Astrometeorology—the study of correlations between weather and the Sun, Moon, and planets—dates back to the origins of Western astrology some four thousand or more years ago. Because astrology originated in the early agricultural centers of the Near East, there was great interest in knowing what the weather might be like in the future, as the success of crops was of vital importance. Over time, sky watchers in the great cities of the region, such as Babylonia, observed and recorded correlations with the moving planets, Sun, and Moon. These early observations were the start of a long tradition that was documented by Ptolemy, the famous scientist of the early Roman period, more than a thousand years later. Five to seven hundred years after Ptolemy, the great Arab astrologers of the Middle Ages sought ways to predict rain using astrology; a thousand years after that, the almanac writers of the Renaissance issued forecasts based on astrometeorology for years ahead. Today, a few atmospheric scientists are busy rediscovering the Moon's effects on the weather.

In his great work on astrology from about 150 CE called the Tetrabiblios, Ptolemy described a way of predicting weather that was based on close scrutiny of the New or Full Moons that occur near the equinoxes or solstices, the markers of the four seasons. The methodology he recorded was to chart the time of these special New or Full Moons and note what signs they fell in and how they related to the other planets. With this information, a forecast for the season's weather could be made. Ptolemy also had some insights into a kind of lunar tidal effect on the atmosphere. He compared the ebb and flow of the tides (which he correctly attributed to the phases of the Moon), to the changes in air currents that occur when the Sun or Moon were rising, setting, or directly overhead or under the Earth.

Arab learning during the height of the Islamic Empire (c. 630–900 CE) was extensive and included astrometeorology as a subject of central importance. For religious reasons, Arab astrology concerned itself primarily with those parts of astrology that did not deal with individuals. Natal astrology was not practiced, but astrometeorology was a major theme, as was astrology applied to history, interrogations and elections, and the medical field. The contents of a work by the great Muslim philosopher Al-Kindi (800–873), titled De Mutatione Temporum (On the Changes of the Weather), are almost exclusively practical techniques and methodologies for predicting weather, especially rains—which makes good sense given the dry nature of the eastern and southern Mediterranean region. According to Al-Kindi, the probability for rain in the Middle East becomes greater near a New or Full Moon when all the planets are retrograde in a specific quadrant of the year, usually winter. Also, the motion of the planets must be moving in the zodiac toward the Sun and Moon. Further details amplify or decrease the possibility of rain. Other techniques, including zodiacal sign positioning, aspects between planets, aspects to the quarters of the Moon (which naturally involve the Sun), and ingresses into the equinoctial sign Libra all contribute to the art of forecasting rain.

During the Renaissance, almanac writers nearly always included weather forecasts for the year. Leonard Digges was the author of a popular almanac first published in 1553, The Prognostication Everlasting of Right Good Effect. It contained the standard astrological methodology for predicting the weather in England. With this almanac came an ephemeris of the year's planetary positions along with a "do-it-yourself" manual. Digges noted that, "the conjunction, quadrature and opposition of the Moon with the Sunne in moist signes, rayny weather: the more if the Moon go from the Sunne to Saturne." He said one should look to the quarters of the Moon and pay attention to what signs they occur in, as this will tell you how much rain will fall. The moist signs are the water and air signs. He also said the chances for rain increase after the New, Full, and quarter Moons if the Moon's next aspect is to Saturn.

In 1686, John Goad (1616–1687) published a major work on astrology entitled Astro-Meteorologica, or Aphorisms and Discourses on the Bodies Celestial, their Natures and Influences. Astro-Meteorologica is a comprehensive work of over five hundred pages, many of them samples of his weather log. His book was probably the most scientific work that focused on astrometeorology, and certainly the most ambitious, to appear during the entire seventeenth century. After a lengthy introduction covering his basic principles, Goad examined the various Sun-Moon aspects. Beginning with the conjunction (New Moon), he analyzed its correlations with the weather over a seven-year period of eighty-seven conjunctions. He next considered the opposition between the Sun and Moon (Full Moon). Next were the quarters (90 degrees), the trines (120 degrees), and the sextiles (60 degrees). Goad sought to examine the frequencies of various kinds of weather patterns occurring during the range of time that he thought each aspect was effective. If there was a correlation between aspect and weather pattern more than half the time, he maintained that the influence of the aspect was proved.

At the conclusion of his study of the five Sun-Moon aspects, Goad summarized his findings in a table. His records for the Full Moon showed that some form of moisture was recorded in 75 of the 87 aspect events during the seven-year period. His weather record suggested that there were more wet days at the Full Moon than at the New Moon, and that the second half of the Sun- Moon cycle (the later trine, square, and sextile) were generally warmer than the first half. He noted there was much rainfall at the first sextile (60 degrees after the New Moon) and the second trine (60 degrees after the Full Moon). This result, more rain about five days after the New and Full Moon, surprised him; he had expected that the traditionally more powerful aspects—the conjunction and the opposition—would account for the most moisture.

