NICOLAS-LOUIS de La CAILLE (1713-1762)

[glenc (Glen)]

Lacaille was the first to catalogue the star clusters and nebulae in the far southern sky.

LIFE SKETCH OF LACAILLE
(https://upload.wikimedia.org/wikiped...a/Lacaille.jpg)
The biographical information on Lacaille’s life was principally obtained from the book
Lacaille: Astronomer, Traveler, by David S Evans.

Previous to Nicolas-Louis de La Caille work, Edmond Halley (1656 – 1742) catalogued 341 stars from the island of St Helena in the South Atlantic Ocean in 1677. Halley’s interest was not just astronomy, it also included finding accurate time for navigation at sea. His star catalogue was designed to help in this regard.

In 1714 the British Board for the Discovery of Longitude offered prize money of £20,000 for an accurate method for measuring east west longitude at sea. Although Evans indicates that Lacaille was not interested in personal wealth, this prize may have provided some impetus for Lacaille. For the next 47 years there was competition amongst astronomers until the money was finally awarded.

Lacaille’s father, Charles Lewis La Caille (1679-1731) and his mother Barbe Rebuy had 10
children, 6 daughters and 4 sons. Six children died young, three girls and three boys. The other
3 girls become nuns and Nicolas-Louis became an astronomer.

(Lacaille monument at Rumigny. https://maps.app.goo.gl/nMKSVXYkKtM7RTGq5)
Nicolas-Louis de La Caille was born in March or December 1713 at Rumigny (195 km NE of Paris), France. His education began with his mechanically minded father tutoring him. At age sixteen, Nicolas-Louis attended the college of Mantes-sur-Seine, where he studied humanities. He studied rhetoric for the next two years in Paris, but his interests included history, antiquities, mythology and Latin poetry. His father’s death in 1731, when he was eighteen, left Nicolas-Louis with large debts.

After completing his philosophical studies in Paris, he went on to three years of theology, intending to become a priest. It was during his theological studies that Lacaille became interested in mathematics, especially Euclid’s Elements, which he studied on his own. He also taught himself astronomy. Lacaille passed his examinations with distinction, but when he failed to answer the Vice-Chancellor’s questions satisfactorily, he was denied his Master of Arts, and this turned him from theology. He was never ordained.

Lacaille spent the next year at Paris Observatory working with Cassini II and Maraldi II. For
some reason, Lacaille lost the esteem of Cassini II, but in July 1739, Lacaille and Cassini III
began a geodetic survey of a 930 km long meridian from Perpignan, on the French
Mediterranean Coast (near Spain) to Dunkirk on the English Channel, to establish the
circumference of the earth.

Later Lacaille moved to the Collège Mazarin where he studied the earth’s orbit, parallaxes, planetary orbits, comets and stars. He also wrote textbooks on mathematics, practical astronomy, mechanics and optics for the college. It was during this time that Lacaille became interested in the work of Father Feuillée who observed from the Canary Islands in 1724. This probably inspired his later trip to the Cape of Good Hope.

LACAILLE’S WORK IN THE SOUTHERN HEMISPHERE, 1751-1754

Lacaille set himself several goals for his stay in the southern hemisphere.
* He wanted to find the exact positions of the fixed stars, especially first, second and third magnitude stars near the ecliptic;
* the length of a simple seconds pendulum to provide a value for gravity at that location and hence information on the shape of the Earth;
* the longitude and latitude of important places, especially Cape Town;
* measure an arc of the meridian; and
* measure the parallaxes of the Moon, Mars and Venus which he did in conjunction with astronomers in Europe. Lacaille made plans with other astronomers for simultaneous observations of solar
system objects to determine their distances from the Earth, before he left for the Cape.

The authorities at the Cape were reluctant at first to allow Lacaille to observe there, because he
was a French Catholic, while they were Dutch Protestants. Previously there were problems with
a German astronomer, Peter Kolbe who invented false stories against these authorities.

Lacaille left Paris on October 20, 1750 and sailed from L’Orient on November 21 on the ship Le Glorieux. His trip was to span three years and eight months. The ship was uncertain of its position on the way from France to Africa, but a lunar eclipse on December 13, 1750 (totality from 5:32 to 7:11 UT) showed they were 4 degrees off course. They stayed at Rio de Janeiro from January 25 to February 25, 1751 where Lacaille did experiments on magnetism and pendulums. He also determined Rio de Janeiro’s longitude using the moon’s position with respect to the stars as an accurate clock.

