ARISTARCHUS

Our planetary spaceship, Earth, is a little more than 4½ billion years old. Consistent with the age of the Solar System and about one-third of the estimated age of the universe, the elapsed time since the Big Bang (an event that was neither large in spatial terms nor loud in the sense of a sound wave travelling) marked the beginning of time and space within which our home coalesced.

Current understanding is that our species, Homo sapiens, has existed for about 300k years. Our presence on this middle-aged planet has therefore been fleeting; using the 24-hours in our day as an analogy, we have been here for less than six seconds.

Nevertheless, progress has been rapid, particularly recently. Most remarkably, we, as self-replicating sentient beings, composed of nothing more than the star-stuff pervading the universe, have deduced the principles and rules that govern its evolution.

In early times, our ancestors must have surveyed the firmament and wondered. Later, structured observation began, sometimes to comply with belief systems. Astrology was widely believed; royal households often retained an in-house guru. Practical uses also developed, notably in navigation and agriculture, though a true understanding of the cosmos remained elusive for millennia.

Until the sixteenth century and the acuity of Nicolaus Copernicus, the accepted cosmological models were geocentric systems, meaning with Earth at the hub and heavenly bodies arranged around the periphery. Designs with a central position for humankind. Various schemes existed, emanating from thinkers including Aristotle and Ptolemy, reflecting long-held anthropocentric perspectives.

The Copernican model, heliocentric, meaning with the Sun at the centre, was a radical departure, resisted by the Church and some scientists, but subsequently supported by Galileo’s observational evidence. His advantage was to possess a telescope. While revolutionary in its day, heliocentricity was not a new idea. Maybe forgotten, but an alternative with origins long before Copernicus, awaiting rediscovery and the weight of empirical proof.

Living in the era of Greek pre-eminence, about 2,300 years ago, was an exceptional astronomer and mathematician named Aristarchus of Samos. Today, his reputation is slight, partly because time has passed, but more particularly because little of his thinking has survived. Study of Archimedes’ writings gives some insights. His book, The Sand Reckoner, referenced in Jimbo’s Assumption, quotes Aristarchus:

“the fixed stars and the Sun remain unmoved, that Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit”

The source of his intuition is unknown. A combination of observation and contemplation might be our conclusion. A potent brain was the only significant tool available, underscoring the power of abstract thought in scientific discovery.

In his era, it was widely held by Greek scholars that the Earth rotated daily, which explained the motion of the stars. Aristarchus posited that these stars were far removed, compared to the distance between Earth and Sun. If the stars were considered as distributed around a sphere, he deemed its radius to be immeasurably greater than Earth’s orbit. This assumption was essential to his heliocentric thesis, otherwise parallax effects would be discernible. While not literally immeasurable, as later demonstrated by Friedrich Bessel in determining the distance to a nearby star, Aristarchus was correct in his assessment of comparative distances, revealing a universe of immense scale, far beyond the immediate vicinity of our local system.

The sole remaining fragment of his cleverness is also astronomical but reveals impressive mathematical skills. His surviving work, On the Sizes and Distances of the Sun and Moon, is a geometric treatment of the relationship between these bodies and Earth. His conclusions, that the Sun is 20x more distant than the Moon, and is 20x the Moon’s size, are both underestimates. On average, the Sun is circa 390x more distant and is approximately 400x larger in diameter. (This coincidence explains why the Sun and Moon appear to be the same size, given the disparity in their distances from Earth.) The reasoning elaborated by Aristarchus was impeccable, the weakness lay in the paucity of his instruments.

As later scholars have suggested, perhaps Aristarchus was less interested in accuracy but rather in demonstrating that mathematical reasoning is central to astronomy, and to science. This is a fundamental notion, the bedrock upon which subsequent research and technology have been built.

Aristarchus’s obscurity suggests his thinking did not excite the contemporary intelligentsia. Had heliocentricity taken hold then, we can only speculate on the consequences. More rapid astronomical advances, perhaps, although the telescope remained a gift awaiting centuries to pass. Social effects, once humanity was removed from a pedestal around which the universe revolves, are imponderable.

As for Aristarchus, he is now a faint figure distant in our history, but not quite forgotten. A lunar crater commemorates his endeavours; 40 km in diameter and visible from Earth. A fitting tribute, a celestial landmark honouring a mind that dared to see nature differently.