Astronomy through the agesThe history of the Observatory fits easily into the history of astronomy. The chronological time-line includes a few key milestones and curious stories, which highlight the rich and continuous activity of French astronomers, as well as the breadth of this scientific discipline.
The restoration of the Perrault building
Initially, a vast, slightly sloping, terrace covered the observatory building. It was supported just by the walls of the vaulted ceiling, which were reinforced by a thick cement screed. The settling of the massive walls led to a gradual deterioration of the terrace, resulting in increasingly abundant water seepage.
In 1757, Grandjean de Fouchy, perpetual secretary of the Académie Royale des Sciences, who occupied the old apartment of abbé Picard on the second floor, was obliged to move. The following year, La Condamine noted that « le mur de 7 pieds d’épaisseur qui paraissait inébranlable, s’était séparé du pavé, et laissait un vide de plus d’une ligne » ("the 7 foot thick wall, which had seemed immovable, had separated from the paving, and there was now a significant gap between them").
Preserving the external shape of the monument
Finally, through the stubbornness of Cassini IV, director of the observatory, the repair work was financed and begun in the summer of 1786 with the complete demolition of the terrace and the vaulting. The restoration was carried out by Brébion and Renard.
Cassini IV would have liked to demolish everything above the second floor; this was refused, since the King wished to preserve the external appearance of the monument. The Observatoire was completely evacuated in March 1787. Cassini moved to a house in a small neighbouring road, the rue Maillet, which had already been occupied by his ancestors; today the road is named after Cassini. The work was finished in 1791.
New vaulting was built over other brick vaulting, carrying wide overlapping stone slabs, sloping to ensure water run-off. Below the brick vaulting, a large airy space with huge arches was fitted out. The large meridian room was divided into three parts spanning its width, and two massive pillars were installed.
Cassini’s project to install an instrument on each of the two towers, thanks to the construction of the double vaulting, was in fact fulfilled during the following century, first by Arago, with the erection on the east tower of the large Brunner equatorial, then by Le Verrier with the erection in 1858 of the Secrétan-Eichens equatorial on the west tower.
The Small Northern Observatory (Le Petit Observatoire du Nord)
Finally, on the northern part, were laid the foundations of a building which Cassini called the Small northern Observatory (Petit Observatoire du Nord). It was finished later by the Bureau des Longitudes. Reichenbach’s circle was installed there in 1811, followed by Gambey’s 10cm equatorial in 1826.
Le passage de Vénus devant le Soleil de 1769
Si le rayon de la Terre et la distance Terre-Lune ont pu être calculés dès l’Antiquité par des méthodes géométriques, il n’en est pas de même pour la distance Terre-Soleil. Cette distance permet, à l’aide de la troisième loi de Kepler, d’avoir accès à l’ensemble des distances planétaires du système solaire, ainsi qu’à la mesure de l’Univers grâce aux parallaxes stellaires.
Vénus et la distance Terre-Soleil
En 1677, l’astronome Edmond Halley après une observation d’un passage de Mercure devant le Soleil imagina une méthode pour déterminer la parallaxe du Soleil, donc la distance Terre-Soleil à l’aide des passages de Vénus devant le Soleil. Pour cela on devait mesurer, en des lieux situés à des latitudes différentes les durées du passage de la planète devant le Soleil.
Les passages de Vénus devant le Soleil sont des phénomènes rares, ils se produisent en suivant le cycle de 8 ans, 121,5 ans, 8 ans et 105,5 ans. Un premier passage avait été observé en 1639, les deux suivants devaient se produire en 1761 et en 1769. Le passage de 1761 donna lieu à une grande campagne d’observations internationales en pleine guerre de Sept Ans. L’expérience acquise lors de ces observations permit d’améliorer les méthodes d’observation pour le passage de 1769.
Le Gentil, Chappe, Pingré
En 1769, la France organisa trois expéditions. L’astronome Le Gentil de La Galaisière, après un premier échec en 1761 au large de l’Inde, était resté sur place et se rendit à Pondichéry mais il ne put observer le passage, car un nuage fatal le priva de l’observation.
L’abbé Chappe d’Auteroche, avec une équipe d’observateurs, se rendit en Basse-Californie où il put observer le passage, mais restée sur place pour observer une éclipse de Lune afin de calculer la longitude de son lieu d’observation la quasi-totalité de l’équipe fut décimée par une épidémie de typhus.
La troisième expédition fut une expédition maritime, où l’on testa également les premiers chronomètres de marine, l’abbé Pingré et le Comte de Fleurieu observèrent le passage depuis le Cap François à Saint-Domingue.
La caméra électronique André Lallemand
Le principe de cette caméra a été présenté pour la première fois à la séance de l’Académie des Sciences du 20 juillet 1936 : dans une enceinte sous vide l’image astronomique est projetée sur la photo-cathode, les photons provoquent l’émission d’électrons qui forment une image électronique grâce à l’optique électronique, cette image est enregistrée sur une plaque photosensible. A un photo-électron correspond une trace formée de plusieurs grains d’argent ; le récepteur est donc linéaire et a une sensibilité de 30 à 40 fois supérieure à la plaque photographique.
photons et électrons
A. Lallemand et M. Duchesne développent cet instrument à l’Observatoire de Paris dans leur laboratoire installé au « petit coudé », les premières photos d’objets célestes avec la caméra électronique ont été obtenues au foyer de la lunette équatoriale.
