Heraklides of Pontus

388-315 b.c.

Greek Philosopher and Astronomer

Heraklides of Pontus was one of the many early scientists to come from ancient Greece. An astronomer, Heraklides made many important observations during his life, and may have been one of the first to suggest that some of the other planets (although probably not Earth) revolve around the Sun. This was a major departure from the thinking of the day and, in fact, it would not be until the time of Nicolaus Copernicus (1473-1543) that this would again be suggested.

Like so many others born over two millennia in the past, virtually nothing is known of Heraklides's early life. In fact, very little is known about his life at all, with the exception of some of his cosmological observations.

Apparently, in addition to his musings on philosophy, Heraklides spent a great deal of time watching the skies and plotting the positions of the planets in the heavens. Cosmology, the study of the origins and structure of the universe, was an important field of philosophical speculation to the ancient Greeks, and Heraklides tried to make his observations fit into the day's cosmological framework—that Earth was the center of the universe and everything rotated around Earth.

Today, this cosmology, called geocentric (or Earth-centered) seems silly. However, we make such an assessment with the perspective of several centuries of scientific observation with instruments and theories of which Heraklides could not even conceive of, let alone use. The ancient Greeks could hardly be faulted for devising a cosmology based on observations with their most sophisticated scientific instrument—the naked eye. And what the eye sees is an endless cycle of stars and planets rising in the east, passing overhead, and setting in the west. No wonder so many believed for so long that the universe rotated around us, especially because, not understanding gravity, many were convinced that a spinning Earth would fling everyone off into space.

However, Heraklides seems to have been interested in the fact that the planets moved against the backdrop of the fixed stars. Consider that over 2,000 stars are clearly visible at any one time in a dark sky, and about 6,000 or so stars are clearly visible during the course of the year. All of these stars appear fixed to the sky; over a human lifetime (over several lifetimes) they remain unmoving with respect to each other. Of all these stars, only five are ever seen to move: Mercury, Venus, Mars, Jupiter, and Saturn.

The Greeks were seemingly born to speculate, and it is no surprise that they would try to find a reason why only five "stars" were allowed to move. Over the following centuries, this question plagued many of the early scientists, and ever-more elaborate theories were constructed to try to explain this phenomenon. Heraklides was the first to suggest that, instead of orbiting Earth, Venus and Mercury actually orbited the Sun. In so doing, he made an intellectual leap of astounding proportions, because it was so different from what the eye saw.

Heraklides appears to have limited his theorizing to the planets closer to the Sun than Earth, and their motions are certainly easier to plot. Both of these planets appear only in the morning or the evening, and both are usually close to the Sun in the sky. Their motions across the sky are by far more simple than those of the outer planets. Nonetheless, Heraklides's suggestion was revolutionary, so much so that it was not to be taken up again for nearly 2,000 years.

Heraklides died in about 315 b.c., and his ideas do not seem to have taken hold during his lifetime. However, they did inspire Copernicus who, in his book introducing the Sun-centered (heliocentric) universe, cited Heraklides as one whose work supported his own. Few scientists can ask for more.

P. ANDREW KARAM