Lecture 4 and 5, ASTA01

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Lecture 4 and 5, ASTA01
Chapter 2Users Guide to the SkyPatterns and
Cycles
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On 13 Sept 2012, NASA Martian rover Curiosity
doubles up as astronomer

• Eclipse (or transit) of moon Phobos in front of
  the Sun in the sol 43 of robot (Martian day 43)

Watch also the 2010 eclipse at
http//www.youtube.com/watch?vViiriVhIhjE
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Eclipses

• Eclipses are due to a seemingly complicated
  combination of apparent motions of the Sun and
  Moon.
• However, they are actually easy to predict once
  all the cycles are understood.
• Watch a total eclipse of the sun seen from Turkey
  in 2006 on Youtube

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Eclipses

• For some, eclipses are alsoamong the
  mostspectacular of naturessights one can
  witness.

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Solar Eclipses

• From Earth, you can see a phenomenon that is not
  visible from most planets.
• Because the Sun is 400 times larger than our Moon
  and, on the average, 390 times farther away, the
  Sun and Moon have nearly equal angular apparent
  diameters.
• Thus, the Moon is just about the right size to
  cover the bright disk of the Sun and cause a
  solar eclipse.
• In a solar eclipse, it is the Sun that is being
  hidden (eclipsed) and the Moon that is in the
  way.

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Geometry of solar eclipses

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Solar Eclipses

• A shadow consists oftwo parts.
• The umbra is the regionof total shadow.
• For example, if you werein the umbra of
  theMoons shadow, you wouldsee no portion of
  the Sun.
• The umbra of the Moonsshadow usually just
  barelyreaches Earths surface andcovers a
  relatively small circular zone.

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Solar Eclipses

• Standing in that umbral zone, you would be in
  total shadow unable to see any part of the
  Suns surface.
• This is called a total eclipse.

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Solar Eclipses

• If you moved into the penumbra, you would be in
  partial shadow, but could also see part of the
  Sun peeking around the edge of the Moon.
• This is called a partial eclipse.

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Solar Eclipses

• If you are outside the penumbra, you see no
  eclipse at all.

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Solar Eclipses

• Because of the Moons orbital motion and
  Earths rotation, the Moons shadow sweeps
  rapidly across Earth in a long, narrow path of
  totality.
• If you want to see a total solar eclipse, you
  must be in the path of totality.

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Solar Eclipses

• When the umbra of the Moons shadow sweeps over
  you, you see one of the most dramatic sights in
  the sky the totally eclipsed Sun.

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Solar Eclipses

• The eclipse begins as the Moon slowly crosses in
  front of the Sun.
• It takes about an hour for the Moon to cover the
  solar disk.

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Solar Eclipses

• As the last sliver of Sun disappears, dark falls
  in a few seconds.
• Automatic street lights come on, drivers of cars
  turn on their headlights, and birds go to roost.
• The sky usually becomes so dark you can even see
  the brighter stars.

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Solar Eclipses

• The darkness lasts only a few minutes.
• This is because the umbra is never more than 270
  km in diameter on Earths surface and sweeps
  across the landscape at over 1600 km/hr.
• The period of totality lasts on average only 2 or
  3 minutes and never more than 7.5 minutes.

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Solar Eclipses

• During totality you can see subtle features of
  the Suns atmosphere.
• These include red flame-like projections that are
  visible only during those moments when the
  brilliant disk of the Sun is completely covered
  by the Moon.

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Solar eclipse
2012
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Solar eclipse
2012
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Solar eclipse the active sun
1980
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Solar eclipse - the active sun
The white streamers are the upper layers of the
solar atmosphere called Corona (that is,
crown). It is a nonuniform, outflowing part of
the sun, gradually becoming the rarified solar
wind that flows past the Earth and
planets (mainly, ionized hydrogen and helium, and
the electrons) Streamers emanate from
regions called coronal holes, where the
magnetic field lines leave the sun flow out
into space
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Solar eclipse - the active sun

• How does the sun get its corona?
• Hot temperature causing the
• outflow 1 mln degrees C,
• Maybe due to shock waves
• (ii) Magnetic fields, the so-called
• Reconnection of loops.
• Loops are regions where gas
• flows along magnetic field tubes.

Ultraviolet picture of the coronal loop
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The sun is an active star
Solar flare
Ultraviolet picture of the coronal loop
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The sun is an active star (here, in extreme UV)
Solar Dynamics Observatory satellite
observatory
http//www.youtube.com/watch?vQrmUUcr4HXg
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Coming back to.. Solar Eclipses

• As soon as part of the Suns disk reappears, the
  fainter features vanish in the glare.
• The period of totality is over.
• The Moon moves on in its orbit and, in an hour
  the Sun, is completely visible again.

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Solar Eclipses

• Sometimes, when the Moon crosses in front of the
  Sun, it is too small to fully cover the Sun.
• Then, you would witness an annular eclipse.
• This is a solar eclipse in which an annulus
  (ring) of the Suns disk is visible around the
  disk of the Moon.