En 1959 la caméra est installée au foyer du télescope de 3m05 de Lick et permet de mesurer pour la première fois, avec l’astronome américain M. Walker, la rotation du noyau de la galaxie d’Andromède.
- MM. André Lallemand et Roger Alexandre (verrier) du laboratoire de physique astronomique de l’Observatoire de Paris
Observatoire de Paris / J Counil
Au vue des performances de ce nouvel instrument et afin d’en assurer son développement un nouveau laboratoire est construit en 1960. C’est le laboratoire « André Lallemand », qui regroupe en dehors des bureaux et divers laboratoires de montage et d’expériences un atelier de mécanique et un atelier de verrerie.
Le laboratoire Lallemand
Gérard Wlérick associé à A. Lallemand et M. Duchesne transforme ce récepteur d’images en un véritable compteur de photons à deux dimensions : réduction drastique de la lumière parasite, mesure de la sensibilité locale des photo-cathodes et de l’émission parasite résiduelle, liaison avec les télescopes des Observatoires du Pic du Midi et de Haute Provence.
L’instrument est alors utilisé de façon régulière par les astronomes G.Wlérick, A.Bijaoui, G.Lelièvre aux observatoires du Pic du Midi, de Haute Provence, du CFH (télescope Canada,France,Hawai) et du Chili. Par ailleurs deux nouvelles générations de caméras sont développées à l’Observatoire de Paris à partir des années 65 une camera à vanne par P. Felenbok et une camera magnétique grand champ avec une photo-cathode de 90mm de diamètre par A. Lallemand, B. Servan et L. Renard.
L’exploitation régulière de ces caméras a nécessité la création en 1977 d’un laboratoire de fabrication des photocathodes dirigé par F.Gex. L’image astronomique étant projetée directement sur la couche photosensible l’homogénéité en sensibilité et la qualité de propreté de son support était fondamentale. Dans ce but ce nouveau laboratoire situé au 77 av. Denfert-Rochereau fut équipé dès l’origine d’une salle blanche à flux laminaire contrôlé.
L’observation aux foyers des télescopes avec une caméra électronique était assez complexe si on la compare avec l’observation aujourd’hui avec les détecteurs modernes que sont les CCD. En effet la grande difficulté de cet instrument était de maintenir dans un même vide poussé la photo-cathode et les plaques photosensibles en gélatine. La photo-cathode était maintenue sous vide dans une enveloppe en verre intermédiaire et était libérée juste avant l’installation de la caméra au foyer du télescope.
Pour maintenir le vide dans l’enceinte de la caméra, l’optique électronique et le porte plaques étaient refroidis à l’air liquide, les réservoirs d’air liquide de type Dewar devaient être remplis en permanence pendant l’observation ce qui représentait une sorte de prouesse lorsque vous vous trouviez par exemple à plusieurs mètres de hauteur sur la passerelle du foyer Newton du 193 de l’OHP.
- Caméra électronique Lallemand-Duchesne au foyer Newton du télescope de 193 cm de l’O.H.P.
Après l’observation la caméra était démontée du télescope et transportée dans un laboratoire situé dans une salle annexe afin d’ouvrir la caméra et récupérer les plaques photos. L’ouverture de la caméra entrainant la mise à l’air de la photo-cathode celle-ci était détruite immédiatement et il était nécessaire de préparer une nouvelle caméra avec une nouvelle photo-cathode pour la prochaine observation.
La mise en exploitation d’une caméra nécessitait l’intervention de deux techniciens de jour relayés par deux autres techniciens la nuit ainsi que la rotation de trois tubes de caméras avec à chaque fois une nouvelle photo-cathode et de nouvelles plaques photosensibles
Les résultats scientifiques
Les résultats scientifiques les plus marquants sont :
la mesure du noyau de la galaxie d’Andromède (G.Walker)
la mesure de l’épaisseur des anneaux de Saturne et la démonstration que les quasars sont bien au centre des galaxies (G. Wlérick).
l’étude à haute résolution des jets de Messier 87 et 3C273 (G.Lelièvre, G.Wlérick)
Jules Janssen’s photographic revolver
In 1873, Jules Janssen (1824-1907) presented to the Commission for Venusian solar transits, his «revolver method» for solving in an objective and permanent way the difficult problem of determing the exact moment of contact of the planet Venus with the solar disc. In effect, the accuracy with which can be determined the moment of transit, measured in different places on the Earth, is directly related to the accuracy with which can be known the mean distance of the Earth from the centre of the Sun.
Taking photographs automatically
After the failure of earlier visual observations, the idea was to make an automatic system, which would register a series of 48 successive pictures on an annular photographic plate, together with the exact time of the first image.
