Scientist of the Day - Pierre-Charles Le Monnier

Headpiece showing Cancer (the Crab) for a catalog of stars in the constellation Cancer, engraving in Nouveau zodiaque, réduit à l'année 1755, by Pierre-Charles Le Monnier, 1755 (Linda Hall Library)

Pierre-Charles Le Monnier, a French astronomer and mathematician, died Apr. 3, 1799, at the age of 83. He came from a family of astronomers, well established at court and at the Paris Observatory, and Pierre-Charles entered the profession early. By the time he was 21, his reputation was well enough established that he was asked to join the French expedition to Lapland, led by Pierre-Louis Maupertuis and Alexis Clairaut, to measure the length of a degree of latitude, and thus help determine the shape of the Earth. We have told some of this story in posts on Clairaut, and Réginald Outhier.

Portrait of Pierre-Charles Le Monnier, age 61, oil on canvas, by Nicolas Bernard Lépicié, Calouste Gulbenkian Museum, 1777 (Wikimedia commons)

When he returned home, Le Monnier began a long career as a mathematical astronomer. Much of his work was devoted to the "lunar problem," which had vexed Isaac Newton and Edmond Halley and continued to occupy astronomers and mathematicians up through Leonard Euler. Focused on trying to work out the precise orbit of the Moon around Earth, it was an extraordinarily technical and difficult problem, and we will not pursue it here. We wish rather to look at two of Le Monnier's publications that are more accessible to the general reader.

Title page, Histoire céleste, by Pierre-Charles Le Monnier, 1741 (Linda Hall Library)

The first is his Histoire céleste (1741), a history of French astronomy since the establishment of the Paris Observatory in 1671. There had not been many histories of astronomy written in any language before 1741, although the discipline would become quite popular in France later in the century – perhaps because of Le Monnier. For a frontispiece, Le Monnier showed a view and a ground plan of the Paris Observatory (fourth image). This would not be a big deal, except that I know of only one other large printed illustration of the Paris Observatory in the entire 18th century, that of Jacques Cassini (Cassini II) in a book of 1740. We showed that other engraving in our second post on Giovanni Domenico Cassini; we will show it again, in context, when we profile Jacques Cassini later this month. There are some tiny vignettes of the Paris Observatory in the corners of star maps, such as those of Johann Doppelmayr, but I don’t count those here.

Paris Observatory, view and ground-plan, engraved frontispiece, Histoire céleste, by Pierre-Charles Le Monnier, 1741 (Linda Hall Library)

There are several plates in Le Monnier’s Histoire that are of interest; I show one of them here, recording telescopic observations of the rings of Saturn and Jupiter’s moons by various observers in Paris, such as Christiaan Huygens and Giovanni Cassini (fifth image). The Histoire is a quarto-sized book, so this is a large engraving, with more detail than you would find in the original publications in the Journal des Sçavans. There is also a plate of the constellation Aquarius, being traversed by a comet, which I do not have room to include here, but which is notable for being copied right out of John Flamsteed’s star atlas (1729). Le Monnier was a great admirer of Flamsteed, indeed of all British astronomy, so this is not a great surprise.

Views of Saturn, Jupiter, Mars, and Venus, made by French astronomers, 1667-1673, engraving in Histoire céleste, by Pierre-Charles Le Monnier, 1741 (Linda Hall Library)

The other book I wish to introduce to you is much smaller than the Histoire, and you might not even pull it from the shelf to look inside. It is called Nouveau zodiaque, réduit à l'année 1755 (1755), and should you take the plunge and open it up, you will be greatly rewarded. It attempts to provide the exact position of the hundeds of stars in the 12 constellations of the Zodiac, which was of great importance to planetary (and lunar) astronomers, because the planets pass through this region, and when you record their positions, you do so with reference to one of the stars catalogued here. The more precisely they are known, the more accurate your planetary positions will be.

Engraved title page, Table de toutes les etoilles fixes zodiacales, in Nouveau zodiaque, réduit à l'année 1755, by Pierre-Charles Le Monnier, 1755; this is not the title page of the volume, which is type-set, but the separate title page to the catalog (Linda Hall Library)

Star catalogs are ordinarily not that attractive, unless you really like tables of longitude and latitude. But Le Monnier’s little compilation is different. First of all, except for the lengthy introduction, the entire production, tables and all, is engraved. That must have been expensive. And secondly, each of the 12 sections of the catalog is graced by a small engraved headpiece, representing the constellation being catalogued.

Map of the stars of the Pleiades, engraving, Nouveau zodiaque, réduit à l'année 1755, by Pierre-Charles Le Monnier, 1755 (Linda Hall Library)

I show three images here – one of the entire engraved page containing star positions and the vignette for Virgo (eighth image); another of the top half of the page showing Sagittarius (last image), and the third (and our opening image), a detail of just the vignette of Cancer (the Crab). This engraving is only 1” x 3”, but it is delightfully detailed, as you can see in my iPhone close-up. And these are decorations, not maps – there is not a star in sight. Note that Virgo and Sagittarius are lying down, unusual for both, but they do have to fit their horizontal confines.

First page of the star catalog for the constellation Virgo, with engraved vignette, Nouveau zodiaque, réduit à l'année 1755, by Pierre-Charles Le Monnier, 1755 (Linda Hall Library)

Nouveau zodiaque also contains two regional star maps, of the Hyades cluster in Taurus, and the Pleiades. I show the latter (seventh image), because they were a surprise to me, and I have organized two entire exhibitions around star maps. They are very precise and attractive maps, and I would like some future star-map-exhibit organizer to know of their existence.

First page of the star catalog for the constellation Sagittarrius, detail of engraved vignette, Nouveau zodiaque, réduit à l'année 1755, by Pierre-Charles Le Monnier, 1755 (Linda Hall Library)

We have quite a few other books by Le Monnier in our collections; we will return to him one day and take a further look. I will also search for a portrait of Le Monnier when he was younger; I have not been able to find one so far.

William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor emeritus, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to ashworthw@umkc.edu.