1. 1 Transits of Venus: the history Luxembourg, January 2004 Jean-Eudes Arlot IMCCE/observatoire de Paris-CNRS

2. 2 The transit of June 8, 2004 On June 8, 2004, the planet Venus will pass in front of the Sun. Nobody alive today has seen such an event. Why this event retained the attention of the astronomers in the past? 5h40 UTC 11h05 UTC

3. 3 The transit of Venus In the history of humanity, the observation of the transits of Venus was one of the most important mean to measure the universe. This explains all the efforts made in the past to observe this event, even in the difficulties.

4. 4 Looking at the universe Look at the sky? Is is possible, just looking at the stars, to know their distances to the Earth?

5. 5 Measuring distances Parallax or triangulation or how to measure a distance to somewhere without going there?

6. 6 Measuring a distance with angles : the parallax the triangulation a b A ? B ? c? c a’ C Base

7. 7 The parallax of the Sun To measure the distance from the Earth to another body of the solar system, we will use the parallax effect from two different sites on Earth a R  Terre One measure  and R to calculate a R = 6400 km and a ~ 150x10 6 km Then  ~ 10" ==> difficult to measure

8. 8 Measuring distances Measure of the distance Earth-Sun or measure of the radius of the Earth? This shows the necessity to have a good model before measuring anything

9. 9 The parallax The parallax method allows to measure only distance to close bodies to the Earth since the base of the triangle may not be larger than the Earth. The Sun is too far: only the Moon, Mars and Venus are accessible. The Earth and the Moon at the relative scale.

10. 10 The parallax of Mars (1672) Cayenne   Paris R D Mars First measures of distance using parallax: Cassini and Richer  s = 9.5" ( a = 138x 10 6 km) Flamsteed  s = 10" ( a = 130x 10 6 km) First measures of distance using parallax: Cassini and Richer  s = 9.5" ( a = 138x 10 6 km) Flamsteed  s = 10" ( a = 130x 10 6 km)

11. 11 The parallax of Venus : Halley’s method a a b b c c The relative positions of the chords provide the parallax of Venus. The solar disc is used as a reference frame. The measure of the length of the chords is replaced by the measure of the duration of the transits. The measure of a duration is more accurate. But remember that the Sun itself has also a parallax.

12. 12 Kepler’s first law Kepler (1571-1630) Each planet describes an ellipse of which the Sun is at one of the focus (1605).

13. 13 Deuxième loi de Képler The surfaces described by the radius-vector planet-Sun, are proportional to the time used to make them (Astronomia Nova, 1609);

14. 14 Kepler’s third law The semi_major axis a and the period of revolution T are linked by a 3 /T 2 =constant for all the planets (1618).

15. 15 The astronomical unit The third law of Kepler implies that it is sufficient to measure only one distance in the solar system to know all the distances between the planets and the Sun, especially the distance Earth-Sun, known as astronomical unit. Let us measure the distance from Earth to: Mars, Venus or … Eros. The astronomical unit will allow us to measure the distance from Earth to the nearest stars.

16. 16 Measuring distances The distance to stars: The base of the triangle will be 300 millions kilometers: the diameter of the orbit of the Earth thanks to two observations made after a 6 months interval.

17. 17 The transit of Venus: a rare event Three conditions : The Earth and Venus should have the same heliocentric longitude The Earth and Venus should have the same heliocentric longitude The frequency of this configuration is the synodic revolution ofThe frequency of this configuration is the synodic revolution of Venus (RS). The Earth, Venus and the Sun should be on a same line (Venus should not The Earth, Venus and the Sun should be on a same line (Venus should not be above or below the Sun as seen from the Earth. The frequency of this configurartion is the draconitic revolution The frequency of this configurartion is the draconitic revolution of Venus (RD). due to the size of the Sun, the Earth and Venus, the event occurs even the two conditions are not completely satisfied. due to the size of the Sun, the Earth and Venus, the event occurs even the two conditions are not completely satisfied. 1  2  3333 + these two conditions may not be satisfied very often