The eclipse never becomes total. It never
quite gets totally dark.
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Solar Eclipses

• Annular eclipses occur because the Moon follows a
  slightly elliptical orbit around Earth.
• If the Moon is in the farther part of its orbit
  during totality, its apparent diameter will be
  less than the apparent diameter of the Sun, and
  you see an annular eclipse.
• Also, Earths orbit is slightly elliptical.
• As a result, both the Earth-to-Sun distance and
  the apparent diameter of the solar disk vary
  slightly.
• These contribute to the effect of the Moon's
  varying apparent size.

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Solar Eclipses

• Solar eclipses can be misleading tempting you
  to look at the Sun in spite of its brilliance and
  thus risking your eyesight.

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Solar Eclipses

• During the few minutes of totality, the brilliant
  disk of the Sun is hidden, and it is safe to look
  at the eclipse.
• However, the partial eclipse phases and annular
  eclipses can be dangerous, and people dont
  necessarily know which type to expect.

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Solar Eclipses

• If you plan to observe a solar eclipse, remember
  that the Sun is bright enough to burn your eyes
  and cause permanent damage if you look at it
  directly.
• This is true whether there is an eclipse or not.

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Solar Eclipses

• The figure demonstrates a safe way to observe the
  partially eclipsed Sun.
• Or look through very much darkened glass like
  welders hood
• Dont look through sunglasses

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Solar Eclipses

• upcoming solar eclipses

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Lunar Eclipses

• Occasionally, you can see the Moon darken and
  turn copper-red in a lunar eclipse.
• Time lapse
• video

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Lunar Eclipses

• A lunar eclipse occurs at full Moon when the Moon
  moves through Earths shadow.
• As the Moon shines only by reflected sunlight,
  you see the Moon gradually darken as it enters
  the shadow.

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Lunar Eclipses

• If you were on the Moon and in the umbra of
  Earths shadow, you would see no portion of the
  Sun.

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Lunar Eclipses

• If you moved into the penumbra, you would be in
  partial shadow and would see part of the Sun
  peeking around the edge of Earth so the
  sunlight would be dimmed but not extinguished.

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Lunar Eclipses

• In a lunar eclipse, it is the Moon that is being
  hidden in the Earths shadow and Earth that is
  in the way of the sunlight.

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Lunar Eclipses

• If the orbit of the Moon carries it through the
  umbra of Earths shadow, you see a total lunar
  eclipse.

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Lunar Eclipses

• As you watch the Moon, it first moves into the
  penumbra and dims slightly, then more

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Lunar Eclipses

• In about an hour, the Moon reaches the umbra, and
  you see the umbral shadow darken part of the
  Moon.
• It takes about an hour for the Moon to enter the
  umbra completely and become totally eclipsed.

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Lunar Eclipses

• The period of total eclipse may last as long as 1
  hour 45 minutes however, the timing of the
  eclipse depends on where the Moon crosses the
  shadow.

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Lunar Eclipses

• When the Moon is totally eclipsed, it does not
  disappear completely.
• Though it receives no direct sunlight, the Moon
  in the umbra receives some sunlight that is
  refracted through Earths atmosphere.

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Lunar Eclipses

• If you were on the Moon during totality, you
  would not see any part of the Sun.
• It would be entirely hidden behind Earth.
• You would be able to see Earths atmosphere
  illuminated from behind by the Sun.

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Lunar Eclipses

• The red glow from this ring consisting of all the
  Earths simultaneous sunsets and sunrises
  illuminates the Moon during totality and makes
  it glow coppery red.

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Lunar Eclipses

• If the Moon passes a bit too far north or south
  of Earths shadow, it may only partially enter
  the umbra.
• Then, you see a partial lunar eclipse.

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Lunar Eclipses

• The part of the Moon that remains outside the
  umbra in the penumbra receives some direct
  sunlight.
• The glare is usually great enough to prevent you
  from seeing the faint coppery glow of the
  part of the Moon in the umbra.

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Lunar Eclipses

• Lunar eclipses always occur at full moon but not
  at every full moon.
• The Moon's orbit is tipped about 5 degrees to the
  ecliptic.
• So, most full moons cross the sky north or south
  of Earths shadow and there is no lunar eclipse
  that month.
• For the same reason, solar eclipses always occur
  at new moon but not at every new moon.

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Lunar Eclipses

• Although there are usually no more than one or
  two lunar eclipses each year, it is not difficult
  to see one.
• You need only be on the dark side of Earth when
  the Moon passes through Earths shadow.
• That is, the eclipse must occur between sunset
  and sunrise at your location to be visible.

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Lunar Eclipses

• upcoming lunar eclipses

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Lunar Eclipses

• The orientation of the Moon's orbit in space
  varies slowly.
• As a result, solar and lunar eclipses repeat in a
  pattern called the Saros cycle lasting 18 years
  11 days 8 hours. Ancient people who understood
  the Saros cycle could predict eclipses without
  understanding what the Sun and Moon really were.

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Eclipses in historical fiction

• Boleslaw Prus Faraon (1897) described a
  turbulent period of Egyptian history in the 9th
  century B.C., when the priests took power from
  the pharaohs. They used the knowledge of an
  upcoming solar eclipse to pretend only they are
  favored by gods staged an uprising.