Since the instrument made by Deschiens was not satisfactory, Janssen asked Redier father and son to make a new device: while the platform carrying the annular daguerreotype turns through 1/48 of a complete circuit, then stops to take the picture, the disc which carries 12 radial shutter slits (regularly spaced and with adjustable widths) turns continuously four times more rapidly. Thus, while the photographic plate has gone trough a complete circle (in 72 seconds), the «shutter» disc will have gone through four. Janssen was able to test this instrument to his satisfaction, and he took it in 1874 to Japan.
The 1874 transit of Venus
In England, as soon as he heard about the project, the «Astronomer Royal» Airy had built instruments of the same type, called the «Janssen», and with which all the British expeditions were equipped.
- Janssen’s photographic revolver: picture taken during the 1874 solar transit of Venus
- La Nature - vol. 3, 1875.
Even though the results of classical photography and those obtained using the eight or so revolvers working in 1874, were disappointing, Janssen (for whom the Physical Astronomy Observatory of Paris, situated in Meudon, had been created the preceding year), Janssen had nevertheless designed and created the first cinematographic camera.
Astronomy through the ages
Léon Foucault at the Observatory
Léon Foucault (1819-1868) is without any doubt one of the greatest experimental physicists of all time.
He became famous through his pendulum experiment, which showed the rotation of the Earth; this experiment was mounted for the first time at the Observatory in 1851, then mounted almost immediately in the Panthéon and in hundreds of other places.
He was hired in 1855 at the Observatory as a «physicist», through pressure exerted by Napoléon III on the director, Le Verrier, who did not want him! Nevertheless, Le Verrier surely did not regret the decision, since Foucault’s work was outstanding.
He created the modern type of telescope, in which a silvered glass mirror replaces the poorly reflecting and rapidly oxidized bronze mirrors used before. At the same time, he perfected all the techniques for making telescope mirrors, techniques which are still used today. Two of his telescopes were used for a long time at the Observatory; the largest one he built, with an 80cm mirror, was used for a century at Marseille.
In 1862, Foucault made the first precise measurement of the speed of light, in the Cassini room.
Le Verrier and the creation of modern meteorology
On the 14th of November 1854, a violent storm destroyed many French warships, including the Admiral’s, which were besieging Sebastopol. Shocked by this catastrophe, Napoleon III asked Le Verrier if it would not be possible to predict the arrival of storms.
Why Le Verrier? Because the study of meteorology had always been considered to fall within the astronomer’s terrain.
The inquest he organized showed that the perturbation responsible for the storm had taken several days to move from the Atlantic to the Black Sea, and that it would in fact have been possible to predict its arrival had it been possible to transmit to a central point meteorological data gathered along its trajectory.
Taking advantage of the development of the electric telegraph, Le Verrier created a national, then international, meteorological service: the meteorological data were measured every morning in many towns and telegraphed to the Paris Observatory, where they were combined to make true meteorological maps of Europe, with lines of equal pressure and information about winds, thereby enabling rough predictions to be made.
This service developed rapidly, and became one of the Observatory’s principal activities, until it flew on its own wings in 1878. It is in fact the ancestor of Météo-France.
Cassini I (1625-1712)
Gian Domenico Cassini, born in Perinaldo on the 8th of June 1625, finished his studies in 1650 and was soon after appointed Professor of astronomy in Bologna. He observed from an observatory belonging to an amateur, the Marquis de Malvasia, and installed in the church of San Petronio a large meridian to replace the one made a century earlier by Danti. Cassini’s notoriety in Europe grew through his work on the Sun, the planets and the Jovian satellites. In September 1668, his published ephemeris for the eclipses of the Jovian satellites was sent to the Royal Academy of Science, and attracted the attention of the astronomers for whom the Royal Observatory had been created in 1667.
From the University of Bologna to the Royal Observatory in Paris
Invited to be a member of the Academy and to direct the Observatory, Cassini arrived in the spring of 1669, and found there, in particular, Picard and Huygens. The pendulum clocks whose movement Huygens had learnt how to regulate, combined with Cassini’s ephemeris, were the principal reason, mainly through the work of Picard, for the rapid development of geodesic astronomy and cartography in France.
While continuing to collaborate with Picard, La Hire and Richer, Cassini continued his own research using the long focal length lenses he had brought from Italy; he was soon able to order them from famous opticians in Rome and also in France.
The ring and satellites of Saturn
Cassini’s main successes in this domain pertain to the Saturnian system with the discovery, in 1671 and 1672, of two new satellites; the first had been discovered by Huygens. In 1675, he realized that the ring, whose existence had enabled Huygens to explain the apparently strange shape of the planet, has two differently coloured regions. In 1684, Cassini discovered two new satellites.
These discoveries were the reason for the name given to the NASA/ESA orbiter launched in 1997.
Cassini I (to distinguish him from his three successors) died in Paris on the 14th of September 1712; his tomb-stone is in the church of Saint-Jacques du Haut-Pas. His name is still associated with a lunar map published in 1679, based on observations made with two engravers, Leclerc and Patigny, and which the latter had created under his supervision. This map was virtually unmatched until the middle of the XIXth century.
