Phases of the Moon

							    Lecture 3
Phases of the Moon
Moon
         Phases of the Moon
• The Moon orbits the Earth ~ every month.

• The Moon appears to have different phases
  because we see more or less of the lighted
  side of the Moon depending on where it is
  with respect to the Sun.
               First Quarter
                                          Phases of the Moon



     Waxing
                               Waxing
     Gibbous
                               Crescent




Full Moon                         New Moon




    Waning                     Waning
    Gibbous                    Crescent

               Last Quarter
Phases - Photos
        Rotation of the Moon
• We only see one side of the moon
• Moon rotates at the same rate as it orbits the
  Earth

           Pspin  Porb
• When spin and orbital periods are correlated
  = synchronous orbit
Rotation of the Moon
Moon -- Part of Far Side
       Orbit of the Moon – Lunar
                  Month
• Sidereal Month = amount of time for the moon to
  come to same spot with respect to the celestial
  sphere. (27.3 days)

• Solar Month = amount of time for the moon to
  come to same position with respect to the sun
  (29.5 days)
  – One Lunar phase cycle
Siderial and Solar Month
                  Eclipses
• Umbra = darkest shadow of and object.
  – Sun completely blocked in umbra
• Penumbra = lighter shadow
  – Sun only partially blocked

                         Umbra

Sun                                   E

                        Penumbra
                 Eclipses
• Solar eclipse = when the Earth passes
  through the shadow of the moon

• Lunar eclipse = when the Moon passes
  through the shadow of the Earth
         Total Solar Eclipse
• Sun completely covered by the moon --
  seen only in the umbra

                      Umbra

 Sun                               E

                      Penumbra




             View from the Earth
         Partial Solar Eclipse
• Partial Eclipse: Part of Sun is blocked by the
  Moon -- only seen in penumbra

                       Umbra

 Sun                                  E

                       Penumbra




              View from the Earth
       Annular Solar Eclipse

• Moon too far from Earth to cover Sun
  completely -- umbra does not reach the
  Earth
                           Umbra

 Sun                                           E

                          Penumbra

              Umbra does not touch the Earth



                  View from the Earth
Solar Eclipse
Solar Eclipse -- August 11, 1999
         Total Lunar Eclipse
• Moon pases through umbra of the Earth.

                                        Umbra
                  Total Eclipse


 Sun                     E




                                    Penumbra
                  View from Earth
         Penumbral Eclipse
• Moon only passes through penumbra.
                                        Umbra
                  Total Eclipse


Sun                      E



                Penumbral Eclipse
                                    Penumbra

                 View from Earth
        Partial Lunar Eclipse
• Moon partially passes through umbra.
                         Partial Eclipse
                                               Umbra
                   Total Eclipse


Sun                       E



                 Penumbral Eclipse
                                           Penumbra


                View from Earth
Lunar Eclipse
   Why are there few eclipses?
• Orbit of moon is tilted with respect to
  Earth’s orbital plane by 5°
• Line of Nodes = position where moon and
  earth’s orbital planes intersect
             Line of Nodes
• When the moon is on the line of nodes it is
  crossing the Earth’s orbital plane
• At other times moon is slightly above or
  below the Earth’s orbital plane
• If moon crosses the line of nodes near a new
  or full phase an eclipse will occur
Orbit of the Moon