Great Star of the South

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

  • Full Moon Fever
  • Astrophotography 101
  • Sky Calendar & Moon Phases
  • 10 Steps to Rewarding Stargazing
  • Understanding the Ecliptic and the Zodiac
  • False Dawn: All about the Zodiacal Light
  • Reading Weather in the Sun, Moon and Stars
  • How and Why the Night Sky Changes with the Seasons
  • Night Sky Main Page: More Skywatching News & Features

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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.

DEFINITIONS

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.