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By James Pugsley
Astronomy North
(YELLOWKNIFE, NWT) Possibly my favourite question
to ask budding skywatchers is "What is the biggest
object in the night sky that we can touch but cannot
see?"
As shoulders begin to shrug, heads will jolt back
for a final squint at the stars. Some youngsters have
even been known to make outstanding attempts at grabbing
the moon.
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That's
my cue to repeat the question. "What is
the biggest object in the night sky that we
can touch, but cannot see?"
Soon after, the energetic youngsters finally
realize that they can, in fact, see the moon,
and that no matter how high they leap, they
most likely will never touch it.
The answer, of course, is the atmosphere –
and despite being so close to home, it is an
essential piece of the astronomical puzzle.
Blue
by day, invisible by night, it is the dynamic
systems in Earth's troposphere, stratosphere,
mesosphere, thermosphere and exosphere that
determine what a skywatcher will and will not
see each evening.
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RETURN
TO AURORA FORECAST
COSMIC FILTER • A clear night of viewing
depends on the troposphere (0-12 km), stratosphere
(12-48 km), mesosphere (48-80 km – home to
noctilucent clouds and meteors), ionosphere (80-640
km – where auroras form) and the exosphere
(640-1,300 km – where satellites roam).
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That's
because almost all of the light we see at night is
distorted by the relatively thin layer of oxygen,
nitrogen and other gases in our atmosphere.
For example, when you see the faint light from a distant
star begin to twinkle, it is logical to assume that
the star itself is, well, twinkling.
Truth is, the star's energy output isn't changing
at all, rather, what you are seeing is a distortion
of starlight caused by disturbances in Earth's atmosphere.
In
the Arctic, this bending and twisting of light is
usually caused by turbulent upper troposheric air
currents (i.e. the polar jet stream). Closer
to home, changing levels in moisture and/or low altitude
winds can make a difference too.
When
all systems are steady, the night is ripe for stargazing.
This is known as good "seeing", which refers
to an observer's ability to see stars, planets, the
Milky Way and even the aurora without atmospheric
distortion getting in the way.
Heck, on those perfect nights I might just take a
few good leaps at the moon myself.
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