Eclipses

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					               Lunar Phase Review

Lunar phases
repeat
through the
Lunar cycle-
                            6am
certain
                     12am     12pm
phases are
visible only                6pm
at certain
times of the
day (or
night).
                                     1
Lunar Phase Review




                     2
Lunar Phase Review




                     3
                         Eclipses
                                       • What is an eclipse?
                                          – Any time one
                                            astronomical object
                                            casts a shadow on
                                            another we say that an
                                            eclipse is occurring.
                                       • The Earth can cast a
                                         shadow on the Moon
                                       • The Moon can cast a
                                         shadow on the Earth.
Jupiter’s Moon Io is causing a solar
eclipse to occur on part of Jupiter
                                                             4
             Angular vs. Actual Size or
          Which is Bigger Your Thumb or the Moon?
                                        • In astronomy we often talk
                                          about the angular size of an
                                          object.
                                        • Angular sizes are measured in
                                          degrees or fractions of a
                                          degree. (1 arcminute = 1/60 of
                                          a degree, 1 arcsecond = 1/60 of
                                          an arcminute)
Your thumb, held at arm’s length,       • For eclipses it is important to
appears to be about 2 degrees wide        note that the angular size of the
or 4 times the width of the Moon. (Try    Moon and the angular size of
this yourself).                           the Sun are the same, both are
The Moon is much larger than your thumb   about 1/2 a degree. This does
but it is also much farther away!         not mean the actual sizes are
                                                                     5
                                          the same!
What do you need for an Eclipse?
                   • You need the Sun,
                     Earth and Moon (in
                     some order) to line
                     up.
                   • During a New Moon
                     the Sun, Moon and
                     Earth are lined up.
                   • During a Full Moon
                     they are lined up too.
                   • But are they really
                     lined up?
                                      6
             The Moon is Far Away
                                      • Compared to their sizes,
                                        the Earth and Moon are
                                        far apart.
                                      • In fact it would take 30
Earth                          Moon     Earths to span the
                                        distance between the
                                        Earth and the Moon!
   The Earth-Moon system to scale
                                      • And the Moon is only a
                                        third the size of Earth!
                                      • Since the Moon is far and
                                        small it’s easy for its
                                        shadow to miss the Earth
                                        if the two aren’t closely
                                        aligned with the Sun.
                                                           7
        The Moon’s Orbital Plane is Tipped
                                      • Compared to their sizes,
                                        the Earth and Moon are
                                        far apart.
                                      • In fact it would take 30
Earth                          Moon     Earths to span the
                                        distance between the
                                        Earth and the Moon!
   The Earth-Moon system to scale
                                      • And the Moon is only a
                                        third the size of Earth!
                                      • Since the Moon is far and
                                        small it’s easy for its
                                        shadow to miss the Earth
                                        if the two aren’t closely
                                        aligned with the Sun.
                                                           8
 Two Planes Crossing
              • The orbital plane of the
Node            Earth around the Sun (the
                Ecliptic) and the orbital
                plane of the Moon around
                the Earth are tilted by 5
                degrees.
              • The two planes cross at
                two points called nodes.
              • Eclipses can only occur
                when the Moon is at one of
       Node     the nodes, that is when the
                Moon is on the Ecliptic!
                                     9
So What Do You Need for an Eclipse?
                            The Moon must
                              be either at
                               – Full Moon or
                               – New Moon
                               and
                               – be crossing
                                 the Ecliptic
                                 (be on one of
                                 the nodes)
                            This happens at
                              most twice a
                              year
                                       10
Solar and Lunar Eclipses
                 • When the shadow
                   of the Moon
                   strikes the Earth it
                   is called a Solar
                   Eclipse
                 • When the shadow
                   of the Earth
                   strikes the Moon it
                   is called a Lunar
                   Eclipse


                                11
Solar and Lunar Eclipses




                           12
Solar Eclipse




                13
Lunar Eclipse




                14
                    Eclipse Seasons




• Eclipse seasons are roughly 6 months apart
   – May have two eclipses (1 lunar, 1 solar) 2 weeks apart
   – Eclipses may not be total
      • Solar: total, annular, partial
      • Lunar: total, partial, penumbral

                                                              15
       Lunar Node Precession
• Lunar orbit precession
• Lunar nodes precession period is 18.6 years
• Eclipse seasons come about three weeks
  earlier in subsequent years




                                            16
Annular Eclipse
 15 Jan 2010




                  17
Annular / Partial Eclipse Shadows




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