New Horizons

Matt Lowry

As an astronomy teacher, one thing I strive to do is give my students some feeling for the historical significance of how the science of astronomy has influenced their lives. Often this influence is felt in ways they don't consciously acknowledge, and at times, the future influence is yet to be felt.

Consider that in classical Greece, the study of the Earth's shape (known as "geodesy") ranged from Homer's ideas of a flat disc to those proposed by Pythagoras that the Earth was a sphere - was the horizon a flat, straight line as it appeared, or was it part of a larger curve that circled back upon itself?

It wasn't until the third century BCE that another Greek, Eratosthenes, was able to make definitive measurements proving that the Earth was indeed spherical in shape with a circumference of 252,000 stadia, or roughly 39,690 km. One amazing result of Eratosthenes' measurement, which can be examined at, is that it was amazingly accurate for having used such crude methods.

In short, Eratosthenes determined the Earth's circumference based upon his astronomical knowledge of the summer solstice. He knew that at noon local-time on the summer solstice in the town of Syene, the sun would appear at the zenith, directly overhead. He could confirm this by noting accounts of how at this specific date & time, a stick oriented vertically in the sand would cast no shadow. He also knew that in his hometown of Alexandria, a similarly oriented stick would cast a shadow angled at about 7.2 from the vertical. From this he inferred that the angle of elevation of the Sun would be 7.2 south of the zenith at the same time in Alexandria.

From this information, he concluded two things:

1. The Earth had to be spherical in shape. Otherwise, how is it possible that two sticks at two different locations could possibly cast shadows of differing lengths at the same instant in time?

2. Having concluded the spherical shape of the Earth, he determined that the distance between Alexandria and Syene had to be about 7.2/360 of the Earth's circumference (recall there are 360 around a sphere).

Once Eratosthenes had this information in hand, and since he knew the distance from Syene to Alexandria (about 5000 stadia or 800 km), he could perform a simple calculation to determine the distance around the Earth. Impressively, his calculations were only 0.79% off from today's accepted values.

The next time you take a plane flight at high altitude or watch a video from the International Space Station, consider this: when you are observing the slight curvature of the Earth along the horizon, the whole idea of a spherical Earth might seem a mere trivial fact. However, to one confined to the surface it isn't obvious, so it is understandable that the ignorant would conclude a flat Earth. It wasn't until Eratosthenes' proof that the fact of a spherical Earth was established, and that fact radically altered how our ancestors viewed the universe. It astonishes me to know how such a simple experiment, involving nothing more than sunlight, sticks in the sand, and shadows, could yield such a reality-altering fact.

And the facts kept coming: Copernicus, Galileo, Newton, Einstein, Hubble, and others have all contributed to expand our horizons of knowledge in ways that we could never have anticipated prior to their discoveries. Just as soon as we think we know enough about the universe, and humanity's place within it, we are often surprised (and shocked) by some new scientific discovery that expands our horizons ever farther into unforeseen territory.

Where will the new horizon lead us next?

Ad Astra - Matt Lowry

Published in the September 2006 issue of the NightTimes