Event driven astronomy
[11] The so called “Diamond Ring” effect caused by sunlight shining through depressions in
the edge of the Moon. There are two diamond rings during a total eclipse – one just before
totality begins and one that occurs just as it ends.
have been a rather lengthy wait for the partial eclipse to become almost
total, everything suddenly starts to happen at once!
As the initial partial phase progresses to totality, the remaining bit of Sun
takes the form of a thin crescent. Sunlight passing through small apertures
such as a pinhole in a piece of card, project the shape of this crescent onto
the ground. Even dappled sunlight passing through the leaves of a tree
shows this effect. As the crescent gets ever thinner, shadows take on a
strange appearance, looking sharp in one direction but distinctly fuzzy at right
angles to that direction. This is because the Sun’s thin crescent acts like a slit
of intense light, producing sharp shadows in the direction of the slit’s shortest
dimension. Along the length of the slit, it’s as if you have lots of point sources
of light all lined up. Consequently, the shadows that are cast can look fuzzy in
this direction.
The effect gets more pronounced as the crescent starts to get very thin
and it’s at this time that other physical effects make themselves known. The
light from the Sun takes on a distinctly twilight feel, dimming ever quicker as
totality approaches. The temperature also drops noticeably at this time and
animals may react strangely at this point too.
As the Moon makes its final move across the solar disc, the demise of
the thin crescent quickens, its horns accelerating towards one another until
there is just a hint of sunlight left visible at the edge of the Moon. On the
ground the strange phenomenon known as "shadow bands" may be seen.
Caused by the thin strip of sunlight being affected by the Earth’s moving
atmosphere, the bands appear like undulating thin waves of light and dark
that move and shimmer in parallel. These are best seen against a plain
background such as a white bed sheet.
As this time, the remaining sunlight just visible at the edge of the Moon
pours through irregularities on the Moon’s edge caused by mountains, crater
rims and valleys seen in profile. The brilliance of the Sun looks intense as
it appears through the depressed regions, producing bright spots of light
known as Bailey’s Beads.
The appearance of the beads is coupled with a marked darkening of
the sky around the Sun’s disc, a time when the inner part of the Sun’s
atmosphere, the chromosphere and solar corona, start to become visible.
At this point in the proceedings the edge of the Moon is surrounded by a
glowing ring of light and with the Bailey’s Beads resembling a brilliant jewel,can choose whether it’s an annular or total solar eclipse you want to watch.
For reasons which should appear obvious after reading the description of a
total solar eclipse below, most people opt for the total solar eclipse.
A Total Eclipse of the Sun
Assuming you have done your homework and positioned yourself in the
right place at the right time and the weather is being kind, it is possible to
witness the majesty of a total eclipse of the Sun. The eclipse starts as the
Earth enters the penumbral shadow of the Moon. At this point, the Sun
can only be viewed with a certified protective solar filter. Using a filtered
telescope, the first evidence that something will happen is a tiny "dent" at
the edge of the Sun, the tell tale evidence that the hitherto invisible Moon is
indeed in the predicted position. This event is called first contact.
Over the tens of minutes that follow, the Moon’s disc gradually encroaches
over the Sun, leading to an ever increasing partial solar eclipse. Amazingly,
it’s not until 90 percent or more of the Sun’s disc is covered, that there’s any
noticeable change to the surrounding light level. Then after what seems to
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