| By
Pierre Chastenay, astronomer
Montreal
Planetarium
The Antarctic sky is quite different
from the one we know in the Northern Hemisphere.
The Big Dipper and the North Star are not visible from
there. On the other hand, from that far-off continent
one can see some constellations unfamiliar to us here
in the north: Crux the Southern
Cross, Tucana the Toucan,
Musca the Fly and others.
Unfortunately for them, the inhabitants of the Southern
Hemisphere don't have a "south star," like
our own North Star, to mark the position of the celestial
pole. In fact, it is purely by chance that the North
Star (its real name is alpha Ursa Minoris) is
located along the axis of the Earth's rotation. No star
happens to be in the corresponding position in the Southern
Hemisphere.
Why are the constellations "down under" so
different from ours? Because the Earth is round, and
the Antarctic is almost directly on the other side of
the Earth from us. If the globe were perfectly transparent,
like a glass marble, you could see the constellations
of the Southern Hemisphere just by looking down between
your feet. But our planet is rather opaque, in reality,
and we cannot see them. We would have to follow Bernard
all the way to the Antarctic to admire the southern
skies.
But Bernard will see only one
star during his Antarctic expedition: the Sun. That's
right, the Sun is a star, and in November, it
is the only one shining over the Antarctic. Have you
ever heard of the midnight sun?
In the Northern Hemisphere, above the Arctic Polar Circle,
the Sun shines constantly in the sky for six months,
from the equinox in March to the equinox in September.
During this time, the Sun never goes down! For the rest
of the year, though, the Sun remains below the horizon,
and it is night for six months. Needless to say, the
people who live in these far northern regions find the
winter very long!
Exactly the same phenomenon occurs in the Antarctic,
except that the seasons in the Southern Hemisphere are
the opposite of ours. When summer begins here, it is
winter there, and vice versa. So Bernard will be embarking
on his expedition in the midst of the southern summer,
when the Sun never sets over Antarctica. He'll enjoy
daylight around the clock. Indeed, it's much better
that way-had Bernard tried to make this climb in July,
in the middle of the southern winter, he would have
had to lug along a lot of batteries for his flashlight!
If Bernard had decided to set off on this adventure
in the middle of the southern winter, though, what would
he have seen in the sky? Here is a map of the sky as
it appears in the Antarctic
in July, in the depths of the polar night. It shows
the sky as you would see it if you were lying on your
back exactly at the South Pole and looking toward the
zenith (the highest point above the horizon). Do you
recognize any of the constellations? On the left, Scorpius
the Scorpion seems to
be standing straight up on its tail. On the right, you
can see the lower half of Orion.
And just above the centre of the image, the Southern
Cross is leaning slightly to
the left.
You can also see a shimmering grey band with ragged
edges stretching across from one horizon to the other.
This is the Milky Way,
our galaxy, which we are seeing from our position inside
the disc of our island of stars. Everyone agrees that
the brightest and most colourful parts of the Milky
Way are visible only from the Southern Hemisphere. The
centre of the Milky Way is more or less in the constellation
Sagittarius the Archer.
The two small grey patches near the middle of the image
are the Small and Large Magellanic
Clouds, two dwarf galaxies orbiting our Milky
Way. The Large Magellanic Cloud is 163,000 light years
from Earth; the Small Cloud is a bit farther away, nearly
200,000 light years. They owe their names to one of
Bernard's forerunners, the famous navigator Ferdinand
Magellan, who was the first to circle the globe, in
1520.
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