Le Glorieux arrived off Cape Town on April 19, and Lacaille presented himself to the governor the next day. A leading citizen by the name of Bestbier gave Lacaille the use of his house. He spent the next six weeks in May and June constructing an observatory.

LACAILLE’S CAPE TOWN OBSERVATORY

Lacaille’s observatory was at the lower end of Strand St, in Cape Town behind Jan Lourens
Bestbier’s house. The governor, Ryk Tulbagh helped with the building of the observatory, which was only 2.5 metres above sea level.
(Lacaille’s observatory was here. https://maps.app.goo.gl/Epu1gxKoGEASMGXk7)

The observatory was a 4.1 m square with the corners pointing north, east, south and west. The sector was placed on the western pedestal, the quadrant on the northern pedestal and the sextant on the eastern pedestal. A bed, table and chairs filled the southern part of the crowded room. The door was near the southern corner on the western side and there was a window in the southeast wall. The wooden roof was covered with a tarpaulin.

INSTRUMENTS IN LACAILLE’S OBSERVATORY

The instruments used in Lacaille’s observatory are listed below:
1. A sector, with 1.95 m radius.
2. A sextant, with 1.95 m radius, which was equipped with two telescopes perpendicular
to each other. The focal lengths of the telescopes were 2.11 m and 1.79 m.
3. A quadrant, of 0.97 m radius with a 13.5 mm aperture refractor telescope attached. This was the main instrument used for the star catalogue.
4. A sidereal clock made by Julien le Roy.
5. Telescopes of various sizes, with focal lengths 14, 15 and 18 feet. Unfortunately their
apertures are not known. The 14-foot telescope was used to examine parts of the Milky
Way and the Magellanic clouds.

Lacaille’s aim was to replace the catalogue of 341 southern stars made by Edmund Halley in
1677 with a better one. He used a quadrant telescope, 75.7 cm in length with a three degree field
of view to make his star catalogue. The observations for the star catalogue were made “with a
clock regulated to the revolutions of the stars and with a telescope equipped with different
reticles.”

This telescope had a length of 866 mm. It was applied parallel to the fixed telescope of a quadrant of 974 mm radius, very heavy and of a very solid construction. The object glass of the telescope had a focal length of 712 mm: its aperture was 13.5 mm. The focal length of the eyepiece was 88 mm to give the telescope an extended field, very clear, and free from parallaxes at its edges. The field was almost 3 degrees. This gave eight times magnification. An expert workman by the name of Poitevin maintained and modified the instruments.

[glenc (Glen)]

CATALOGUE OF 9766 STARS

The weather in Cape Town was much better than in Europe. Lacaille categorised the year’s
weather as follows: one fifth was cloudy (winter), one fifth variable, one fifth calm and clear, and the
remaining two fifths were clear with a strong, south-easterly wind (September to March).

Because of the strong winds, Lacaille viewed through a 10 cm opening in the roof of his
observatory. The proposed catalogue required one hundred nights of uninterrupted viewing, for
six hours at a time, to complete. Each clear night he observed a section of sky covering 6 hours in
Right Ascension and 2.7 degrees in Declination. Two bright stars were observed during each session to
establish accurate positions for the fainter stars in the catalogue. “He used a clock star, usually
Sirius, to check on the performance of his clock. He always observed a reference star for which
the Right Ascension had been determined by observations of corresponding altitudes on the
same or nearby day…. The clock was illuminated by a feeble light provided by a dark lantern
placed opposite.”

“As soon as a star entered or left the plates of the reticle, Lacaille, closing his right eye, which was only used to look in the telescope, and keeping his left eye open, turned a little to present a little paper to the light of the dark lantern at the clock. He recorded his observation on it and quickly returned to the telescope.” To systematically cover the whole area, Lacaille divided the sky into twenty-five zones, between the south celestial pole and declination 23d 18’16”S.