18. 18 A transit as seen from space Sun planet Below the shadow cone Axis of the shadow cone Shadow cone Penumbra cone summit of the cone of penumbra of penumbra summit of the shadow cone Plane of Bessel Sun (1) Central transit (1) (2) Non- central transit (3) Partial transit (2) (3) (4) (4) No transit

19. 19 Observation of a transit: what we see Duration of a transit of Venus 5 to 8h Duration of a transit of Mercury 3 to 8 h t 1, t 4 : external contacts t 2, t 3 : internal contacts The external contacts are very difficult to observe t1t1 t 1 :1 e contact t2t2 t 2 :2 e contact t3t3 t 3 :3 e contact t4t4 t 4 :4 e contact t 1 - t 2 : entrance of the planet t 3 - t 4 : exit of the planet

20. 20 Who, first, has the idea to observe the transits ? Ptolemeus noted this possibility in his system A transit of Mercury is mentionned in 807 –but no transit occurs at that time ! –nearest dates : 23/04/806 and 24/10/809 Copernic tells that transits are possible –but invisible because of the size of the planet Kepler predicts a transit of Mercury for May 29 1607 –he observed on May 28. –he noted a black spot on the Sun and announced an observation –in fact no transit occurs at that date –nearest dates : 01/11/1605 and 03/05/1615 The spots of the Sun were observed by projection after 1610

21. 21 First observation of a transit: Gassendi in Paris First observation of a transit Use of a darkroom ( and may be a lens ) Observation from Nov 5 (bad weather on 5 and 6) Starting from the sunrise on Nov 7, Gassendi saw a black spot –Measured diameter of Mercury : 20" (true value : 10") Error of 5h from Kepler’s predictions Three other observations in Europe Transit of Mercury on Nov 7, 1631 Calculation for Paris hourSun (true solar time) 2e contact5h 06 -21° 3e contact10h28+22° "Le rusé Mercure voulait passer sans être aperçu, il était entré plus tôt qu'on ne s'y attendait, mais il n'a pu s'échapper sans être découvert " Mercurius in sole visus et venus invisa Parissiis anno 1631.

22. 22 Visibility of the Mercury transit of 1631

23. 23 First observations of a transit of Venus: J. Horrocks First observation of a transit of Venus Use of a darkroom with a refractor Observations on Saturday 3  nothing visible On Sunday 4 he observed from the morning, through clouds He stopped observing for religious obligations At 3h15 he continues his observations and the weather became fair Transit of Venus on Dec 4. 1639 local timeSun 2e contact15h15+ 4° 3e contact21h30- 47° sunset15h50

24. 24 Observation of J. Horrocks (Venus in Sole Visa) He made three measures in a hurry before the sunset tdistance (") 3h15864 3h35810 3h45780 3h50sunset Diameter of Venus: 1' 16"

25. 25 Latin text from Horrocks

26. 26 Observations of W. Crabtree Observations made at Manchester Cloudy until 3h35  10 min of observation possible only ! Amazed by the transit, he made no measure ! Painting of F. M. Brown, visible at the City Hall of Manchester

27. 27 Visibility of the Venus transit of 1639

28. 28 Transits during the XVIIIth century Longitudes are not yet well known. Clocks are not good time keepers. Traveling is slow (sailing). Voyages are very expensive. Nobody has never observed a transit of Venus. Two methods of measure of the parallax : Method of Halley : The durations of the transits are compared => no problem with longitude. Method of Delisle : The times of contacts are compared => more observations but longitudes have to be known. Two methods of measure of the parallax : Method of Halley : The durations of the transits are compared => no problem with longitude. Method of Delisle : The times of contacts are compared => more observations but longitudes have to be known.

