SOLAR ECLIPSE
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Solar Eclipses – Short Video
• http://www.youtube.com/watch?v=HrloqdXrz
N4
• This video touches on the various types of
solar eclipses.
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An Eclipse from Another World
• Mars’ Phobos eclipses the sun as seen from
Curiosity Rover.
• http://www.youtube.com/watch?v=OyZoD7B
RTtg
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Solar Eclipse
• A solar eclipse occurs when the moon moves
between Earth and the sun.
– There are total solar eclipses – the moon covers
the disk of the sun completely.
– Partial solar eclipses – less dramatic, when the
moon only covers part of the solar disk.
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A total
eclipse
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Partial Eclipse
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Angular Diameter (the moon’s and
sun’s disks are nearly the same size)
• Solar eclipses are spectacular because the
moon and sun have nearly the same angular
diameter.
– This means, simply, that the moon appears big
enough that it covers almost exactly the sun’s disk.
– This is from your frame of reference on Earth –
looking out into space at the moon and sun.
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Angular Diameter
• Angular diameter of an object is the angle
formed by lines extending toward you from
the opposite edges of the object and meeting
at your eye.
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Angular Diameter
Calculating angular
diameter
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Angular Diameter
• The formula for angular diameter is:
• Angular diameter/206.265 = linear diameter/distance
• 206.265 is the number of arc seconds in a
radian. This allows us to, when we divide, to
convert the angle from arc seconds to radians.
– A radian is a unit of angular measure.
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Angular Diameter
• Angular diameter/206.265 = 3476 km/384,000
km.
– This gives us an angular diameter 1,870 arc
seconds. When we divide 1,870/60 we get 31 arc
minutes. An when we divide that by 60, 31/60,
we get degrees.
– The angular diameter of the moon is
approximately .5 degrees.
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Angular Diameter
• Using this same equation, you can find the
angular diameter of the sun.
– AD/206.265 = 1.39 X 106km/1.5 X 108km
– AD = 1,900 arc seconds
– This, after dividing by 3600, gives as
approximately 0.5 degrees.
– In other words, the sun is actually a touch bigger
but not by much
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Angular Diameter
• As luck would have it,
our moon is the right
size, more often than
not, to almost exactly
cover up the sun –
leaving the sun’s
atmosphere, its
corona, visible.
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Moon’s Distance-to-Earth Effect
• At apogee, the moon is at its farthest point
form Earth and its angular diameter is 6% less
than average.
At apogee, the moon’s disk
may not cover the sun’s.
This produces an annular
eclipse – an eclipse in
which a ring (an annulus)
is visible around the
moon’s disk.
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The Moon’s Shadow
• To see a solar eclipse you must be in the
moon’s shadow.
• Like in a lunar eclipse, there is a penumbra
and an umbra.
The moon’s umbral shadow produces a
spot of darkness about 170 miles in
diameter on the Earth’s surface.
The picture at right is of the umbral
shadow over Alaska.
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The Moon’s Shadow
• Due to the orbital motion of the moon and
the rotation of Earth, the moon’s shadow
moves across Earth at high speeds (1,600
mph).
– This called its “path of totality.”
– People in the path of totality see a total eclipse.
– Outside the umbral shadow, there is a penumbral
shadow, which is a partial eclipse.
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Totality is Short
• It lasts no longer than 7.5 minutes, but is
usually only 2 to 3 minutes long.
– When totality ends, the corona and chemosphere
vanish.
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Features of a Solar Eclipse
• When there is a total eclipse, it is dark
causing: birds to go to roost (bed) and
automatic streetlights to come on.
• The sun’s photosphere (the bright surface of
the sun) is completely covered in a total
eclipse.
– This allows you to directly view the corona, the
sun’s outer atmosphere.
– At the point of totality, seeing only the corona, it is
safe to look directly at an eclipse.
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Features of a Solar Eclipse
• The corona is made up of hot, low-density gas,
which has a wispy appearance due to the solar
magnetic field.
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Studying the Corona
• At a total solar eclipse observed from the
ground, we can see part of the corona that is
not accessible in any other way.
– Though we do now have space crafts that are able
to send us pictures of the corona, eclipses were,
pre-space age, the only opportunities to observe
the corona.
– It was especially important for measuring the
temperature of the solar corona.
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Studying the Corona
• The corona has a temperature of one to three
million degrees Kelvin (water boils at 373
degrees Kelvin).
– This is significantly hotter than the sun’s surface
which is only 5,800 degrees Kelvin.
• We also use total solar eclipses to study how
sun’s magnetic field shapes the corona.
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Why is the sun’s magnetic field
important
• Much solar activity appears to be directly
connected with the properties of the sun’s
magnetic field.
• The sun has a magnetic field cycle that
changes in polarity every 22 years, which is
exactly twice the length of time of the sunspot
cycle.
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The sun’s magnetic field and sunspots
• They are caused by intense magnetic activity,
which inhibits convection currents, forming
areas of reduced surface temperature. They
usually appear as pairs, with each sunspot
having the opposite magnetic pole to the
other.
• Most solar flares and coronal mass
ejections originate in magnetically active
regions around visible sunspot groupings.
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Why is the sun’s magnetic field
important
• The number of sunspots correlates with the
intensity of solar radiation.
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Why is the sun’s magnetic field
important
• The increase in the sun’s ejections can be seen
on Earth as the Aurora Borealis and the Aurora
Australis.
– Video: Aurora Borealis:
https://www.youtube.com/watch?v=FcfWsj9OnsI
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Why is the sun’s magnetic field
important
• “The sudden increase [from the sun’s ejections] in
power can damage sensitive electronic equipment.
Power transformers can overload, causing long-lasting
blackouts. Long metal structures like oil and gas
pipelines can carry currents, which can enhance their
corrosion over time and lead to devastating effects if
proper safety measures are not in place. The resulting
variations in the ionosophere can disrupt GPS signals,
giving inaccurate readings.”
• https://www.youtube.com/watch?v=GSVv40M2aks
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Sun’s Magnetic Field Photographed
During an Eclipse
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A Misconception
• Sunlight is not more dangerous during a solar
eclipse.
– It is always dangerous to look at the sun.
– Eclipses are more dangerous only because it is
when people are tempted to ignore common
sense and look at the sun directly burning their
eyes.
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The First Evidence for General
Relativity
• In May 1919, a team led by the British
astronomer Arthur Stanley Eddington claimed
to have confirmed Einstein's prediction
of gravitational lensing (bending) of
starlight by the Sun while photographing the
solar eclipse with dual expeditions in Sobral,
northern Brazil, and Príncipe, a west African
island.
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Next Total Solar Eclipse in US
• From what I was able to find, August 21, 2017
will be the date for the next solar eclipse
visible near us.
• Takes place at noon CDT.
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Questions
• Why do solar eclipses happen only at new
moon?
– Why not every moon?
• Why isn’t the corona visible during partial or
annular solar eclipses?
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