Jean Picard was born on the 21st of July 1620 in La Flèche where he went to school at the Collège Henri IV, and we know of his observations of the 1645 solar eclipse, done in Paris under the supervision of Gassendi. We also know that he observed, still under Gassendi’s supervision, a lunar eclipse in 1646 another in 1647, as well as an occultation of Jupiter by the Moon.
Right from the beginnings of the Royal Academy of Science in 1666, Picard was a member, together with three other astronomers, Auzout, Huygens and Roberval. The following year, these members of the Academy established a work programme, which included the determination of the size of the Earth. Picard was assigned to this task, and for it he created three instruments:
• a telescope mounted on an 18 inch radius quadrant, equipped with a micrometer designed together with Auzout
• an 18° sector with a radius of 10 feet
• a spirit level with two telescopes.
As an experienced field worker who could make measurement even while travelling, he continued his observations in Paris, in 1669 and 1670; Picard published the results of this work in 1671 in his book "Mesure de la Terre" (the Measurement of the Earth). He adopted and generalized the so-called triangulation method, which had been studied and tried in Holland. When he went to Denmark in 1671, Picard used the eclipses of the jovian satellites to link the meridian of Tycho Brahe’s observations to that of Paris by a determination of the longitude; Cassini made the corresponding observations in Paris.
In 1668, Colbert asked the members of the Academy to create a general map of France. A first attempt to map the region around Paris was made, and enabled various methods to be tested under the supervision of Roberval and Picard. Starting 1669, an engineer worked on site; the map was finished in 1674 and published in 1678. In the years between 1676 and 1681, Picard and La Hire, during many trips around the west coast of France, and Cassini in Paris or on the Mediterranean coast, worked out the general shape of Louis XIV’s kingdom. When this work was finished in 1682, Cassini continued the triangulation of the Royal Observatory’s meridian; this would constitute the basis on which was established, in the XVIIIth century, the map of France.
From 1674 on, Picard went frequently to Versailles where Louis XIVth’s château was being built, for the supervision of the levelling work needed so that the pools and fountains could be supplied with water. The creation of two artificial pools in the Bois d’Arcy and in Trappes is due to Picard. His text on levelling appeared, thanks to la Hire, in 1728.
The inspection of the cellars by the revolutionaries
Staring on the 16th of July 1789, the Observatory is plunged into the revolutionary fever. At six o’clock in the morning, three hundred arment men descended on the place and mounted guard all the exits. Cassini IV, who since March 1787 no longer lived at the Observatory because of the restoration of the ageing Perrault building, was searched for in this house in the rue Maillet (now called the rue Cassini) by an officer accompanied by six men.
He was shown the warrant to make a detailed search of the Observatory, suspected of hiding flour, gunpowder and guns. No corner of the building was spared, leaving only the enormous network of galleries, a leftover from the old quarries situated 25 m underground.
As soon the construction work for the Observatory had been terminated, a Mariotte thermometer had been placed there on the 24th of September 1671, and showed that the subterranean temperature was constant. On the 7th of July 1783, another extremely sensitive thermometer, the so-called cellar temperature thermometer, built by Mossy under Lavoisier’s supervision, was installed: it registered 11.42°.
- The so-called Lavoiser thermometer, in the cellars of the Observatory.
- From Œuvres de Lavoisier, vol. III.
Cassini, lighting the way with a flaming torch, led a hundred men and commissioners into the depths of the cellars. The door of the room housing Lavoisier’s thermometer (the name given to it by Arago a century later) was forced open and to Cassini’s great displeasure, a very curious experiment concerning the mouvements of a magnetised needle which had been going on for years, was knocked over.
Cassini dragged them right up to the Montrouge quarries, via tunnels flooded for most of the year: Avancez, Messieurs, avancez ; vous venez chercher des poudres et des farines sous l’eau, convenez que la cachette ne serait pas mauvaise … ! (Come on, gentlemen, come on, you are looking for gunpowder and flour, you must admit that the hiding place is pretty good!)
Finally, their only trophy, which they took, was the canon ball which was used as a counterweight for the roasting spit, found in the Cassini’s kitchen!! It was triumphantly exhibited in a net and carried through the Paris streets by a cohort of citizens. The "boulet de l’Observatoire" - the Observatory’s cannon ball - was thus the second success of the Revolution, after that of the fall of the Bastille.
Cassini exclaiment ironically: The roasting spit and its ball, that is my arsenal!
After this visit, Cassini declared that he would never again go underground; he henceforth refused to even carry the keys.
From this episode, we still have the fractured door and a pot, possibly abandoned there by one of the armed men…
Arago and Biot in the Baleares: the extension of the measurement of the Paris meridian
The first measurement of the meridian arc between Dunkirk and Barcelona along the Paris meridian was made between 1792 and 1799 by Delambre and Méchain. However, the latter was not satisfied with the results of this first expedition; he suggested that the measured meridian be really symmetrical about the mean latitude of 45°. C’est pourquoi il y a nécessité à prolonger la mesure jusqu’aux Baléares (The measurements should thus be extended up to the Baleares (increasing the angular size of the arc to 12°).
