Earths Orbit and the Seasons by dffhrtcv3


									Earth’s Orbit and the Seasons
Seasons on the Earth
                                               Receives only half
                                               the heat and light

1 KW/m2                                            1 KW/m2

                1 m2                      2 m2
   In winter a bundle of light is spread over a bigger area than in summer
   owing to the inclination away from the Sun
 Phases of Moon: Earth-Sun Positions

                                                                 What time
                                                                 of day does
                                                                 the new moon
                                                                 rise and set ?

What time of
day does
the full moon
rise and set ?

          ‘Horns’ of a crescent moon always point away from the Sun
                    Full Moon rises
               at dusk and sets at dawn

The New Moon rises and sets with the Sun (lies in the same direction as the Sun)
   Lunar Eclipse (around Full Moon)
Umbra and Penumbra- total or partial obscuration

 Umbra – dark part of the shadow; Penumbra – lighter part of the shadow
 Earth’s maximum umbra at Moon’s distance is 9200 Kms; the penumbra is
 16000 Kms across
Total Solar Eclipse: Total obscuration of the
  Sun by the Moon, possible because the
    angular size of Moon equals Sun’s
             Solar Eclipse (at New Moon):
    Visible from a narrow ‘band’ across the Earth

                Diameter (Moon) = 3476 Km = ¼ Diameter (Earth)
Moon’s umbra reaches only a small area < 270 Km where solar eclipse is visible
       Annular Solar Eclipse:
(when the tip of moon’s umbra doesn’t quite
              reach the earth)
Eclipses occur on Line of Nodes:
Earth-Moon-Sun must be in line

  Why do eclipses NOT occur each new and full moon?
  The E-M-S line can may deviate by up to 5 degrees
      Solar and Sidereal Day
• How long is one day ?
• From noon (Sun directly overhead) to
  noon ?
• How long does it take for the Earth to
  complete one rotation on its axis ?
Orbital and angular motion of the Earth
Solar & Sidereal           T=24h
Days                     (Solar day)

                           T=23h 56m 04s
                           (Sidereal Day)


   Not to Scale
• The earth moves each day by 1 degree in its
  orbit around the Sun. So each day the earth has
  to rotate a bit more to reach the noon position
• To rotate one extra degree requires
             24 x 60
             ---------- = 4 minutes
• Solar day (noon-to-noon = 24 hours) is
  rotation period with respect to the Sun
   4 minutes longer than the true rotation period
  of the Earth with respect to the stars called the
  Sidereal day = 23h 56 m
• 366 sidereal days per year
       Solar and Lunar Calendar
• Rotation of the earth = 1 solar day
• Revolution around the sun = 365.2422 d
• Orbital period of moon = 1 month = 29.5306 d
• Lunar OR solar calendars are possible, but not
  both since a month is not (i) exact no. of days,
  and (ii) year is not exact no. of lunar months
  12 x 29.5306 = 354.3672 days, not one year
• Solar calendar is the one most widely used
• Roman Julian Calendar year = 365 d + leap yr
   Gregorian-Julian Calendar
• Problem with Julian Calendar  average yr
  is 365.25 d, 11 minutes too long !
• By the 1500’s, the time of Pope Gregory,
  the calendar was ahead of astronomical time
  keeping by more than two weeks
• Gregorian reform:
  Century years not divisible by 400 are
  ordinary years, not leap years
   (Example: 1700 AD was not a leap year, but
  2000 AD was)
              Motions of the Earth:
   Rotation on its axis (day), Revolution or orbit
   around the Sun (year), and Precession of the
                   Polar N-S axis

Position of the north star changes due to the slow precession of the Earth’s axis
due to the gravity of the Sun – just like the wobbling axis of a spinning top
Precession and the location of Polaris:
         Period 26,000 Yrs

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