Earth’s Orbit and the Seasons

• Around Full Moon

  Lunar Eclipse
• Total or partial obscuration


  Umbra and Penumbra
• Dark part of the shadow


  Umbra
• Lighter part of the shadow

  Penumbra
• Total obscuration of the Sun by the Moon, possible because the

  angular size of Moon equals Sun’s

  Total Solar Eclipse
• Eclipses occur on Line of Nodes:

  Earth-Moon-Sun must be in line
• When the tip of moon’s umbra doesn’t quite reach the earth

  Annular Solar Eclipse
• (Noon-to-noon = 24 hours) is rotation period with respect to the Sun


  Solar day
• 4 minutes longer than the true rotation period of the Earth with respect to the stars called the

  Sidereal day
• Rotation of the earth

  1 solar day
• Revolution around the sun

  365.2422 days
• Orbital period of moon

  1 month = 29.5306 days
• Roman Julian Calendar year 


  365 days + leap year
• Gregorian reform


  Century years not divisible by 400 are ordinary years, not leap years
• 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