sun is at its farthest South
          of the Celestial Equator (23.5° S
          declination). This is known as
          the Winter Solstice.
       – The Northern Hemisphere is
          tilted away from the Sun & we
          experience the shortest day of
          the year (about December 21).
       – The Southern Hemisphere is
          tilted towards the Sun. This is
          the beginning of Summer and is
          the longest day of the year there.
       – Sun rise and set is at its most
          southerly.
                                       10
Spring and Autumn
         • The Sun crosses the Celestial
           Equator heading North on the
           first day of Spring (the Vernal
           Equinox, about March 21).
         • It crosses the Celestial
           Equator again heading South
           on the first day of Autumn
           (the Autumnal Equinox,
           about September 21).
         • In both the Northern and
           Southern Hemisphere days and
           nights are of equal length

                                   11
         Seasons Interactives
• Seasons Interactive I - McGraw Hill
• Seasons Interactive II - University of
  Nebraska Lincoln (UNL)
• Sun Motion – UNL
• Seasons Interactive III - Iowa State



                                           12
      Summary – The Seasons
• The rotation of the Earth on its axis
  determines the length of the day
• The orbit of the Earth around the Sun
  (revolution) determines the length of the
  year
• The tilt of the Earth with respect to the
  ecliptic causes the seasons.

                                              13
           Summary - Equinoxes
• The two equinoxes occur when the Sun crosses the
  Celestial Equator
   – In March, as the Sun is heading North, and
   – In September, as the Sun is heading South
• Equinox means “equal night”
   – Length of night is the same as the length of daylight for the
     equinoxes
• The Sun will rise due East and set due West.
• For an observer on the Earth’s equator
   – The sun will be at the observer’s zenith at local noon.
• For an observer on a pole: the Sun circles the observer on
  the horizon…16 hours from half-to-all or half-to-gone.
                                                                     14
                       Equinoxes



                                   At lat. = 20
On equator, lat. = 0

At lat. = 50                       At pole lat. = +90
           Summary - Solstices
• The two solstices occur when the Sun reaches its
  extreme North or South positions in the sky.
   – December Solstice: farthest South
   – June Solstice: farthest North




                                                     16
Solar Altitude Variation
   Over Six Months




                           17
                     Analemma




• Earth's axis is tilted 23.5 degrees
• Earth's orbit is elliptical, not circular   18
        Analemma Interactive
• http://www.analemma.com/SunGraph/index.html
• APOD - Analemma Movie over New Jersey




                                                19
Analemma for Greenwich, England
         Latitude +51




                                  20

								
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