29. 29 The transit of June 6, 1761 for this first transit, all the astronomical communiuty was ready to observe. for this first transit, all the astronomical communiuty was ready to observe. voyages were difficult and the 7-years war (a world war) set ablaze seas and colonies. voyages were difficult and the 7-years war (a world war) set ablaze seas and colonies. the coordination of all the astronomers was made by the french astronomer Joseph-Nicolas Delisle (1688-1768) who sent his mappemonde to more than 100 astronomers in the world. the coordination of all the astronomers was made by the french astronomer Joseph-Nicolas Delisle (1688-1768) who sent his mappemonde to more than 100 astronomers in the world. General circonstances first contact of the penumbra : 1h 55m 17.1s first contact of the shadow : 2h 13m 9.7s Maximum of the transit : 5h 19m 16.1s Last contact of the shadow : 8h 25m 20.1s Last contact of the penumbra : 8h 43m 12.6s General circonstances first contact of the penumbra : 1h 55m 17.1s first contact of the shadow : 2h 13m 9.7s Maximum of the transit : 5h 19m 16.1s Last contact of the shadow : 8h 25m 20.1s Last contact of the penumbra : 8h 43m 12.6s

30. 30 Le passage du 6 juin 1761 Projection de Hammer

31. 31 The transit of June 6, 1761 the Académie Royale des sciences organized three campaigns of observation. the Académie Royale des sciences organized three campaigns of observation. Two of these voyages took oplace in countries allied of France. the one of César-François Cassini de Thury (1714-1784) in Vienna who observed the one of César-François Cassini de Thury (1714-1784) in Vienna who observed the transit with the archduke Joseph (successful observation). the transit with the archduke Joseph (successful observation). the one of the Abbot Jean-Batiste Chappe d'Auteroche (1728-1769) to Tobolsk in the one of the Abbot Jean-Batiste Chappe d'Auteroche (1728-1769) to Tobolsk in Siberia invited by the empress Elisabeth I (successful observation). the one of Alexandre Guy Pingré who went to Rodrigues Island (north of the one of Alexandre Guy Pingré who went to Rodrigues Island (north of Madagascar), Thanks to the compagnie des Indes (observation partially successful). a fourth astronomer, Guillaume Joseph Hyacinthe Jean-Batiste Le Gentil de a fourth astronomer, Guillaume Joseph Hyacinthe Jean-Batiste Le Gentil de La Galaisière (1725-1792), left by sea in order to observe the transit in Indies at Pondichéry. Unfortunately the city of Pondichéry was taken by the English and he Saw the transit from the ship, unable to make a measurement; he decided to wait until the next transit in 1769 At last Joseph-Jérôme Lefrançois de Lalande (1732-1807 ) observed from Luxembourg Palace in Paris. The French

32. 32 The voyage of Chappe d’Auteroche The travel of Chappe d’Auteroche to Tobol’sk

33. 33 Le voyage de Le Gentil Guillaume Joseph Hyacinte Jean Baptiste Gentil de la Galaisière (1725-1792). Départ de France le 26 mars 1760 et arrivée à l'Île de France en mai. Problème pour repartir vers Pondichéry. Sac de Pondichéry par les Anglais en janvier 1761. Départ pour Mahé en mars 1761. Temps calme ! Arrivée le 24 mai : occupation anglaise  Demi-tour vers l'Île de France Le 6 juin : temps magnifique … en mer. Passage observé, sans valeur astronomique Il décide de rester dans l'océan indien pour des explorations géographiques, d'histoire naturelle et d'attendre le passage de 1769.

34. 34 Relation of the voyage of Le Gentil

35. 35 Le voyage de Pingré Alexandre-Gui Pingré (1711-1796), astronome français Astronome, théologien, latiniste, historien, poète… Envoyé à l'Ile Rodrigues par l'Académie Possibilité d'observation entrée et sortie Départ en janvier 1761 ; Navire réquisitionné au Cap. Arrive finalement le 28 mai 1761. Le 6 juin : pluie toute la matinée  entrée manquée. Beau temps pendant le transit. Pluie lors de la sortie ! Arrivée des anglais sur l'Île peu après Retenu sur place pendant 3 mois (étude du milieu naturel) Son navire est attaqué au retour et il est débarqué à Lisbonne ".. nous fûmes réduits à la seule boisson ignoble de l'eau …"