The stars’ positions were determined using a clock and four different copper rhomboid reticles in his telescope. The first was used for zones 1 and 2 near the pole; the second, the small reticle (35 mm in diameter) was used for zones 3 to 7; the third, the large reticle (70 mm in diameter) was used for zones 8 to 21 and the fourth (also 70 mm in diameter but reversed) was used for zones 22 to 25 which were north of the zenith (dec -33.92d). Silk threads formed a cross in the middle of the rhomboid field.

Lacaille began observing in June 1751 but discovered errors in his observations made between
June 1 and August 21, 1751 and these objects were re-observed. He started again on August 23,
1751 and by July 18, 1752 had catalogued all the stars he could easily see, totalling 10,035. This
took 100 sweeps and 76 nights. It can be estimated that Lacaille was observing to approximately
magnitude 7.7 when the total number of stars is compared with the 1997 Hipparcos-Tycho
catalogue which contains 9,409 stars to magnitude 7.6 and 10,353 stars to magnitude 7.7 south
of Declination –23 degrees.

The formula m = 9.1 + 5logD in inches gives Lacaille’s half-inch aperture telescope a limiting magnitude of 7.6 which corresponds well with the estimate from the Hipparcos catalogue.

PUBLICATION OF THE LACAILLE STAR CATALOGUES

Lacaille’s raw observations of 10,035 stars were published in two parts. He reduced the data for
eleven of the twenty-five zones, namely zones 6-16 (Declination 47d to 77d south), and
produced a catalogue of 1942 stars to an estimated magnitude limit of 6.8. (The Hipparcos Catalogue has 1858 stars to magnitude 6.8 in this Declination range.) The first catalogue containing 1942 bright stars was published posthumously in 1763, a year after Lacaille died, as Coelum Australe Stelliferum. It was edited by J D Maraldi.

“The remaining stars were reduced much later in 1844 and published as A Catalogue of 9766 Stars in the Southern Hemisphere for the Beginning of the Year 1750 from observations of Lacaille in 1847, eighty-five years after his death. This was decades after Dunlop and Brisbane completed the Parramatta catalogue in 1827. The British Association for the Advancement of Science paid £200 for Lacaille’s second publication. Mr Wallace did the reductions and John Herschel wrote the preface.

SOUTHERN CONSTELLATIONS DEFINED BY LACAILLE

Lacaille constructed a chart before leaving the Cape that introduced fourteen new constellations.
He placed 1,000 stars in his fourteen new constellations and rejected a constellation called
Robur Carolinum (Charles’ Oak), which Edmund Halley had introduced in 1677 “to pay
homage to the king of England.” This was because nine of its twelve stars were already in
ancient catalogues. French English rivalry also probably paid a part.

The New Constellations were:
1. Apparatus Sculptoris (the Sculptor's Tools)
2. Fornax Chemica (the Chemical Furnace)
3. Horologium (the Clock)
4. Reticulus Rhomboidalis (the Rhomboidal Net)
5. Caela Sculptoris (the Sculptor's Chisel)
6. Equuleus Pictoris (the Painter's Easel)
7. Pyxis Nautica (the Mariner's Compass)
8. Antlia Pneumatica (the Air-Pump)
9. Octans (the Octant)
10. Circinus (the Compasses)
11. Norma, alias Quadrans Euclidus' (Euclid's Square)
12. Telescopium (the Telescope)
13. Microscopium (the Microscope)
14. Mons Mensae (Table Mountain)

[glenc (Glen)]

OBSERVATIONS OF 42 NEBULAE AND CLUSTERS

While determining the positions of stars for his catalogue, Lacaille also noted the positions of some double stars and nebula. He produced a catalogue of 42 nebulae, which was published in 1755 on his return to France in Memoires de l’Academie Royale des Sciences, 1755. The following is an extract from his journal article, On the Nebulous Stars of the Southern Sky.

“The Stars which are called nebulous offer to the eyes of Observers so varied a spectacle that their exact and detailed description could occupy an Astronomer for a long time and cause philosophers to make many curious reflections. As strange as are those nebulae that we can see in Europe, those that are in the vicinity of the southern Pole concede them nothing either in number or form. I am going to outline here an account and a list: this essay may help those who have the equipment and leisure to study them with long telescopes. I would have greatly wished to give something more detailed and instructive for this article but, other than ordinary telescopes of 15 to 18 feet [4.57 m to 5.49 m] focal length, those that I had at the Cape of Good Hope were not adequate or convenient for this kind of research. Those who would take the trouble to examine what occupied me during my visit to that country will easily see that I did not have enough time to make these kinds of observations.

