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Can the meteorology of Posidonius be called "science"?

John Hall

(University of Cambridge)

What relation did Posidonius’ aims and methods in meteorology have to those of modern scientists? He regarded meteorology as a branch of philosophy and was uninterested in any practical use of it, e.g. he did not study weather-signs. His motive was intellectual curiosity and he believed that meteorological events have naturalistic causes. In this, and in that his aim was a genuine desire to know that nature and the causes of meteorological events, his aim was that of a scientist. His methods I shall illustrate with two examples.

First, tides. In a 30 day visit to Cadiz he confirmed the basic facts about tides: that the tide rises each day as the moon approaches its zenith, and as it approaches the point opposite its zenith, and that the tide falls as the moon moves on from those points; and also that the tides rise highest at full moon and new moon, and rise least at half moon. He also said, erroneously, that tides are greatest at the solstices and least at the equinoxes, but took care to explain that this was an inference from what he had been told, not an observation. He also measured, rather roughly, the height of a tidal rise at Cadiz harbour.

So Posidonius observed tides with some accuracy. He also tried to explain their cause. Accounts of his explanation differ. Likeliest, I think, is that he thought the moon slightly heats the sea, causing it to expand, like water in a moderately heated kettle. This was a hypothesis, supported only by an analogy of unproven relevance.

Second, the height up to which weather phenomena occur. According to Pliny, Posidonius thought that this height is less than (or, possibly, not less than) 40 stades, i.e. between 6 and 8 kilometres – a figure within the range of heights calculated by modern meteorologists for the troposphere, the region in which most weather phenomena occur. How did Posidonius arrive at that figure?

Aristotle says that winds do not blow above the highest mountains, and numerous later ancient writers express the same view, which seems ultimately based on a mythological conception of Mount Olympus as the home of the gods, “neither shaken by winds nor wetted by rain” (Odyssey 6.42-45). This was evidently the general view, likely to have been shared by Posidonius. But, how high were the highest mountains?

Various ancient authors, from about 300 B.C. on, give figures for the heights of the highest mountains, or of particular mountains – sometimes fantastic, sometimes reasonable, some of them apparently the result of measurement; some are accurate, though very imprecise, some are not. The more reasonable figures all relate to Greek mountains, and the highest of these figures is 20 stades – between 3,000 and 4,000 metres – for Mount Cyllene in the Peloponnese.

Posidonius had travelled in western Europe, and so was probably more aware than most Greek writers, even if only from hearsay, of the great height of the Alps – higher than any Greek mountain. He had no chance of measuring their height, but (I suggest) he assumed that the highest Alp might be up to twice the height of the highest Greek mountain, i.e. twice 20 stades, whence his figure of 40 stades for the height up to which weather phenomena occur.

If I am right, he had reached this roughly correct figure with the aid of a few systematic measurements combined with some unverified assumptions. This example illustrates the difficulties with which ancient investigators had to contend, with limited opportunities for gathering information, virtually no instruments, and no easy way to communicate new ideas and discoveries to each other.

watch the recorded talk: click the image

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