36. 36 The transit of June 6, 1761 The English The english astronomers organized two campaigns far from England to observe the event. Nevil Maskelyne (1732-1811) went to Sainte-Hélène where he was not able to observe because of clouds. Nevil Maskelyne (1732-1811) went to Sainte-Hélène where he was not able to observe because of clouds. Charles Mason (1728-1786), James Bradley and Jeremiah Dixon (1733- 1779) was supposed to observe from Bencoolen (Sumatra). They were not able to make the observation because the French took the city. They observed then at Capetown.Charles Mason (1728-1786), James Bradley and Jeremiah Dixon (1733- 1779) was supposed to observe from Bencoolen (Sumatra). They were not able to make the observation because the French took the city. They observed then at Capetown. John Winthrop, professor in Harvard went to St-John (Terre-Neuve) where « surrounded by billions of insects " he succeeded to observe the last contact of the transit.John Winthrop, professor in Harvard went to St-John (Terre-Neuve) where « surrounded by billions of insects " he succeeded to observe the last contact of the transit.

37. 37 Voyages organized for the transit of 1761

38. 38 Results from the transit of 1761 The number of observers was 120, on 62 sites (S. Newcomb, 1959). The number of observers was 120, on 62 sites (S. Newcomb, 1959). Note that some sites of observations were previously selected (Bencoolen, Pondichéry, Batavia) by Halley in 1716. Note that some sites of observations were previously selected (Bencoolen, Pondichéry, Batavia) by Halley in 1716. The large error is due to: - a bad knowledge of the longitudes of the sites of observation - the black drop effect which decreases the precision of the measurement of the time of the contacts. 8.5" <  < 10.5" Disappointing results : no improvement of the measures from Mars.

39. 39 The transit of Venus of June 3-4, 1769 The organization of the observations for 1769 were made by Lalande in France and Thomas Hornsby in England. The organization of the observations for 1769 were made by Lalande in France and Thomas Hornsby in England. They took benefit from the observations of the transit of 1761. They took benefit from the observations of the transit of 1761. 27 refractors were used, only 3 were used in 1761.27 refractors were used, only 3 were used in 1761. General circonstances First contact with penumbra : le 3 à 19h 8m 31.2s First contact with shadow : le 3 à 19h 27m 6.7s Maximum of the transit : le 3 à 22h 25m 20.3s Last contact with shadow : le 4 à 1h 23m 35.7s Last contact with penumbra : le 4 à 1h 42m 11.2s General circonstances First contact with penumbra : le 3 à 19h 8m 31.2s First contact with shadow : le 3 à 19h 27m 6.7s Maximum of the transit : le 3 à 22h 25m 20.3s Last contact with shadow : le 4 à 1h 23m 35.7s Last contact with penumbra : le 4 à 1h 42m 11.2s

40. 40 Visibility of the transit of 1769

41. 41 The transit of 1769 The French Pingré studied the best sites of observationPingré studied the best sites of observation Le Gentil still in Madagascar, went to Manila, then Pondichéry where a cloud prevents the observation Le Gentil still in Madagascar, went to Manila, then Pondichéry where a cloud prevents the observation Chappe accompanied by Pauly, Noël and Dubois and by two spanish astronomers Vicente de Doz et Salvador de Medina went to California on the west coast of Mexico, near Cape Lucas today named San José del Cabo. Chappe accompanied by Pauly, Noël and Dubois and by two spanish astronomers Vicente de Doz et Salvador de Medina went to California on the west coast of Mexico, near Cape Lucas today named San José del Cabo. the observation by Chappe was successful the observation by Chappe was successful they observed also the lunar eclipse on June 18 1769 to measure the longitude. Unfortunately theu died from an epidemic of typhus except Pauly by looking after the inhabitants they observed also the lunar eclipse on June 18 1769 to measure the longitude. Unfortunately theu died from an epidemic of typhus except Pauly by looking after the inhabitants Pingré and the Comte de Fleurieu, observed the event from Cape François in Saint-Domingue.Pingré and the Comte de Fleurieu, observed the event from Cape François in Saint-Domingue.