“I first observe that three kinds of nebulae can be distinguished in the heavens;
The first is no more than a whitish, ill-defined area, more or less luminous and of a very irregular shape: these patches are quite similar to the nuclei of faint, tail-less comets. [Most turned out to be globular clusters.]
“The second class of nebulae comprises Stars which are only nebulous in appearance to the naked eye, but when seen in the telescope, show up as a cluster of distinct stars, although very close to each other. [Usually open clusters.]
“The third class is that of Stars which are actually accompanied by or surrounded with white patches or by nebulae of the first class.

“I have found a large number of these three types of nebulae in the southern part of the sky but I do not flatter myself that I have observed them all; especially those of the first and third classes, because they can scarcely be seen except out of the twilight and in the absence of the Moon. However, I believe that the list I give here is passably complete in regard to the more outstanding in these three classes.

“In frequent examination with a 14-foot [4.27 m] telescope of the areas of the milky way where its whiteness is most noticeable, and comparing them with the two clouds commonly called the Magellanic Clouds and which the Dutch and the Danes call the Clouds of the Cape, it is obvious that the white portions of the sky resemble one another so perfectly that one believes, without too much conjecture, that they are of the same nature, or, if you like, that these clouds are no more than detached portions of the milky way which are themselves composed merely of parts often interrupted. It is not certain that the whiteness of these portions could be caused, as is commonly supposed, by clusters of small Stars, more closely packed than in other parts of the sky, for with such attention as I have observed the better defined extremities, whether of the milky way or of the clouds, I have not seen anything there with a 14-foot telescope, other than a whiteness against the background of the sky, without seeing there more stars than elsewhere, where the background becomes darker.

“I will not venture further than to suggest that the nebulae of the first class are no more than small portions of the Milky Way, spread throughout different regions of the sky and that the nebulae of the third class are only stars which are found relative to us, in a straight line as we observe these luminous patches.

“The list which I am going to give here is an extract from the Catalogue of Southern Stars which I put before the Academy: I was not able to distinguish in the Catalogue, the different nebulae except by brief notes which are explained in the discourse which I have included here; but in order to satisfy the curiosity of those who may find these notes too vague, I will give here a short description of each nebula in particular.”

There are 24 open clusters, 7 globular clusters, 7 asterisms, 3 nebula (M8, NGCs 2070 and 3372) and one galaxy (M83) in Lacaille’s catalogue. http://www.messier.seds.org/xtra/history/lacaille.html

[glenc (Glen)]

Diameter of the Earth
With his star catalogue completed, Lacaille turned his attention to measuring an arc of the meridian, to determine the shape of the Earth. Between September 9 and October 23, 1752, Lacaille measured an arc 135.8 km long with the southern end at his Cape Town observatory and the northern end at Klipfontein (the modern town of Aurora). This involved measuring the angles in two large triangles and two small triangles and also measuring the altitude of stars from the two ends of the arc.
(Aurora. https://maps.app.goo.gl/wf6RL6jn7QTiHN5B8)

Lacaille took two friends, two wagons, one drawn by six horses and the other by ten oxen, and eight slaves with him. From Klipfontein on September 24, he measured the angles to two signal fires on two mountains, Riebeek Kasteel and Kapokberg. On October 14, from Riebeek Kasteel Mountain, he measured the angle to a fire at Klipfontein and also the angle to his observatory at Cape Town. On October 16, Lacaille made similar measurements from Kapokberg Mountain to Klipfontein and Cape Town. Between October 17 and 21, he measured a base line 12.6 km long across flat ground just north of Kapokberg using pine rods 5.85 m long. He also measured the angles from the eastern and western ends of this line to the Kapokberg and Riebeek Kasteel mountains.

By measuring the angular altitude of stars he calculated that the celestial angle between Cape Town and Klipfontein was 1d 13’17.3” and this meant that one degree equalled 111.165 km. Klipfontein was 5.1 km east of the meridian line through Cape Town.

