Solar Eclipses

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Solar Eclipses Through Space and Time
Cycles in the Sky Lou Mayo, NASA/GSFC
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Chinese astrologers wrote of an eclipse occurring
over 4000 years ago. Historians and astronomers
believe that this was an eclipse that happened on
22 October 2134 B.C. Two astrologers at the
time, Hsi and Ho, had apparently failed to
predict this eclipse, and so were beheaded.
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"Nothing can be sworn impossible since Zeus made
night during mid-day, hiding the light of the
shining Sun." - Archilochus 648 BC
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Ancient Meaning

• Solar eclipse have been generally explained in
  one of four ways
• A celestial being, usually a monster, attempts to
  destroy the Sun
• The Sun fights with its lover the Moon
• The Sun and the Moon make love and discreetly
  hide themselves in darkness
• The Sun grows angry, sad, sick, or neglectful
• Littmann and Willcox, Totality

Norse mythology the wolflike giant Sköll follows
the Sun hoping to devour it. Ancient Egypt the
evil god Set was thought to have leapt into the
eye of the Sun god, Horus. Ancient China A
heavenly dog ate the Sun. Chippewa Indians shot
flaming arrows at the Sun hoping to rekindle the
flames.
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BASIC ECLIPSE GEOMETRY
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Perigee
Apogee
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When do we get an eclipse?

• Whenever the Sun is within 18.5 of a node.
• The Sun travels along the ecliptic at about 1
  per day
• It takes about 37 days to cross through the
  eclipse zone centered on each node. 
• A New Moon occurs every 29.5 days and therefore
  at least one solar eclipse must occur during each
  of the Sun's node crossings.

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Saros Cycle

• Saros Greek meaning Repetition
• 1 Saros 18 years, 11 1/3 days
• Line of nodes drifts westward at 19 deg / year
• Eclipses repeat because the moon and the nodes
  return to the same place wrt the sun
• The 1/3 day means you must go through 3 Saros to
  have an eclipse at the same location on the Earth
  (54 years, 1 month)

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Fun Eclipse Facts

• The moons shadow moves at 1700 km/hour (1,048
  mi/hr) .
• Maximum totality is 7 ½ minutes.
• Every place on Earth will see a total solar
  eclipse about every 400 years.
• Solar Eclipses occur more frequently than lunar
  eclipses ( by 53).
• There must be at least two solar eclipses every
  year.
• There can be two solar eclipses in back to back
  months with a total lunar eclipse in between.
• This triple eclipse can occur twice during an
  eclipse year (1935, 2160).
• Seven eclipses is the maximum - 4 solar, 3 lunar
  (1982, 2485).

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Will we always have total solar eclipses?

• D(sun) 870,000 mi (1.4M km)
• (32.7 to 31.6)
• D(moon) 2,160 mi (3,476 km)
• (33.5 to 29.4)
• The moon is receding from the Earth by 3.8 cm /
  year.
• When it has drifted another 12,552 mi (20,200
  km), it will always be smaller than the sun (1/2
  billion years)
• Earths day lengthens by 0.0016s / century

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ECLIPSE SCIENCE
August 16, 1868 Helium is discovered in solar
corona. May 29, 1919 General relativity is
verified Total solar eclipses provide
opportunity to study composition of corona.
Accurate timings allow calculation of solar
dimensions. Studies of ancient records reveal
0.001s slowing of Earths rotation
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1919 Solar Eclipse Proving General Relativity
Oh leave the Wise our measures to collateOne
thing at least is certain, LIGHT has WEIGHTOne
thing is certain, and the rest debate
--Light-rays, when near the Sun, DO NOT GO
STRAIGHT.- Arthur S. Eddington (1920)
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A Perfect Day
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Solar Eclipse August 11, 1999 Soissons, France
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March 29, 2006
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1882 Transit (USNO)
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Application to Exo Planet Studies
HD 209458 (mv 7.7) in Pegasus
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Solar Eclipse Activity

• GOALS    To simulate a solar eclipse    To
  understand the concept of angular size    To
  make estimates of absolute and
• relative size
• MATERIALS    Yard or meter stick (don't
• confuse your units!)    Construction
  paper    Tape    Scissors    CD-ROM    Pencil
     Black and yellow markers

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• PROCEDURE
•    1. MAKE THE SUN Lay the CD on the
  construction paper and trace around its outer
  edge. Then trace around the center hole.
•    2. Draw two lines (a tab) down from the CD
  and fanning out so the CD circle and tab look
  like the picture on this slide. The tab will be
  used to mount the CD circle on the yard/meter
  stick.
• 3. Cut out the large CD circle and connected
  tab. This will represent the sun. The small
  circle in the center will represent the size of
  the moon (of course, this is not to scale).
•    4. Color the CD circle yellow (for the sun)
  and the small center circle black.
• 5. MAKE THE MOON Now, on a different piece of
  construction paper, trace just the center hole in
  the CD. Make the same kind of tab for this circle
  as you did for the sun circle. Make the tab a bit
  longer than the sun's tab. Color the moon black
  and cut it out.
•    6. ASSEMBLE Bend the sun and moon back 90
  degrees from their tabs at the BASE of the tab.
  Wrap the fanned out portion of the tabs around
  the yard/meter stick and tape the ends together.
  The sun should be near the end of the stick and
  the moon should be near the front. The sun and
  moon should now be able to slide up and down the
  stick.

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Sun
Moon
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• Now, holding the yard/meter stick against your
  cheek, sight down the stick. The smaller moon
  circle will cover some portion of the sun circle.
  Slide the moon back and forth to a place where it
  just covers the sun.
• Looking at the yard/meter stick, note the
  distance (in inches or cm) of the moon. Then note
  the distance of the sun. Finally, measure the
  diameter of the moon. You can now create similar
  triangles that will help you answer the following
  questions

• On the yard/meter stick, how much further away is
  the sun than the moon?
• 2. Given the diameter of the moon, can you
  predict the diameter of the sun?
• In space, our real moon has a diameter of 3,476
  km and is on average 384,400 km from Earth. The
  sun is about 149,600,000 km from the Earth. How
  many times further is the sun then the moon? What
  would you estimate to be the diameter of the sun?
• What is the angular size of the sun? moon?
• (hint construct right triangles and use
  trig)