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					      IS1101
   Nothing to
   Something

Mr Goh Hock Leong
                                Content
 Celestial Sphere
   declination, right ascension
   vernal equinox, autumnal
    equinox, winter solstice &
    summer solstice
   celestial poles, celestial equator,
    ecliptic
   Seasons
             Movement of Earth

 Revolution of Earth around the Sun
  (Heliocentric model)
 Earth rotates about its own axis
 Axis of rotation tilted at 23.5o
View of the night sky
changes as the earth
orbits and rotates
Stars on celestial sphere
These star cant be seen as it is   Horizon
above the horizon




                                   This star cannot
                                   be seen as it is
                                   below the
                                   horizon
Wishing under the same
          bright stars?
                        Celestial Sphere
 The Earth blocks half of this celestial
  sphere from view at any instant.
 As the Earth rotates different stars come
  into view. The stars you see depend on
   the time of night (i.e. which way the Earth is
    turned).
   whether the Sun is up (the stars are still there
    but you can't see them).
   where you are on the Earth. For example, if you
    are at the Earth's North Pole the bulk of the
    Earth will always block the Southern
    Hemisphere's stars from your view.
Zenith and Meridian
               Zenith and Meridian
 Zenith is the point directly over an
  observer's head.
 Meridian is an imaginary line running
  north to south through your zenith and
  dividing the sky in half.
   "Meridian" is from an ancient word meaning
    midday, which is when the Sun crosses your
    meridian.
   When telling time, we use the letters a.m. to
    indicate morning for ante meridiem, which
    is Latin for "before midday."
   We use p.m. for post meridiem, or "after
    midday."
Celestial Sphere

    Earth at the centre
     (geocentric model)
    Imagine the sky as
     a great, hollow,
     sphere surrounding
     the Earth. The
     stars are attached
     to this sphere
    Sphere rotates
     around the
     stationary Earth
     roughly every 24
     hours (retrogade
     motion)
                      Celestial Sphere
 The celestial sphere is an imaginary sphere
  of infinite size surrounding the stationay
  Earth with the stars embedded in its
  surface.
 Even though we now know that this ancient
  model of a stationary Earth is incorrect, we
  still use this model because it is a
  convenient way to predict the motions of
  the stars and planets relative to a location
  on the Earth.
Reference points and lines
             Important/Reference points
              North Celestial Pole (NCP)
              South Celestial pole (SCP)

             Important/Reference lines
              Celestial Equator
              Ecliptic
             Celestial
                 Pole
     Projection of the
      Earth’s pole out to
     the sky.
     The stars rotate
      around the NCP &
      SCP.
     These are the points
      in the sky directly
      above the
      geographic north
      and south poles,
      respectively.
             Celestial
                Poles
     The Earth's axis of
     rotation intersects
      the celestial sphere
      at the celestial
      poles.
     The number of
      degrees the celestial
      pole is above the
      horizon is equal to
      the latitude of the
      observer.
                    Celestial Equator
 Same plane as
  Earth’s equator
        Celestial
         Sphere
 Imaginary sphere
 Surrounds the Earth
 Maps the location of
  stars
 Stars are visualised as
  to be on the surface of
  the Celestial Sphere
 Infinite size
What would the sky be like at
             the North Pole?
What would the sky be like at
             the North Pole?
What would the sky be like at
        other part of Earth?
What would the sky be like at
        other part of Earth?
Visibility of Stars
What would the sky be like at
               the equator?
What would the sky be like at
               the equator?
       Globe
We use lines
of latitude and
longitude to
pinpoint
position on
Earth.
Lines of Longitude

   Imaginary lines running
    from pole to pole
   ‘Tied’ to Earth
   Measures angular distance
   Measured in degrees
   East or west of Prime
   Meridian (0 degrees)
Lines of Latitude

 Imaginary lines running
  parallel to equator
 ‘Tied’ to Earth
 Measures angular distance
 Measured in degrees
 North or south of Equator
 (0 degrees)
                        Coordinate System
 Like the longitude and latitude system here on
  Earth, the celestial sphere has a system to
  pinpoint locations.
Right Ascension and
         Declination
                                    Declination

 tells us how far an object is
  north or south of the celestial
  equator, measured in degrees
 Measures angular distance
 Expressed as an angle rather
  than in metres
 Expressed in degrees
    + (above celestial equator)
   - (below) celestial equator)
           Right
       Ascension
• Measured in hours,
minutes and seconds
from a zero point
(* Note: angular measures,
not units of time!)

• Position of sun at
vernal equinox
• Increases from west
to east
                                       Right
                                   Ascension
 Right ascension tells us how far an object is to
  the east of the Vernal Equinox (chosen
  arbitray), that point midway between winter and
  summer when the ecliptic (the Sun's path)
  crosses the celestial equator.
 Latitude is measured to both the east and west
  of its starting point, but right ascension is
  measured only to the east.
 Also, right ascension is not measured in
  degrees, but in hours, minutes, and seconds.
 Each hour is equal to 15'.
                            Food for
                            Thought

  If the celestial poles, celestial
   equator, and declination are
projections of earthly coordinates,
  why not simply imagine right
 ascension as projections of lines
           of longitude?
       Answer




View of the night sky
changes as the earth
orbits and rotates
Ecliptic
      Ecliptic
• A circular path
taken by the sun
• Inclined at an
angle of about 23.5
degrees to the
celestial equator
• In the course of a
year
• Against a
background of
stars
   Why do we have seasons?
 Different distance from the Sun?
 Tilt of Earth?
   Area effect?
   Atmosphere effect?
 Height of the sun above the horizon
 Length of the day due to the
  position of the sun
Equinoxes and
     Solstices

 • Earth’s axis is tilted
 • Our planet orbits
 the sun
 • Each hemisphere
 gets varying amounts
 of sunlight during the
 year
Equinoxes and Solstices

                    Solstice:
                     Days of
                   longest or
                     briefest
                    daylight
                 Sol : Sun
                 -stice : Stand Still




                Equinoxes:
                  Days of
                equal night
                 and day
                   Summer Solstices
 Occurs on June 22
 In the northern hemisphere
    Longest day
    Sun is the highest in the sky
 Marks the beginning of summer
 A point in the earth’s orbit
 Planet’s north pole points closest to the sun
 Locations spend the greatest portion of
  their time exposed to the sun
                   Winter Solstices

 Occurs on December 21
 In the Northern Hemisphere
   The shortest day
   The sun is the lowest in the sky
 Marks the beginning of winter
 Six months after summer solstice
                                  Equinoxes
• Days of equal night and day
• When the ecliptic intersects the celestial
equator as the sun passes into the northern
hemisphere
• Vernal Equinox
 Occurs March 21
 Marks the beginning of spring
• Autumnal Equinox
 Occurs September 23
 Marks the beginning of fall
                       Equinoxes and
                            Solstices
  Summer Solstice



                    Vernal Equinox


Autumnal Equinox


                         Winter Solstice
                                   Reference
 Astronomy Fifth Edition– A self teaching guide by
  Dinah L. Moche
 Astronomy Today Second Edition by
  Chaisson/McMillan
 The complete Idiot’s Guide to Astronomy Second
  Edition
 http://csep10.phys.utk.edu/astr161/lect/celestial
  /celestial.html
 http://www-astronomy.mps.ohio-
  state.edu/~pogge/Ast161/Unit4/movearth.html
 Iowa State University, Polaris Project north Star,
  http://www.polaris.iastate.edu/NorthStar/Unit4/
  unit4_sub1.htm

				
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posted:12/19/2012
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