The weather and the stars

My attention was recently drawn to the MacTutor history of maths website article on The History of Weather Forecasting. The article is largely concerned with the mathematics of weather forecasting from the nineteenth century onwards but has some short introductory paragraphs covering the prehistory of meteorology, which unfortunately displays a woeful ignorance of the subject. Under the heading Early Attempts we get served up the following:

It is not known when people first started to observe the skies, but at around 650 BC, the Babylonians produced the first short-range weather forecasts, based on their observations of the stars and clouds. The Chinese also recognised weather patterns, and by 300 BC astronomers had developed a calendar which divided the year into 24 festivals, each associated with a different weather phenomenon. Generally, weather was attributed to the vagaries of the gods, as the wide range of weather gods in various cultures, for example the Egyptian sun god Ra and Thor, the Norse god of thunder and lightning, proves. Many ancient civilisations developed rites such as rain dances and animal sacrifices in order to propitiate the weather gods.

The ancient Greeks were the first to develop a more scientific approach to explaining the weather. The work of the philosopher and scientist Aristotle (384-322 BC) is especially noteworthy, as it dominated people’s views on and their knowledge of the weather for the next 2000 years. In 340 BC, Aristotle wrote his book Meteorologica, where he tried to explain the formation of rain, clouds, wind and storms. In addition, he also described celestial phenomena such as comets and haloes. Many of his observations were — in retrospect — surprisingly accurate. For example, he believed that heat could cause water to evaporate. But he also jumped to quite a few wrong conclusions, such as that winds form “as the Earth exhales“, which were rectified from the Renaissance onwards.

Throughout the Middle Ages and beyond, the Church was the only official institution that was allowed to explain the causes of weather, and Aristotle’s Meteorologica was established as Christian dogma. Besides, weather observations were passed on in the form of rhymes, which are now known as weather lore. Many of these proverbs are based on very good observations and are accurate, as contemporary meteorologists have discovered.

This brief synopsis, which covers approximately one thousand years actually almost completely ignores the main form of weather forecasting practiced throughout the period covered astrometeorology. As any astute reader will have already deduce astrometeorology is a branch of astrology and is in fact astrological weather forecasting. This is one of the more rational forms of astrology; weather comes from the heavens, be it sunshine, fog, wind, or one of the many forms of precipitation (rain, snow, sleet, hail), so it would seem fairly logical to assume that the heaven cause or control the weather. This is exactly what people in antiquity did in many different cultures and actually what the article above is referring to in both of the first two quoted sentences although the author doesn’t seem to or doesn’t want to know it. It was not Aristotle’s views as expressed in the Meteorologica that “dominated people’s views on and knowledge of the weather for the next 2000 years” but astrometeorology. There is a slight irony here as a quote from Aristotle’s Meteorologica delivered one of main justifications for astrology in Western thought up to the Early Modern period. Astrometeorology is along with astro-medicine one of the branches of natural astrology and as such was even accepted throughout the Middle Ages and the Renaissance by people who rejected other forms of astrology, for example judicial or horoscope astrology.

This very widespread acceptance meant that it was astrometeorology that was the dominant form of weather forecasting in the Middle Ages accepted even by the Church at a time when judicial astrology was, at least official, heavily frowned upon. One might well say, so what? What does it matter what people believed before the emergence of scientific meteorology in the seventeenth century, when this superstitious twaddle got drop anyway? The answer is quit simple; astrometeorology played a significant role in the emergence of that scientific meteorology.

During the Renaissance astrology reached its highest level of popularity in the history of Western culture. Almost all mathematicians and astronomers (mostly one and the same) were also practicing astrologers and they were not just doing it for the money as is often falsely claimed by those who try to deny the significance of astrology in the Early Modern period; they really believed in it. However these were the people who also laid the foundations of the modern empirical approach to the sciences and they were often painfully aware of the lack of empirical justification for the science of astrology that they practiced. To counter this weakness they set about developing various projects to give astrology a solid empirical base, one of the principle projects involving astrometeorology. This project consisted of keeping accurate and continuous weather diaries. They thought that by recording the weather over long periods of time on a daily basis they could then distinguish the correlation, that they were sure existed, between the weather and the movement of the celestial bodies. The oldest known such weather diary was kept by Roger Bacon in the thirteenth century. Bacon was of course not only a fervent believer in astrology but also an early proponent of empirical methods in science. There are other scattered medieval weather diaries but the process first really kicked off at the end of the fifteenth century. The keeping of weather diaries was greatly furthered by the introduction of printed ephemerides, which provided the potential meteorologist with a convenient place to record his observations directly next to the astronomical/astrological information for the day.

A notable writer of weather diaries was Johannes Stöfler (1452-1531), who taught and influenced Philipp Melanchthon (a powerful advocate of Renaissance astrology), Sebastian Münster and others. Another was the Nürnberger mathematicus Johannes Werner (1462-1522), who first suggested the chronometer method of determining longitude. Probably most well-known as weather diary keeper was Tycho Brahe (1546-1601), without doubt the important observational astronomer in the pre-telescope era. Tycho is possibly also responsible for David Fabricius (1564-1617), discoverer, amongst other things, of the first variable star, Mira, keeping a weather diary. Fabricius help Johannes Kepler (1571-1630) to formulate his theory of elliptical planetary orbits in an extended correspondence; systematically criticising Kepler various formulations. Kepler a passionate astrologer also kept a weather diary. He famously established his reputation as an astrologer by correctly predicting an especially hard winter in his first prognostications as district mathematicus in Graz in 1594. Weather diaries were also kept by many other less well-known figures. It is significant that Pico della Mirandola (1463-1494) the most strident Renaissance critic of astrology also kept a weather diary and used the results as one of his arguments for rejecting astrology.

Pico della Mirandola was of course right the systematic keeping of weather diaries did not, as hoped, provide an empirical basis for the science of astrology but did exactly the opposite showing that astrometeorology, at least, was a refuted theory. However the results were not all negative. The systematic empirical weather observations contained in the diaries laid the foundations for scientific meteorology in the seventeenth century. The data collected by those Renaissance astrologers is still used by modern meteorologists to help establish long-term weather patterns.