The expedition was brought to fruition, in the period 1806-1808, by two other persons, Jean-Baptiste Biot and the youthful François Arago. The context was not too favourable: the Napoleonic wars had created much anti-french feeling in Spain. Seventeen triangles spanning the islands were nevertheless set up (Majorca, Minorca, Ibiza and Formentera).
- Extension to the Balearic islands of the measurement of the meridian in France by MM. Biot and Arago.
- Œuvres complètes de François Arago. - Paris : Gide ; Leipzig : T. O. Weigel, 1854-1862.
The work done in the Baleares led to no change in the definition of the metre, which had been the objective of the various operations carried out along the Paris meridian. The metre had been defined by the National Convention in 1795, as being the forty millionth part of a quarter of the terrestrial meridian.
On the other hand, a man and a scientist, François Arago, would emerge. His stay in the Balearic Islands was quite the opposite of the languor associated today with this area; it was more in the line of the romantic adventures of a Jean Valjean or an Edmond Dantès.
Disguised as a sailor
Already in the autumn of 1806, in Valence, Arago nearly lost his life in an ambush prepared by the fiancé of a young lady with whom he had - according to him - just dined the preceding evening. After Napoleon crossed into Spain, things became more serious.
Starting on the 27th of May 1808, the arrival at Palma de Majorque of one of Napoleons military officers led to a general uprising. Arago, with his luminous signals, was immediately suspected of sending messages to the French army. He managed to evade the mob which had come for him, by disguising himself as a sailor. He was not recognized because had a perfect command of the Majorcan language. In the end, the authorities decided to imprison him in the château of Belver.
This time, on the way to the château, he was recognized by the crowd, and had to run to his prison to save himself from being lynched. He got away with just a jab from a dagger in his thigh! .
He finally escaped on the 28th of July 1808 and took ship to Algiers on the 3rd of August. On the 13th of August 1808, with the help of a false passport in the name of a commercial traveller obtained by the French consulate, he left for Marseille. On the 18th of August 1808, the ship was boarded and searched by a Spanish privateer with the pretext that they had violated the blockade of the French coast. The judge who examined his case did not recognize who he was, since he spoke, sometimes with the Valencian accent, sometimes with that of Ibiza, and sometimes he spoke in French.
The mistral struck
It was only on the 28th of November that the ship was authorized to continue to Marseille. Unfortunately, an exceptionally violent mistral forced the boat towards Bougie (today’s Béjaïa, 180 km east of Algiers). Arago decided to go on foot to Algiers, where he arrived on the 25th of December 1808.
He stayed there until the 21st of June 1809; he was finally authorized to leave for Marseille after the consulate had paid three hundred thousand francs to the Dey of Algeria. Off the coast of Marseille, an English frigate stood in their way. Nevertheless, the captain carried on anyway, and managed to enter the port of Pomègues island.
It had taken him almost 11 months to go from Algeria to Marseille! Soon after, on the 18th of September 1809, he was elected to the Academy of Science, replacing Joseph Jérôme Lalande. He was then 23 years old.
Charles Delaunay and Le Verrier’s interregnum
Urbain Le Verrier, appointed Director of the Observatory in 1854, was without doubt a very great scientist and a good administrator, but he was also a veritable and intolerable dictator: during the thirteen years of his reign, not less than 68 astronomers left the Observatory: some resigned and others were dismissed.
Finally, exasperated, all the astronomers of the Observatory sent a collective resignation to the Ministry.
Le Verrier reacted badly, was dismissed, and was replaced on the 3rd of March 1870 by his sworn enemy, Charles Delaunay (1816-1872).
- Letter of Emile Segris, Minister of Public Education, relieving Le Verrier of his post - Paris, February 5th 1870 (Observatoire de Paris, Ms 1070-32)
Delaunay was an excellent astronomer, and full of good will, but it was not easy to calm his over-excited troops - war was declared. He would not have the time to re-organize the Observatory, since he drowned in Cherbourg harbour on the 5th of August 1872.
Nobody was keen to replace him, and Le Verrier, who had taken certain precautions vis-à-vis the political authorities was reappointed at the Observatory the following year, flanked, however, by a council which was supposed to keep an eye on him. His second reign went down better than the first one, but was short since he died in 1877.
La Caille at the Cape of Good Hope : 1751-1752
Nicolas-Louis de La Caille was born in 1713. He joined the Paris Observatory in 1736, and left it in 1742 in order to teach mathematics at the Mazarin College, where an observatory was built for him. There, he began to assemble a stellar catalogue, but concluded that for a full coverage of the whole sky, it would be necessary to travel to the Southern hemisphere.
127 nights to survey the Southern sky
He obtained the necessary finance and authorizations, and sailed for the Cape of Good Hope in 1750. He was very well received by the governor, who had an observatory built for him. During 127 tireless nights of observation, he carried out the first ever systematic survey of the Southern sky. He catalogued 9 766 stars, most of them previously unlisted, as well as numerous nebular structures.
