At around 8:30 in the evening
this week, anyone who looks skyward will see mighty Orion dominating the southern
sky, having just crossed the meridian on his nightly journey westward.
Also in this part of the
sky are the two brightest-appearing stars in the night sky, Sirius and Canopus,
themselves soon to cross the meridian. We highlighted Sirius
- the brightest of all the stars - just a couple of weeks ago. This dazzling
blue-white diamond has the distinction of being visible from every well-populated
region on Earth.
In contrast, the star second
only to Sirius in brilliance, Canopus (magnitude -0.62),
is only visible for those south of latitude 37.9 degrees
north - a parallel passing close to San Francisco, California; Wichita, Kansas;
and Richmond, Virginia. Wrote Robert Burnham (1931-1993) in his classic Celestial
Handbook:
"Canopus is the Great
Star of the South - a name and a legend only to many North American observers,
but a dazzling gem to our more fortunately situated neighbors to the south."
Canopus is located in the
southern constellation of Carina, the Keel of the now defunct constellation
of Argo Navis, the Great Ship of Jason and the Argonauts. Canopus lies roughly
due south of Sirius, so when the latter is at its highest, so too is Canopus.
In fact, Canopus arrives at the meridian about 20 minutes before Sirius.
When Sirius reaches its
highest point in the south, Canopus shines about 40 degrees below it; unfortunately
out of sight for Europe, Canada and the northern half of the United States.
Appearing to skim just above
the southern horizon from most of the southern United States, it usually is
immersed in thick horizon haze which is probably the main reason for the popular
misconception that it's a deep yellow or even orange in color; its true tint
is a silvery white.
So it is that latitude 37.9
degrees north marks the northernmost limit for getting just a brief view of
Canopus, presupposing that you have access to a perfectly flat southern horizon,
with absolutely no obstructions and can thus take full advantage of the "lifting
effect" of atmospheric refraction.
Interestingly, there is
a sneaky way to get a view of Canopus even farther to the north. Because the
horizon appears to "dip" at altitudes of 30,000-feet or more, airline
passengers might be able to glimpse this star peeking just above the horizon
possibly from latitudes as far north as Washington, D.C., or St. Louis, Missouri.
Passengers on board commercial
airliners incidentally, also have an advantage of flying above the thick haze
and murk of our atmosphere. I still vividly remember my very first airline flight,
at the age of 10, occurring on a mid-August night. My face was pressed against
the window looking out at a truly splendid view of a dark, starry sky, occasionally
punctuated by the bright streak of a Perseid meteor.
Even today I'll avail myself
of such opportunities to do aerial stargazing, however, I usually have to place
either a blanket or my suit jacket over my head, much like an old-time photographer's
focusing cloth to block the interior cabin lights and their window reflections.
In this post 9-11 era, I also have to make sure to explain to those seated around
me, as well as to the cabin attendants what I'm doing. After all, a person seated
on a commercial aircraft with his head covered by a blanket is no doubt apt
to arouse some strange looks!
Getting back to Canopus,
its location, roughly 15-degrees from the south pole of the ecliptic has given
it an important space-age role. Many of the space probes that have been sent
out into deep space have carried an optical Canopus sensor to stabilize the
craft in the direction perpendicular to its orbit, which is close to the ecliptic
plane. Other celestial bodies as the Sun and Earth are similarly used for stabilizing
such vehicles in other directions during their long flight.
A long-standing controversy
regarding Canopus has been its distance. Were you to check a variety of astronomy
books published over the last 30 or 40 years, you'll likely encounter a surprising
range of distances for this star, anywhere from less than 100 light years to
more than 600. So why all the discrepancy?
One major factor is that
for many years Canopus was always unavailable to the great observatories of
the Northern Hemisphere, so it really wasn't adequately observed until large
observatories began to spring up in the tropics and Southern Hemisphere. At
one time Canopus was thought to be one of the most luminous of all known stars,
with figures of up to 60,000 times that of the Sun being quoted.
More recent observations
do not support such a large estimate. But make no mistake about it; Canopus
is indeed a very large and brilliant star. Based on the latest data obtained
from the HIPPARCOS satellite, we know today that Canopus is 313 light years
away and has a computed luminosity of at least 12,000 times that of our Sun.
Basic Sky Guides
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Astronomy
for the
Entire Family!
Starry
Night software brings the universe to your desktop. Map the sky from
your location, or just sit back and let the cosmos come to you.
LEARN
MORE!
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Joe Rao
serves as an instructor and guest lecturer at New York's Hayden Planetarium.
He writes about astronomy for The New York Times and other publications, and
he is also an on-camera meteorologist for News
12 Westchester, New York.
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DEFINITIONS
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Degrees
measure apparent sizes of objects or distances in the sky, as seen from
our vantage point. The Moon is one-half degree in width. The width of
your fist held at arm's length is about 10 degrees.
1 AU, or
astronomical unit, is the distance from the Sun to Earth, or about 93
million miles.
Magnitude
is the standard by which astronomers measure the apparent brightness
of objects that appear in the sky. The lower the number, the brighter
the object. The brightest stars in the sky are categorized as zero or
first magnitude. Negative magnitudes are reserved for the most brilliant
objects: the brightest star is Sirius (-1.4); the full Moon is -12.7;
the Sun is -26.7. The faintest stars visible under dark skies are around
+6.
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