Eclipses

1
Eclipses

• On multiple occasions throughout the year, the
  Sun, Moon, and Earth will align to create a lunar
  or solar eclipse
• With the use of precise timing and knowledge of
  the orbits of the Earth and Moon, it is possible
  to predict these events in advance
• Not all eclipses are the same, and their
  appearance can change depending on the alignment
  and the apparent size of the Moon

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Apparent Size of the Moon and Sun

• The size of an object on the sky is expressed as
  an angle
• From horizon to horizon is 180o
• The size of your outstretched fist is about 10o
• Astronomical objects are usually quite small
• Remember, 1o is made up of 60 arcminutes
• 1 arcminute is made up of 60 arcseconds
• The apparent size of the Sun is about 32
  arcminutes
• The apparent size of the Moon is about 30-32
  arcminutes
• Remember that the Moon is sometimes closer and
  sometimes farther away, so this number changes
  over the course of a month
• The fact that the Sun and the Moon are so close
  in apparent size creates the spectacular eclipses
  we see here on Earth

3
Solar Eclipse

• For a solar eclipse to occur, the Moon must block
  out the light from the Sun
• This means the Moon must be in between the Sun
  and the Earth
• Notice that this position corresponds to a New
  Moon
• Therefore, a solar eclipse must always occur
  around the time of New Moon

4
Solar Eclipse

• If it has to be a New Moon for a solar eclipse,
  why doesn't one happen every month?
• Remember that the path of the Moon through the
  sky is slightly tilted compared to the path of
  the Sun
• Only when the paths cross are conditions right
  for an eclipse

The points where the paths of the Moon and the
Sun cross are called nodes. The moon passes
through these nodes at least twice per year.
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Types of Solar Eclipse - Total

• When the Moon is closer to us (apogee), its
  apparent size matches the apparent size of the
  Sun
• If an eclipse occurs close to apogee, the disk of
  the Moon will completely block out the light from
  the Sun
• When the disk of the Moon is directly over the
  Sun, a bright halo of light is visible
• This is an outer layer of the Sun called the
  corona (more on this later in the semester)

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Types of Solar Eclipse - Annular

• If the Moon is farther away from us, its apparent
  size will be smaller
• If an eclipse occurs during this time, the disk
  of the Moon will not be able to fully cover the
  disk of the Sun
• This results in an annular eclipse
• Instead of completely blocking out the Sun's
  light, a thin ring (or annulus) of light is still
  visible

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Solar EclipseThe Right Place at the Right Time

• Everyone on Earth does not experience an eclipse
  in the same way
• Only a very few will see the Moon's disk pass
  directly over the Sun (a total eclipse)
• Others will just see the Moon's disk graze the
  Sun (a partial eclipse)

Darkest part of the shadow Umbra Lighter part
of the shadow Penumbra
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Partial Solar Eclipse
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Lunar Eclipse

• For a lunar eclipse to occur, the Moon must pass
  through the Earth's shadow
• This means the Earth must be in between the Sun
  and the Moon
• Notice that this position corresponds to a Full
  Moon
• Therefore, a lunar eclipse must always occur
  around the time of Full Moon

11
Lunar Eclipse

• Like solar eclipses, lunar eclipses only occur if
  the Moon is passing through a node
• The gray circles at right show the Earth's shadow
• Darkest part is umbra
• Lighter part is penumbra
• Depending on the timing, the Moon will pass
  through different parts of the shadow, or
  possibly not at all

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Penumbral Lunar Eclipse

• Penumbral eclipse occurs when the Moon passes
  through the lighter part of the Earth's shadow
• These eclipses are barely noticeable and not
  particularly interesting

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Partial Lunar Eclipse

• Partial lunar eclipse occurs when the Moon passes
  through the Earth's umbra, but never goes
  completely into shadow

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Total Lunar Eclipse

• A total lunar eclipse occurs when the Moon passes
  completely through the Earth's umbra
• Notice below how even when the Moon is completely
  in Earth's shadow, it still appears to have a red
  glow
• The Earth's atmosphere can act like a lenses and
  refract light back onto the Moon's surface
• Depending on the alignment and atmospheric
  conditions, the color of the Moon can change from
  orange to red to simply black

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Upcoming Lunar Eclipses
2006 Sep 07 Partial 01h33m Europe, Africa, Asia, Aus.
2007 Mar 03 Total 03h42m01h14m Americas, Europe, Africa, Asia
2007 Aug 28 Total 03h33m01h31m e Asia, Aus., Pacific, Americas
2008 Feb 21 Total 03h26m00h51m c Pacific, Americas, Europe, Africa
2008 Aug 16 Partial 03h09m S. America, Europe, Africa, Asia, Aus.
2009 Feb 09 Penumbral - e Europe, Asia, Aus., Pacific, w N.A.
2009 Jul 07 Penumbral - Aus., Pacific, Americas
2009 Aug 06 Penumbral - Americas, Europe, Africa, w Asia
2009 Dec 31 Partial 01h02m Europe, Africa, Asia, Aus.
2010 Jun 26 Partial 02h44m e Asia, Aus., Pacific, w Americas
2010 Dec 21 Total 03h29m01h13m e Asia, Aus., Pacific, Americas, Europe
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Other 'Eclipses' in the Solar System

• Occasionally, we can see planets interior to our
  orbit (Mercury and Venus) cross the face of the
  Sun
• These crossings are called transits
• Someone on Mars would have the chance to see an
  Earth transit

Mercury transits the Sun
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Planet Transits Around Other Stars

• Telescopes and cameras have become so sensitive
  that they can even detect planetary transits
  around other stars
• Many extrasolar planets have been detected by
  looking for a dip in the light received from a
  star

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Eclipses Outside the Solar System

• Most stars come in pairs and are called binary
  stars
• Sometime, the alignment of these stars to our
  line of sight lets us see one star eclipse the
  other
• You will learn a lot more about eclipsing
  binaries and their importance in ASTR1020