Before La Caille’s work, large sections of the southern sky had never been mapped. He filled these hitherto virgin regions with 14 new constellations, named after scientific or artistic instruments.
14 new constellations
These names are still in use to-day: Sculptor, Fornax, Horologium, Reticulum, Caelum, Pictor, Pyxis, Antlia, Octans, Circinius, Norma, Telescopium, Microscopium and Mensa (the latter referring to Table Mountain, which overlooks Cape Town). Furthermore, La Caille divided into three parts the enormous constellation of Argo, which thus became Puppis, Carina and Vela.
Making simultaneous observations with Lalande in Berlin, La Caille obtained the best values of his time for the parallax (the distance) of the Moon, Mars and the Sun.
When he returned to Paris, La Caille had made by his friend Anne-Louise Le Jeuneux a magnificent planisphere of the southern sky: this planisphere is now at the Observatory. He took up his teaching duties and continued observing, but died prematurely in 1762.
The meter : a republican measurement
« Double standards! » the very symbol of inequality. In answer to the 1789 lists of grievances, but also to those of the 1576 estates general, the Revolution required the Académie des Sciences in 1790 to standardize the system of weights and measures. The new, and unified, system of measurements should be egalitarian, a means to facilitate exchange and to ensure the integrity of commercial operations.
The ten millionth fraction of a quarter of the terrestrial meridian
A requirement for equality, but also for universality, as proclaimed in Condorcet’s famous slogan, the metric system must be « pour tous les hommes, pour tous les temps » (« for all, for ever »). For this, the meter would be eternal since based on the Earth, itself eternal. It was defined by the law of 18th germinal year III (7th of April 1795): the meter was thus defined as the ten millionth fraction of a quarter of the terrestrial meridian.
Two members of the Academy of Sciences were assigned the task of measuring the length of a specific meridian arc, namely that which crosses France from end to end, from Dunkirk to Barcelona, and traversing the meridian axis of the Paris Observatory. This arc subtends slightly more than 8° in latitude. And so Jean-Baptiste Joseph Delambre (1749-1822) and Pierre François André Méchain (1744-1804) set off in the summer of 1792.
Their trip would take all of 7 years. The adventure of the meter, which began with the capture of the Tuileries by the « sans-culottes » on the 10th of August 1792 and the birth of the first République on the 21st of September 1792, would end with Napoleon Bonaparte’s coup d’état on the 18th Brumaire year VIII (9th of November 1799). The meter is thus wholly a «republican measurement».
An accuracy of several seconds of arc
Over 90 geodetic triangles were set up along the meridian, like the veritable backbone of France. These triangles were measured to an accuracy of a few seconds of arc, thanks to a new instrument created by the chevalier Borda (1733-1799), the repeating circle. In the meantime, a preliminary meter was defined on August 1st 1793 using the measurements of the meridian made in 1740 by Nicolas Lacaille (1713-1762): its value was provisionally fixed at 443,44 “lignes” in the old system.
Finally, the value of the meter was officially defined as 443,296 lignes by the law of December 10th 1799 (19 frimaire year VIII). However, it was only in 1837 that he French government imposed the metric system, making it obligatory in the whole of France and its colonies from January 1st 1840.
Wireless transmission of time signals
Gustave Ferrié (1869-1932), who would be nominated commander, was one of the early leaders in the use of this new technology: in December 1903, he arranged for the installation of a military radiotelegraph unit on the Eiffel Tower: it was composed of the underground laboratory of the Champ de Mars, and an antenna at the top of the tower. The first attempts to establish wireless contact over distances in excess of 400 km were a complete success.
During the same period, Guillaume Bigourdan (1851-1932), an astronomer at the Paris Observatory, was experimenting with this new technique for the purpose of transmitting time signals; his projects were presented at the June 27th 1904 meeting of the Académie des sciences, and were published in the Comptes Rendus.
Determination of longitudes
On the 13th of May 1908, after numerous successful experiments the wireless transmission of time signals, both in France and elsewhere, the Bureau des Longitudes expressed the wish that a daily time signal be transmitted from the Eiffel Tower, to facilitate the determination of longitudes. The organization of this service was put under the joint responsibility of the Paris Observatory, directed since 1908 by Benjamin Baillaud (1848-1934), and the military radiotelegraph unit of the Eiffel Tower under the command of Ferrié.
Since May 23rd 1910, this service operates regularly: every day, at midnight, a time signal, generated by the Observatory, is sent out; since November 21st, a second time signal is sent out every working day at 11h.
The transmission of the time signals involves considerable background work on the quality of the clocks, the transmitters and receivers of the signals, and on problems related to the propagation of radio waves.
At the same time, astronomical observations for the determination of time were intensified; increasingly modern clocks were installed at the Observatory.
A time signal every day at midnight
Quite rapidly, mariners, major users of diverse time signals in order to find their position at sea, noted discrepant values of one or two seconds. This was one of the reasons for creating a central organization for the establishment of a unified time.
