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How Far South Can You See?


Last updated 3/16/2023 at 10:38am

We modern stargazers can use Aristotle's technique to demonstrate easily the Earth's curvature by simply looking skyward while traveling to different latitudes.

Believe it or not, there remain quite a few people who still think 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. Perhaps they think that these are all hoaxes perpetuated by "the guvment." I wish I had an answer.

In any case, the idea of a spherical Earth is not at all new. The concept goes back to the sixth century B.C., when the Greeks began discussing the topic; and then, around 330 B.C., the wise philosopher Aristotle offered some observational evidence to support this round Earth idea.

The matter was essentially resolved about a century later when 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 not only that the Earth was spherical but calculated its circumference to within only a few percent of what we know today – quite a remarkable achievement for the time!

We modern stargazers can use Aristotle's technique to demonstrate easily the Earth's curvature by simply looking skyward while traveling to different latitudes. In fact, a perfect group of stars to help us stands upright in the southern sky around 9 p.m. local time this week. 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. What's nice about Orion is that it lies directly over the Earth's equator and can be seen from everywhere on the planet.

This means that, if we were to stand on the equator and look skyward, Orion would pass directly overhead; from the North Pole, these same stars would appear split by our southern horizon. And from viewpoints in between, Orion would appear at different heights above our southern horizon.

Everyone in North America can see Orion during evening hours right now, and can trace its belt stars eastward toward the star Sirius – the brightest in all the nighttime sky. But another bright star lies south of Orion, and only those who live in, or travel to, more southerly latitudes can see it.

Named Canopus, this bright star is easily 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 would need a perfectly clear view toward the south to get a glimpse of Canopus 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 the nighttime's second brightest star to an already sparkling February sky.

This demonstration is possible only because our Earth is not flat; if it were, 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 in our sky, and brings to our eyes a whole new set of stars to enjoy.

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