42. 42 The transit of 1769 The English Dymond et Wales went to Fort Churchill in the Hudson bay. Dymond et Wales went to Fort Churchill in the Hudson bay. Father Maximilen Hell, with the danish astronomer C. Horrebow and a young botanist Borgrewing, went to Vardö, an island north to Scandinavia. Father Maximilen Hell, with the danish astronomer C. Horrebow and a young botanist Borgrewing, went to Vardö, an island north to Scandinavia. A third group went to the islands of the southern seas as proposed by Thomas Hornsby. This voyage was conducted by a young lieutenant, James Cook, and the observation of the transit was made in Tahiti, -an island discovered two years earlier by Samuel Wallis-, by Charles Green and James Cook. A third group went to the islands of the southern seas as proposed by Thomas Hornsby. This voyage was conducted by a young lieutenant, James Cook, and the observation of the transit was made in Tahiti, -an island discovered two years earlier by Samuel Wallis-, by Charles Green and James Cook. A fourth group, Bayley and Dixon, went to Scandinavia; Bayley observed the transit at Cape North and J. Dixon made the observation on the island of Hammerfest. A fourth group, Bayley and Dixon, went to Scandinavia; Bayley observed the transit at Cape North and J. Dixon made the observation on the island of Hammerfest.

43. 43 The voyage of Cook to Tahiti

44. 44 The voyage of Cook to Tahiti

45. 45 The voyage of Cook to Tahiti

46. 46 The transit of 1769 The Russian The imperial academy of Russia, thanks to the tzarina Catherine II, invited foreign astronomers to observe the transit in Russia The imperial academy of Russia, thanks to the tzarina Catherine II, invited foreign astronomers to observe the transit in Russia The german jesuit C. Mayer, the swiss astronomers Mallet and Pictet and the swedish J. Lexell, L. Euler went in Russia. Observers went also to Yakutsk, Orks and Orenbourg in the south of Oural, to Kola peninsula, and to St Petersbourg. Observers went also to Yakutsk, Orks and Orenbourg in the south of Oural, to Kola peninsula, and to St Petersbourg.

47. 47 Voyages organized in 1769

48. 48 The results from the transit of 1769 The English made 69 observations and the French 34. The English made 69 observations and the French 34. Finally 151 observations, were made from 77 sites. Finally 151 observations, were made from 77 sites. Four observations of the complete transit were made : Finland, Hudson Bay, California and Tahiti. Four observations of the complete transit were made : Finland, Hudson Bay, California and Tahiti. Author(s) Values William Smith 8,6045" (1770) Thomas Hornsby 8,78" (1770) Pingré et Lalande 9,2" et 8,88" (1770) Pingré 8,80 (1772) Lalande 8,55"< P < 8,63" (1771) Planmann 8,43 (1772) Hell 8,70" (1773/1774) Lexell 8.68" (1771) et 8,63" (1772) Author(s) Values William Smith 8,6045" (1770) Thomas Hornsby 8,78" (1770) Pingré et Lalande 9,2" et 8,88" (1770) Pingré 8,80 (1772) Lalande 8,55"< P < 8,63" (1771) Planmann 8,43 (1772) Hell 8,70" (1773/1774) Lexell 8.68" (1771) et 8,63" (1772) The conclusion was that the parallax was from 8,43" to 8,80 ". This was a real improvement regarding the result of 1761 providing a parallax from 8,28 to 10,60".

49. 49 The transits of the XIXth century The longitudes are now well determined (telegraph). The clocks are good time keepers. The travels are faster (steam, Suez channel). The travels are still expensive The photographs appeared (Daguerréotype) The experiences of the XVIIIth century are profitable.