The first speaking clock
Towards the end of 1929, the Paris Observatory, under its Director Ernest Esclangon (1876-1954) was faced with the increasing desire of the general public to know the «exact» time : the one and only available telephone line was all too often saturated. He envisaged as a consequence a completely automatic system: a speaking clock.
The technique that was adopted was inspired by the cinema, which was just starting to « talk »: it involved recording the human voice photographically, with an optical system using a photoelectric cell. This was considered to be more reliable in the long term than using phonographic discs.
A master clock and a speaking machine
The speaking clock thus has two key parts: a master clock which generates seconds (short “strokes”) and the «speaking machine» activated by the clock.
The speaking machine is composed of a rotating drum whose motion is synchronized by a master clock; this drum carries ribbons of photographic paper with the sound recordings, including the «on the 4th stroke it will be exactly ». As the photoelectric cell scans the ribbon, an electric current is produced, which is then transformed into sound via the telephone.
The time strokes are furnished by a chronometer which is installed underground at the Paris Observatory (at a constant temperature and pressure, shielded from all vibrations). In 1933, the master clock was still a pendulum clock.
Au 4ème top il sera exactement…
The time was given by a famous announcer of the radio Poste Parisien, Marcel Laporte, so-called Radiolo. On February 14th 1933, when the first speaking clock in the world was inaugurated, ODEON 8400 received 140 000 telephone calls, of which only 20 000 could be satisfied in spite of the 20 telephone lines reserved for this purpose: what a success!
In 1991, after two different improvements, a totally new speaking clock was put into service, it is completely electronic: the recorded text is stored in a computer memory. Two different voices alternate: one masculine, the other feminine. With this technology, extra information – the date (day, day of the month, month, year) - can be given out.
Hipparcos, the first astrometric satellite
In 1965, just 7 years after the first Sputnik was launched, Pierre Lacroute, who was at that time the Director of the Strasbourg Observatory, together with Pierre Bacchus, initiated the idea of using a satellite to make astrometric measurements of unprecedented accuracy. In effect, making the measurements from space eliminates the deleterious effects of the atmosphere, of telescope distortions induced by terrestrial gravity, and frees one from irregularities of terrestrial motion.
a revolution for astrometry
The CNES (National Centre for Space Research), to which P. Lacroute presented a detailed proposal which already had all the essential elements of what became Hipparcos, recognized the importance of the project: “This project, if realized, will revolutionize astrometry. Virtually all parts of astronomy will be concerned”. Hipparcos was selected by the ESA (European Space Agency) in 1980 after a detailed scientific prospective seconded by international conferences and several in-depth technical studies.
The satellite was launched on August 8th, 1989 and, in spite of a defective orbit, it managed to observe, up to March 1993, the 118 000 stars foreseen in its program. Through this completely innovative project, Europe has emerged as a pioneer in space astrometry.
Precision astrometry for the astrophysics
With Hipparcos, astrometry has become a major astrophysical tool: the accuracy with which distances and motions have been measured has enabled a detailed study to be made of the stars in the solar neighborhood, leading to numerous studies both on stellar physics and on our galaxy. Suffice it to quote the determination of the luminosities and the ages of a large variety of stars; a much improved understanding of stellar structure and evolution; the detailed study of stellar motions in the solar neighborhood, as well as in the whole of our Galaxy, highlighting the distribution of dark matter, the rotation of the spiral arms, or the relics of past collisions.
The success of Hipparcos induced scientists to propose, as early as 1992, a much more ambitious project: Gaia. Gaia was launched on December 19 2013; it is measuring a billion celestial objects with an accuracy 100 times better than Hipparcos; definitive results in 2022!
The polarimeter and astrophysics
In 1808, the young physicist Étienne-Louis Malus (1775-1812), while observing the reflection of the Sun on the windows of the Luxembourg Palace, discovered a new property of light: polarization. Natural light is made up of oscillations in planes uniformly distributed around the direction of propagation, but as a consequence of reflection and other phenomena, the oscillations can be constrained to a specific plane: this phenomenon is called polarization.
Arago rapidly became a specialist in polarization, and built in 1811 a small instrument which could show whether light was polarized or not: this was the polarimeter. With it, he observed the light emitted by an incandescent solid, such as iron under high temperatures, or by a very hot liquid such as melting platinum, and realized that in both cases the light is polarized if one looks obliquely at the surface. He then observed the Sun with his device, and noted that the light emitted by different parts of the surface, and in particular its edges, is not polarized. He deduced that the solar surface is neither solid nor liquid, and so must be an incandescent gas.
Physics in astronomy
This was the first time that one could say something about the nature of a celestial body just by observing it from a distance. In the words of one of his successors at the head of the Observatory, Félix Tisserand, Arago had thus introduced « physics into astronomy », and so created a discipline whose name emerged only much later: l’astrophysique.
