Like many people around the world, last night I took my family out to view the great conjunction of Jupiter and Saturn, affectionately being called the Christmas Star. Even though it wasn’t a spectacular visual display (as one member of my family said, “All I see is two little dots of light”), we all felt that we were in the presence of something profound. We agreed that it was a privilege to be able to witness such a rare event. After all, we had read that these two giant planets had not appeared this close together in the night sky since just before dawn on March 4, 1226! But wait, how is it possible that anyone knows that? The information is not based on some written observational record, rather someone had to figure it out. I’m sure computers helped, but it turns out that people have been pretty good at calculating these kinds of celestial events for a very long time.
Here in our Special Collections, we have a collection of rare astronomy books dating from the late 16th to the early 20th centuries. Among the earlier works are several volumes called Ephemerides, which contain tables predicting the daily positions of the sun, moon, and the planets, as well as other celestial phenomena in our solar system, for years in advance. These were important works during the age of sea exploration as they provided critical information for navigation and astronomical study. However, many were written by astrologers who were more interested in reading the heavens to predict human affairs. We have volumes of ephemerides published in Latin, French, and English dating from the turn of the 17th century. Reliable predictions of the positions of the celestial bodies were possible because of the revolutionary ideas that had only recently been proposed by scientific astronomers such as Nicolaus Copernicus, Galileo Galilei, Tycho Brahe, and Johannes Kepler.
The earliest of these books of ephemerides in our collection is Federici Saminiati’s Tabula Astronomica (Astronomical Tables), published in Antwerp in 1599. Saminiati was an Italian mathematician and contemporary of Galileo. The five volumes of ephemerides in our collection by Andrea Argoli certainly are the most ambitious in scope. They cover the predictions for the years 1631 to 1700. Argoli, who taught mathematics at the University of Rome and the University of Padua, was also an astrologer who may have believed that the relative positions of the celestial bodies also influenced human affairs. The only book of ephemerides we have in English was produced by a far more vehement astrologer, John Gadbury, whose celestial predictions were intermixed with his political views. Gadbury produced many “nativities” or horoscopes that tied a person’s destiny to the placement of the planets in the zodiac at their time of birth. By the time of his death in 1704, astrology was no longer being taken seriously as a science. Antoine de Villon’s 1624 volume L’vsage des ephemerides not only predicts the placement of the heavenly bodies, but also claims to describe “all the other things which depend on them, such as wars, plagues, famines , mortality, and others, as well as all the accidents that can afflict men, both in the body and the mind.”
Accurately predicting the positions of the planets still is important today for nautical navigation, not to mention being able to land a rocket on a distant moon or planet, or predict when some space object might collide with Earth. The successor of these books of ephemerides is the Astronomical Almanac, produced jointly by U.S. Naval Observatory and Her Majesty’s Nautical Almanac Office. Whether produced for science or pseudoscience, the ephemerides are remarkable because they demonstrate that with the use of mathematics the positions of the planets can be predicted with incredible precision. Not only can they be predicted forward in time, but they can also be deduced for past times. In a computer, these mathematical formulas can easily produce the information to tell us that the last time a great conjunction of Jupiter and Saturn was visible in the sky was before dawn on March 4, 1226. A cool visual example of planetary movement through time can be seen on NASA’s Solar System Exploration website.
Like everything else in our collections, these Ephemerides are available for consultation in our reading room by anyone with an interest. We sincerely hope that in 2021 the pandemic situation will change and allow us to return to normal operations. In the meantime, contact us if you want to know more.