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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?"
VIEW
FULL SIZE
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).
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.
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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, ionosphere and exosphere that determine
what a skywatcher will and will not see each
evening.
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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