When Lacaille arrived back at Cape Town, he discovered that the arc was not in accordance with northern hemisphere measurements. His measurements incorrectly indicated that the Earth is pear-shaped. He decided there was no error in the six angles of the two large triangles, so he suspected an error in the base line. He remeasured it on November 3 with a chord 58.5 m long but found no error.

However his problem primarily arose from the mountain’s effect on his plumb line, as first suggested by the surveyor Colonel Sir George Everest. Everest arrived “at an estimate that the sums of the deviations of the plumb-line at Lacaille’s two stations must be 8.99” due to the presence of Table Mountain, Devil’s Peak and Lion’s Head. Lacaille’s meridian length was also later (1841) confirmed to be in error when Maclear from the Royal Observatory at the Cape found a difference of 36 m in the 135.8 km arc from Cape Town to Klipfontein. “He thus verifies Everest’s total effect of the plumb line deviations but attributes the greater part to the close mountains at Klipfontein not to Table Mountain.” Modern measurements give the Earth’s circumference as 40,007.863 km, making one latitude degree equal to 111.133 km.

Longitude at Sea
Another of Lacaille’s goals was to determine longitude at sea using the moon’s motion through the stars. Sailing ships leaving coastal waters needed to navigate in open oceans. This (as already mentioned) prompted the British to offer £20,000 for an accurate method for measuring longitude at sea. Several people suggested using the moon’s motion through the stars, including Sieur de St. Pierre (1674), Bouguer and Halley. Lacaille adapted Halley’s suggestion for finding longitude using the moon’s position with respect to bright stars. “The moon makes a circuit of the sky with respect to the star background on average every 27.322 days.” This can be used to “establish the longitude with an uncertainty equal to the rotation of the Earth in two minutes, that is, half a degree or roughly 30 nautical miles (55 km) on the equator.” There were two major problems: neither the star positions nor the motion of the moon were known with sufficient precision to make the method workable. Two more complications had to be considered: the moon’s position varies depending on the observer’s location on earth and the moon’s position varies depending on its distance from the earth. Lacaille studied refraction, made star catalogues and observed the moon’s orbit in an attempt to solve these problems. He used a graphical method to make it easier for ship’s captains to calculate their longitude.

In 1761, just one year before Lacaille died, John Harrison introduced the marine chronometer which he developed over many decades and this gradually made the lunar method of determining longitude obsolete. With this invention, longitude could be accurately measured at sea, and Harrison eventually received the £20,000 prize money. However the high cost of chronometers meant the lunar method of determining longitude was still used for many years. In the late 1700s, marine chronometers cost £70–£150.

Cartography, mapping
Lacaille left Cape Town on March 8, 1753 and arrived at Mauritius on April 18, 1753. While on the voyage he tested his new method for determining longitude at sea, using the distance of stars from the moon. During the next nine months he made an accurate map of the coastline of Mauritius. On January 16, 1754 Lacaille sailed to the nearby island of Réunion where he made longitude and latitude determinations from January 17 until February 27, 1754. On his return voyage to France, Lacaille visited Ascension Island (St Helena) from April 15 to 20, 1754. This was the place where Halley made his star catalogue in 1677. Lacaille arrived back in L’Orient, France on June 4, 1754.

LACAILLE’S WORK BACK IN FRANCE

The next few years were spent working with his observations from the Cape and in revising his textbooks on mathematics, astronomy and optics. “In 1759 he wrote up his graphical method for the determination of the longitude at sea, by the method of ‘lunars,’ that is the position of the moon with respect to certain bright stars.”

In February 1762 Lacaille had a recurrence of an illness which he experienced at the Cape. On March 19 he was bled by a physician but died on March 21, 1762 aged 49, probably as a result of the bleeding. He was buried at the Collège Mazarin in Paris, but his remains were later moved. Lacaille is honoured by a monument at Rumigny, by a plaque in Cape Town and by a bas-relief bust at Curepipe in Mauritius.

James Dunlop made the second catalogue of southern star clusters and nebulae from Parramatta, NSW in 1826 and John Herschel made the third catalogue from Cape Town between 1834 and 1838.

[Bobbyoutback]

Thanks Glen for posting this !

[Crater101 (Warren)]

Again, a very interesting read, and well put together. Cheers for the information.

[glenc (Glen)]

Thanks Bobby and Warren