Unforgotten sisters: The comet calculator

The birthplace of Madame Nicole-Reine Lepaute was the prestigious Luxembourg Palace in Paris, the ancient seat of the kings of France. Built by the Queen Maria de’ Medici, today it hosts the French Senate and is famous for its wonderful garden.

Nicole, whose full name was Nicole-Reine Étable de la Brière, wasn’t a princess, but she was born there, on January 5, 1723, because her father was a valet to the nobility, and her parents thus lived on site.

The child quickly showed her intelligence, passing her nights reading books. When she was still young, one of her sisters said to her “I am the whitest,” to which she immediately replied, “And I, the wittiest!”

At the age of 25 Nicole married the royal clockmaker, Jean-Andre Lepaute. He was famous all over Europe for technical achievements in his field. For example, he was the first to make a horizontal clock, and a clock with a single wheel. Moreover, many astronomical observatories in Europe were enabled by Lepaute’s pendulum clocks.

Nicole became soon very interested about her husband’s profession, and began to collaborate with him. Her first research concerned the swing of pendulums of different lengths. The result was published in 1755 with the title Table des longueurs des pendules, in Traité d’horlogerie.

The talent of this gifted woman is noted in many statements by the young astronomer Joseph Lalande, pupil of Joseph-Nicolas Delisle. For example, he reported that “Madame Lepaute is immediately entered into this working group, she was too smart not to have the curiosity; she noted, she calculated, she described the work of her husband …”.  

Some sources indicate that during this period Lalande was responsible for the local astronomical observatory at the Luxembourg Palace, so he probably had a privileged and well-informed view of the situation.

From clocks and pendulums, it is but a short step to astronomy, especially if you have an interested astronomer such as Lalande in proximity, and you are a savant calculatrice, or “learned calculator,” in an era where computers did not exist and their mechanical ancestors were rare and of a quite limited use.

Indeed, it is important to note that astronomical calculations were essential in this research, as we will see shortly from Lalande’s report of their collaboration. No scientist of the time was exempted from this long and tedious activity, which required non-trivial physical and mathematical knowledge, not merely the “mechanical” application of some algorithms. Madame Lepaute, therefore, was playing in the same league as her colleague scientists.

Nicole started her first important work in astronomy in 1757, in collaboration with Lalande and the mathematician Alexis Clairaut.

The project was ambitious and complex at the same time. Lalande wanted to accurately determine the date of the return of Halley’s comet – obviously, before the actual event. To do this, it was necessary to calculate the effects of the gravitational pull of Jupiter and Saturn on the path of the comet, which required calculating approximate solutions for the “three-body problem” – specifying the movement of a trio of objects in line with their respective masses, positions and velocities – a challenge Clairaut had previously outlined.

This involved a huge amount of work computing the distances and forces of attraction exerted on the comet by each planet, using Newton’s laws, for a time period of 100 and 50 years respectively.

Nicole and the two scientists threw themselves into this job for more than six months, to the point of getting sick. As Lalande recalled: “For six months we made calculations from dawn to dusk, sometimes even during the meals … The help given by M.me Lepaute was such that without her I would not have been able to complete such a colossal enterprise.”

Today, it looks almost incredible that three people, without computers, had been able to carry out this task by hand. Eventually, they announced to the Academy of Science that the perturbed comet would have reached the perihelion, the point of closest approach to the sun, in mid-April 1759, with an error margin of just a month. This corrected the forecast made by Halley himself, who had placed this event in 1758.

Nicole’s mathematical and astronomical activity continued in subsequent years. In 1761, she published the calculations of all the observations made during the transit of Venus across the sun, and the following year worked on the orbit of a new comet and calculated the duration and the size of an annular solar eclipse set to be visible in Europe in 1764.

In 1774, Nicole took over the responsibility of the Ephémérides of the Academy – the job of computing in advance the positions of the planets, the sun and the moon for observation purposes.

She passed the last years of her life nursing her sick husband, but died a few months before him, on December 6, 1788. Her ephemeris tables, published in 1783, were complete until 1792.

Lalande, after her death, remembered this extraordinary collaborator with these words: “This interesting woman is often in my thoughts, always dear to my heart; the moments that I spent close to her and her family are the ones I most love remembering, and whose memory, mixed with bitterness and pain, spread some sweetness on the last years of my life, as his friendship was the charm of my youth. Her portrait that I always have under my eyes, is my comfort, when I think that a philosopher should not complain about the laws of the imperious necessity, and of the losses that are a necessary consequence of the order of nature.”

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Gabriella Bernardi is a science journalist and author based in Turin, Italy. Her two most recent books are Giovanni Domenico Cassini: A Modern Astronomer in the 17th Century (Springer, 2017), and The Unforgotten Sisters: Female Astronomers and Scientists before Caroline Herschel (Springer, 2016), on which this story is based.