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Solar eclipses tend to be pretty big news.
The 2017 one in North America was hugely memorable
for millions of people, including the SciShow team.
And the 2019 one in South America was equally
amazing and Instagram-worthy.
But during those events, the sky isn’t the
only thing people were looking at.
They were also fascinated by these crescent-shaped
lights that appeared on the ground,
most noticeably underneath trees.
The lights are one of the most fun parts of
any solar eclipse,
and the science behind them is pretty good, too.
The biggest thing causing these lights is
called the pinhole camera effect.
Like the name says, it was originally used
to describe pinhole cameras,
which work by letting light through a tiny hole in a screen.
But the same physics applies when light from an eclipse passes through the spaces between tree leaves.
On the most basic level, light comes from
the Sun, passes through those gaps,
and then hits a projection surface, usually, the ground.
But the important part is that not every ray
of light is able to pass through.
The only rays that reach to the ground are
the ones angled in just the right way
to make it through the spaces between the leaves.
And much of that light takes a very specific
path.
It starts from one side the Sun, travels through
the gap in the leaves,
and hits the other side of the projection surface.
So light that comes from the left side of the Sun ends up on the right side of the patch of ground, and vice versa.
This creates an image of the Sun that’s
both upside-down and backwards.
And it’s why those little blobs of light look so much like the solar eclipse happening way up in the sky:
They’re tiny images of
the eclipse itself.
Technically, this means you can observe this
phenomenon
regardless of whether an eclipse is happening or not.
It’s just that the Sun, without the Moon
in front of it, looks like a circle,
so its projected image is just a circle.
Pretty nondescript.
It’s only once the Moon starts getting in
the way that things become all interesting and pretty.
The cool thing about these images isn’t
just their shape, though,
it’s also the fact that they’re really crisp and clear.
Like, look at these lights compared to the
blobs you normally see underneath trees.
The normal lights are fuzzy, while the eclipse
ones are sharp little moon-shaped things.
That happens for at least two additional reasons.
One is that there’s less ambient light during
an eclipse.
Normally, sunlight gets refracted and bounces
around in our upper atmosphere.
That gives the sky its nice blue glow, but
that glow is another source of illumination,
which makes the lights and shadows we see
pretty fuzzy.
As a solar eclipse approaches totality and
the Moon moves completely in front of the Sun,
that additional glow is dramatically
reduced.
That means you don’t get as much interference,
so you get much cleaner, photo-worthy images.
The other reason these things get so sharp
close to totality
is because the visible portion of the Sun becomes narrower.
Normally, the sharpness of a shadow depends
on how far away you are from a light source:
If you’re really close to it, the shadow
will be sharper, and if you’re farther away,
it will be more fuzzy.
But changing the size of the light source
can have the same effect.
When you decrease the size of your light source
relative to the thing that’s casting a shadow, like,
by covering up part of the Sun, the
shadows change.
The ratio of things that are totally in shadow
versus only partly in shadow increases,
and that gives those eclipse lights on the ground
sharper edges.
So the next time you’re experiencing a solar
eclipse, take some time to look down.
There’s a lot that those funny-looking lights
can teach you.
And hey, if you’re not willing to wait until
the next eclipse, you’re not out of luck.
At night, LED streetlights shining through tree leaves can project some pretty cool pinhole images, too.
So ultimately, solar eclipses are cool in
a lot of ways, but in this case,
they help highlight some of the amazing phenomena that surround us all the time!
I hope you get the chance to check out these amazing shadows the next time you experience a solar eclipse,
especially since looking
at shadows is a great way to see what’s happening
without looking at the sun.
Because, in case you’ve forgotten, that
is a really bad idea,
and we have whole video that explains why which you can watch next.
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