Meanwhile, back on Earth

By Fred Watson 24 June 2009
Reading Time: 2 Minutes Print this page
Astronomy’s lessons about the home planet.

Most people think of astronomy as the science of the night sky. When you go back to astronomy’s roots, however, what’s striking is how much the earliest skywatchers learned about the Earth.

Here’s an example to try at home: a way you can easily observe the Earth’s rotation. Extend the Southern Cross’s main axis four times towards the south and you’ll locate a point in the sky we call the south celestial pole. Our planet turns around this point once every 23 hours 56 minutes 4 seconds and, if you watch the stars for long enough, it’s quite easy to sense that apparent motion. The rotation period is slightly shorter than a solar day (which is the interval between two successive noons) so the sky looks slightly different at the same time on consecutive nights.

Modern astronomy also encompasses far more than stargazing. Information is gleaned from every nook and cranny of the universe by all means at our disposal, and the picture created of things ‘out there’ is remarkably cogent and complete. It encompasses the entire spread of space and time, and it’s been building for millennia – a gradual evolution of an ever-more-complete portrait.

Until about the sixth century BC, the ancient Greeks were completely taken in by the flat-Earth illusion. Then, a certain Anaximander came up with the surprising idea that the Earth is roughly cylindrical in form, boundless towards the east and west, but curved north and south. He based this picture on the simple observation that as one travels to the north or south, the height of the constellations above the horizon changes.

Early in the fifth century BC, Parmenides seems to have refined the picture to that of a spherical Earth. Then, towards the early 200s BC, the observant Eratosthenes spotted sunlight pouring directly down a vertical well at Syene (now Aswan), on the Nile, when he knew with certainty that it was casting oblique shadows back home, 850 km north in Alexandria. That enabled him to estimate the diameter of the Earth. His value was amazingly close to the 12,756 km found by today’s measurements.

One earthly phenomenon that would certainly have baffled some ancient peoples is the aurora. The spectacular northern and southern lights tend to be seen at latitudes well away from the Equator, but southern Australia is occasionally treated to an appearance. Often, it is just a faint red glow on the southern horizon, but the characteristic arcs and streamers of a great auroral display can sometimes be seen.

An aurora is caused by energetic sub-atomic particles from the Sun interacting with the Earth’s magnetic field. They are more common at times of high solar activity, when ‘space weather’ is at its worst. And one thing ancient people certainly didn’t have to put up with is the disruption to power and communications systems that such weather can cause…

Source: Australian Geographic Jul- Sep 2007