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Sky watch: A starry, roundabout view of Earth

Believe it or not, there remain some people who still believe we live on a flat Earth. How they've managed to escape exposure to scientific advances of the past two millennia I'll never know.

The idea of a spherical Earth goes back to the Greeks in 6th century BC; and then, around 330 BC, the great philosopher Aristotle offered observational evidence to support this round Earth idea.

About a century later the mathematician Eratosthenes noticed that, on the summer solstice, shadow lengths were different in Alexandria and Syene in Egypt. From the length of these shadows, he used geometry to show that the Earth was round and to calculate its circumference to within only a few percent of what we know it is today.

We modern stargazers can use Aristotle's technique to demonstrate the Earth's curvature by looking skyward while traveling to different latitudes. A perfect group of stars for this demonstration stands upright in the southern sky after dark this month. Its name is Orion.

Orion represents a great hunter, with its vertical rectangle of bright stars marking his shoulders and knees, and the three equally bright stars in a straight line forming his belt. This constellation lies directly over the Earth's equator and can be seen everywhere on the planet.

If we stood on the equator and looked up, Orion would pass directly overhead; from the North Pole, these same stars would appear on our southern horizon. And from viewpoints in between, Orion would appear at different heights above our southern horizon.

People in North America can see Orion during evening hours now and trace its belt stars east to the star Sirius - the brightest in all the nighttime sky. But another bright star lies below Orion, and only those in more southerly latitudes can see it.

Named Canopus, this bright star is visible to stargazers near and south of the equator. But, if you live farther north than about 37 degrees latitude, you can never see Canopus in your sky.

At a latitude of 37 degrees, you'd need a perfectly clear view toward the south to get a glimpse of this star as it clears the southern horizon for only a few minutes during February evenings.

Farther south of 37 degrees, Canopus appears higher in the south and adds yet another sparkling stellar jewel to an already sparkling February sky.

If our planet were flat, we would see the same stars wherever on Earth we stood. But, because our planet is spherical, traveling north and south changes the positions of familiar stars and brings a whole new set of stars into view.

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