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The Sun

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					PHYS
 205
                    Celestial Sphere

  Imagine the sky as a hollow sphere
  The stars are attached to this hollow sphere, they rise in the east and
  set in the west
  The Sun, Moon and the planets wander on this sphere
   Celestial Sphere
  Very convenient
  But why spherical??
  Pythagoras-> Shadow of the Earth
PHYS
 205
                    Celestial North pole

 Stars rotate around the
 Celestial North and the
 South Poles.
 Stars rotate parallel to the
 Celestial Equator.
 The point straight overhead
 is called the Zenith.
 The arc that goes thru the
 north point, zenith and the
 south point of the horizon is
 called the meridian.
 The position of the zenith
 changes when you move away
 from the North Pole.
PHYS
 205
       We are looking up
                 Your position on Earth will be
                 defined by longitude and
                 latitude:
                 Latitude: distance from the
                 Equator in degrees.
                 Longitude: distance from the
                 prime meridian in degrees.
                 Stars rise in the East, pass the
                 meridian and set in the West.
PHYS
 205
                             On the Equator
 North Celestial Pole
 disappears from sight.
 Stars rise and set vertically.
 Zenith is parallel to the
 celestial equator.


 Now we know our bearings on
 Earth with respect to the
 stars  check out the motion
 of the Sun and the Moon.
PHYS
 205
       Sun’s path


            Earth moves around the Sun once
            every 365.24 days.
            Each day the Sun moves about
            one degree with respect to the
            other stars in the sky because of
            this motion.
PHYS
 205
               Reasons for the Seasons
                                   Earth’s orbit is elliptical, so
                                   sometimes the Earth is closer to
                                   the Sun 
                                   But when we have summer here,
                                   it’s winter in the southern
                                   hemisphere, so this does not
                                   explain the seasons.



 But the tilt of the Earth does.
PHYS
 205
       Precession of the Equinoxes

                       Celestial North Pole points to
                       the star Polaris at the
                       moment.
                       But the position of “North”
                       changes every 26,000 years
                       from Polaris to Vega and back.
                       First observed by Hipparchus
                       in 100 BC.
PHYS
 205
                   Motion of the Moon
                           Moon revolves around the Earth
                           every 27.3 days, and rotates around
                           itself every 27.3 days, so we always
                           see the same face of the Moon.




       New moon
       Crescent
       Quarter
       Gibbous
       Full moon
PHYS
 205
                               Eclipses
                                   Moon’s orbit is tilted by 5o wrt the
                                   ecliptic.
                                   We can have a lunar/solar eclipse
                                   only when the moon cuts the
                                   ecliptic plane.




But the eclipses only happen
when the moon cuts the
ecliptic during a full moon
(lunar eclipse) or new moon
(solar eclipse).
PHYS
 205
                          Solar Eclipse




   Because of the 5o tilt of the Moon’s
   orbit, Moon’s shadow misses the
   Earth most of the times.
   But once in a while (like once in every
   18 months)…
PHYS
 205
                            Total Eclipse




 Moon’s shadow has two parts:
 Penumbra: Faint, outer shadow, only partial
 eclipse can be seen.
 Umbra: Dark, inner shadow with total
 eclipse.

				
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posted:9/8/2012
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