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Solar Eclipses
It's
all about shadows
An
eclipse is the
partial or total hiding of light from a celestial body (the Sun)
as it passes through the shadow cast by another body (the Moon).
(Adapted from the
Philip's Astronomy Dictionary)
If you walk
down the street on a sunny day into the shadow of a building,
you can say that the Sun is being 'eclipsed' by the building.
The same thing happens with much larger objects, like the Moon,
but of course the shadow is much larger! In fact, the Moon's shadow
is nearly 3,500 kilometres in diameter. The time when this eclipse
happens is when the Sun, Moon and the Earth are perfectly lined
up - the time of New Moon.
However, because
the Moon is much smaller than the Sun, the shadow is actually
a cone shape (see diagram below).
An extremely
exaggerated Diagram showing the solar system alignment during
a Total Solar Eclipse with the Sun (left), Moon (middle) and
the Earth (right)
When the Moon
is close enough to us in its orbit we find the shadow-cone of
the Moon can just touch the Earth. This is the time, and place,
of a TOTAL SOLAR ECLIPSE,
or a TOTAL ECLIPSE OF THE SUN.
As you can see from the diagram above, this type of eclipse
is only seen on certain parts of the Earth - the bits at the
end of the shadow-cone.
As the Moon moves
along its orbit the shadow moves across the Earth at a rate
of up to about 2,000 miles per hour. The areas covered are called
the Path of Totality,
and only when you are standing in this path will you see a total
eclipse. Further north or south of this line you will see a
Partial Eclipse where only
a part of the Sun is covered.
Why don't we
see an eclipse every month? Well, it is because the Moon's orbit
around us is tilted about 5 degrees to Earth's orbit around
the Sun. This means the Moon's shadow usually passes above or
below the Sun at New Moon. But at least twice a year everything
lines up so that some part of the Moon's shadow falls on Earth's
surface and a partial, annular or total eclipse of the Sun is
seen from somewhere on the Earth.
Sometimes the
Moon is a little too far away from us in its orbit, and then
it cannot completely cover the Sun. This time we get an Annular
Eclipse. As you can see from the diagram
below, the darkest part of the Moon's shadow (the umbra) is
not long enough to reach the Earth, unlike during a Total Eclipse.
The view of an Annular Eclipse (left), and what's going on (right)
Total
Solar Eclipse Events
FIRST
CONTACT: The instant the Moon starts to
move in front of the Sun. Very slowly you will notice an increasing
'bite' being taken out of the Sun.
Darkening
Skies: For about half an hour you will
not really notice anything unless you know the eclipse is happening.
This is because light levels are falling very gradually and
the Sun's disc is bright enough to overcome the encroaching
Moon.
Trees:
Try and find a tree to watch the dappling of sunlight. Usually
the pinhole effect of the leaves causes circles of light on
the ground. During an eclipse these dapples are transformed
into hundreds of crescents.
Stars
and Planets: The sky just before totality
can be dark enough for the brighter stars and planets to be
visible.
Plants
and Animals: Birds begin to fly back to
their nest, and nocturnal animals may appear. Listen out for
any owls, and watch as some flowers begin to close their petals.
The temperature is also dropping, and it can be quite chilly
by the time of Totality.
SECOND
CONTACT: This is it! But just before the
Sun is covered you can see the Moon's shadow rushing towards
you from the west. During the final moments watch as the Sun
shines through all the valleys mountains of the Moon causing
an effect known as 'Bailey's Beads'. The final view as the Sun
disappears can take on the appearance of a 'Diamond Ring'.
 TOTALITY:
You may have from one second to seven and a half minutes to
enjoy this spectacle. Totality is the only time when the Sun's
outer atmosphere, the Corona, is visible. This is a pearly-white
delicate structure of streamers weaving away from the Sun. Then,
after too short a time, it's all over...
THIRD
CONTACT: The moment the Sun reappears
from behind the Moon. Due to the fact that your eyes have become
slightly dark-adapted during Totality this looks more like a
'Diamond Ring' than Second Contact. Over the next hour and twenty
minutes everything slowly returns to normal.
FOURTH
CONTACT: The final 'bite' of the Moon
disappears, and the Sun is full once again.
Viewing Solar Eclipses
SAFELY
The Sun can only
be viewed safely with the naked eye during the few brief seconds
or minutes of Totality. The partial eclipse and partial
phases of the total eclipse are never safe to watch without
taking special precautions. Even when 99% of the Sun's
surface is obscured during the partial phases of a total eclipse,
the remaining crescent is intensely bright and cannot be viewed
safely without eye protection.
Do
not attempt to observe a partial phase with the naked eye. Failure
to use appropriate filtration may result in permanent eye damage
or blindness!
Click
here for my guide to Safe Sun Viewing
The
only way to safely see an eclipse directly as it happens is
to use specially designed safety viewers that filter out all
of the harmful Ultra-Violet and Infra-Red radiation from the
Sun. Only buy viewers that have been Certified by the European
Union (CE marked) and by the British Standards Institute.
When
using Safe Eclipse Viewers please make sure you follow all instructions
printed on them carefully.
© Anton
Vamplew 2008
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