50. 50 The transit of December 9, 1874

51. 51 An example: the observation at St-Paul July 1874 : departure from Paris.July 1874 : departure from Paris. August 9: Suez channel.August 9: Suez channel. August 30: arrival in Réunion IslandAugust 30: arrival in Réunion Island September 22: arrival in Saint-Paul island in a tempestSeptember 22: arrival in Saint-Paul island in a tempest The probability of fair weather was only 8 to 10% In spite of tempest and bad weather, the observation was a success: 500 exposures of the transit were made The voyage of Commandant Mouchez at Saint-Paul.

52. 52 The voyage to Saint-Paul

53. 53 The observation at Saint-Paul

54. 54 The transit of December 6, 1882

55. 55 The transit of 1882 General circonstances General circonstances Premier contact de la pénombre : 13h 49m 3.9s Premier contact de l'ombre : 14h 9m 1.3s Maximum du passage : 17h 5m 58.5s Dernier contact de l'ombre : 20h 2m 58.3s Dernier contact de la pénombre : 20h 22m 55.7s General circonstances General circonstances Premier contact de la pénombre : 13h 49m 3.9s Premier contact de l'ombre : 14h 9m 1.3s Maximum du passage : 17h 5m 58.5s Dernier contact de l'ombre : 20h 2m 58.3s Dernier contact de la pénombre : 20h 22m 55.7s Les Français organisèrent dix missions : une mission à l'île d'Haïti (d'Abbadie), une mission à l'île d'Haïti (d'Abbadie), une au Mexique (Bouquet de la Grye), une au Mexique (Bouquet de la Grye), une à la Martinique (Tisserand, Bigourdan, Puiseux), une à la Martinique (Tisserand, Bigourdan, Puiseux), une en Floride (Colonel Perrier), une en Floride (Colonel Perrier), une à Santa-Cruz de Patagonie (Capitaine de Frégate Fleuriais), une à Santa-Cruz de Patagonie (Capitaine de Frégate Fleuriais), une au Chili (Lieutenant de vaisseau de Bernardières), une au Chili (Lieutenant de vaisseau de Bernardières), une à Chubut (Hatt), une à Chubut (Hatt), une au Rio-Negro (Perrotin, le directeur de l'observatoire de Nice), une au Rio-Negro (Perrotin, le directeur de l'observatoire de Nice), une au Cap Horn (Lieutenant de vaisseau Courcelle-Seneuil), une au Cap Horn (Lieutenant de vaisseau Courcelle-Seneuil), une à Bragado (Lieutenant de vaisseau Perrin). une à Bragado (Lieutenant de vaisseau Perrin). Le Naval Observatory envoya huit expéditions à travers le monde pour observer le passage.

56. 56 Reduction of photographs The measures on the plates were made through macro-micrometers with an accuracy of one micrometer. In France, 1019 plates were taken. All the measurements were made two times by two different persons. In fact more than 500 000 measurements were made.

57. 57 Recapitulation of the measures of the Earth-Sun distance Methoddateparallaxdistance "millions km Mars16729.5 - 10130 -140 Venus17618.3 - 10.6125 - 160 Venus17698.5 - 8.9145 - 155 Mars18628.84149 Flora18758.87148 Mars18858.78150 Venus1874 - 828.790-8.880148.1 - 149.7 Eros19008.806149.4 Eros19308.790149.7 radar19708.79415149.5978 Viking+radar2000149.597870691

58. 58 The recent transits of Mercury The observation of the transits of Mercury provide us some excperience. Here, the transit of May 9, 1970 at the Solar Tower in Meudon observatory. 12 arcseconds

59. 59 Past transits of Venus The fac-simile of the reports of the observations and voyages made during the past centuries are available on a CD-Rom. More than 10 000 pages of rare books were scanned.

60. 60 Art and the transit of Venus The ceiling of the council room of Paris observatory

61. 61 Passage de Vénus (Paris observatory, Prouha, 1878)

62. 62 Transit of Mercury (Balla, 1914)

63. 63 Comic strip: the transit of Venus

64. 64