Fresnel at the Observatory
In September 1815, François Arago received at the Observatory a letter from someone unknown to him : the letter included the following words:
I think that I have found the explanation and the theory of the colored fringes which can be seen in the shade of bodies illuminated by a point source of light. The results of my calculations are confirmed by observation. [However, to confirm these observations, I need instruments which can only be found in Paris. Before engaging the funds, I would like to be certain that the effort is not useless, and that the law of diffraction has not already been established by sufficiently precise experiments.
- Augustin Jean Fresnel: engraving and drawing by Ambroise Tardieu from life (1825)
- Observatoire de Paris
The name of the unknown person was Augustin Fresnel. He was a young engineer of the department of « Ponts et Chaussées » : having expressed his hostility to Napoleon after the latter’s return from Elba, was confined to house arrest in a small village not far from Caen. He had the time to do whatever interested him, especially physical optics, and he managed to explain diverse optical phenomena via the wave theory of light, which he was in the process of developing.
The law of colored fringes
A month later, Fresnel sent to the Academy of Sciences a note about this theory. Arago, who was assigned as rapporteur, was very impressed. After Napoleon abdicated, Fresnel was freed and Arago wrote to his head, François Marie Riche de Prony (1755-1839), asking him allows Fresnel to work with him at the Observatory. The request was accorded, since Prony could refuse nothing to Arago who was famous for his work in optics.
The wave theory
And so Fresnel came to the Observatory, and began a very fruitful collaboration with Arago. Fresnel was both a very creative theorist and an excellent experimenter. Arago was less theoretically inclined, but on the other hand had considerable experimental skill. Fresnel was thus able to create his masterpiece. Arago did contribute, but generously left all the credit to his young colleague.
Arago, Fresnel and light-houses
Light-houses have existed since Antiquity. Until the XVIIIth century, a fire was lit on top of a tower, but this was somewhat random and extremely expensive. The first lighthouse optics appeared towards 1770 ; they were simply made up of silvered mirrors, which tarnished rapidly and were inefficient.
The Commission for light-houses
In 1811, a Commission for light-houses was created in order to improve the situation, but not much was done until Arago was nominated as its president in 1819. He chose Fresnel to be the secretary. Arago wrote :
This famous scientist considered first that one could use large lenses to light up our coasts; they could be made up of small fragments…. No sooner said than done: various subtle experiments also led to the construction of a lamp with many concentric ridges which was at least twenty times brighter than the best ordinary lamps …. These remarkable results were obtained by combining Fresnel lenses with multiple lamp.
One of the first Fresnel echelle lenses has been conserved at the Observatory, where Fresnel and Arago co-invented the oil lamp with concentric wicks. These items were rapidly adopted and perfected by industrialists, and Fresnel, who became towards 1824 the effective leader of the Light-house Commission, presented an ambitious program involving 58 light-houses along the French coast.
Multiple lamps and echelle lenses
His optics is so successful that his entire program was realized in 1850. It is adopted even abroad. Thanks to these light-houses, the number of shipwrecks decreased dramatically, going in France from 163 per year around 1820 to 39 per year ten years later.
The beginnings of the Royal Observatory
After the passage of the great comet of 1664, Auzout (1622-1691) suggested to Louis XIV, in an epistle dated 1665 and included as a preface to his description of that comet, that a major observatory be created: « Sire, in view of the brilliance of your Reign and the reputation of France, we trust that your Majesty will provide a suitable place from which it will be possible to make all kinds of celestial observations, and that your Majesty will enable it to be equipped with the instrumentation necessary to do this […] and this is maybe the reason why there is no Kingdom in Europe whose geographical maps are so erroneous and where locations are so uncertain. »
Colbert, a veritable protector of the sciences, the humanities and the arts (appointed in 1664 as the surintendent general for buildings and factories, and in 1665 as general financial comptroller), conceived the project made up of a structure which would integrate in one place everything concerning science: he wanted the members of the new society of scholars created in 1666 and baptized soon after "the Royal Academy of Sciences" to have an observatory to house meetings «which would exceed in importance, in beauty and in comfort the observatories in England, in Denmark and in China »
To exceed in importance, in beauty and in comfort
- Visit of Louis XIV to the Academy of Sciences: in the background can be seen the Observatory in the process of being built. - Engraving by Cl. Duflos, based on a picture by Sébastien Leclerc, about 1730
- Observatoire de Paris
In spite of his involvement in the projects of the Academy and of the Observatory - he was one of the first thirteen members of the Academy and he probably sent the plans to the architect Claude Perrault – and his work with Jean Picard to improve the quadrant, Adrien Auzout is nevertheless relatively unknown. He resigned from the Academy of Sciences and left France for Italy towards the end of 1668.
In the final analysis, one of the most beautiful images that has survived Auzout is the dedication made by the great Cassini himself in 1668 in his text Ephemerides Bononienses mediceorum siderum : « Carissimo Doctissimoque D.(Domino) A. Auzotio exterranea haec rudimenta mittit auctor. » (The author offers the great scientist Monsieur A. Azout these celestial elements). The book which has this dedication is conserved in the library of the Paris Observatory.