Goad's observation of rainfall peaks five days after New and Full Moons was confirmed to some extent by a study done in the 1960s, published in the prestigious journal Science. The lead author of the paper was Donald A. Bradley, known to the astrological community as Garth Allen. Under his pseudonym, Bradley published several books (some released by Llewellyn) and numerous articles in astrology magazines such as American Astrology during the later 1940s through the 1960s. He was best known in the astrological community as an advocate of sidereal astrology and has left a considerable legacy in that regard. But he was also an engineer and self-taught scientist who had a paper published (co-authored with Max A. Woodbury) in 1962 that examined precipitation data over the continental United States for a fifty-year period (1900–1949). They found that maximum precipitation appears to be related to the Sun-Moon cycle.

Bradley and Woodbury's study took into account only major rainfall events in a twenty-four-hour period thoughout the continental United States. These were graphed and compared to the lunar cycle over a fifty-year period and analyzed statistically. What they found was a strong tendency for extreme rainfalls near the middle of the first and third weeks of the lunar cycle; that is, the third to the fifth days after New Moon and Full Moon. This result is almost exactly what John Goad had found three hundred years earlier—that rain was more abundant at the first sextile and second trine, which are about five days after New Moon and Full Moon, respectively. In the same issue of the journal Science, Bradley and Woodbury's paper was followed by a report from researchers in New Zealand that they had found the same correlation between heavy rainfalls and the lunar cycle in that part of the world as well.

In 1964 Bradley co-authored a paper (with Glenn Brier of the U.S. Weather Bureau) published in The Journal of Atmospheric Sciences that reported a 14.75-day cycle found in precipitation data—obviously a lunar connection, as that figure is exactly half the full 29.5-day cycle of the Moon. But this was only the beginning of the modern rediscovery of extraterrestrial effects on the Earth's atmosphere. Since the 1960s, many other scientific papers have been published in leading scientific journals that have demonstrated strong correlations between the Moon and atmospheric phenomena such as thunderstorm frequency, atmospheric pressure changes, hurricanes, cloudiness, and surface temperatures.

A connection between the Moon's phases and the frequency of thunderstorms was reported in 1970 in The Journal of Geophysical Research. Mae DeVoe Lethbridge, a meteorologist at Pennsylvania State University, analyzed thunderstorm data for twenty-eight years in the United States and compared it to the days when the Moon was at maximum declination and for the days around the Full Moon. What was found was a peak of thunderstorm frequency two days after the Full Moon, and also when the Moon was at maximum north declination. When these astronomical events combined, a very high increase in thunderstorm frequency occurred. The Moon's declination is always high when it is in Gemini and Cancer, so the Full Moons near the time of the Winter Solstice are the ones to watch. Lethbridge suggests that the cause behind this phenomenon could be the Moon passing through the Earth's magnetic tail, the part of the magnetic field that is blown back into space by the solar wind. The Moon passes through this tail every Full Moon; the disruption could cause electrical changes leading to the formation of thunderstorms.

In 1995 atmospheric scientists and geographers at Arizona State University began publishing scientific papers about the influence of the Moon on weather. Robert C. Balling and Randall S. Cerveny reported in the journal Science that there is a correlation between lunar phase and daily global temperatures. They used daily temperature data from polar-orbiting satellites that cover the entire Earth and compared fifteen years of data with the lunar cycle. It wasn't much of a difference that they found, but they are very confident that the data shows global temperatures to be a little bit warmer at the Full Moon. Exactly how this occurs is not known, but the authors note that the Full Moon reflects a small amount of infrared light back to the Earth, and this causes warming on the Earth. They suggest that this mechanism, a very subtle heating, may also account for the correspondences between lunar phase and precipitation, cloudiness, and storms.

More recently, in 2011, another paper from the group at Arizona State University was published that showed a correlation between the monthly lunar declination extremes and the circulation of the lower atmosphere, the part where our weather occurs. Because the Moon's orbit is tilted relative to Earth's orbit around the Sun, the Moon swings north and south of the celestial equator every month as it orbits Earth. The distance north and south of the equator is measured in degrees of declination, a factor used by astrologers when reading charts. What the scientists found was evidence of greater atmospheric tides in the higher latitudes when declination was high. These tides are apparently strong enough to modulate atmospheric pressure, which could affect the formation of storms that bring rain.

Excerpted from Llewellyn's 2015 Moon Sign Book. Click here for more current-year calendars, almanacs, and datebooks.