1. 1 Transits of Venus: the history Garching, March 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 Looking at the universe Look at the sky? Is is possible, just looking at the stars, to know their distances to the Earth?

4. 4 The transit of Venus In the history of mankind, the observation of the transits of Venus was one of the most important possibility to measure the universe. This explains all the efforts made in the past to observe this event, even in the difficulties.

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 the Earth-Sun distance using parallax: Cassini and Richer138 millions km Flamsteed 130 millions km First measures of the Earth-Sun distance using parallax: Cassini and Richer138 millions km Flamsteed 130 millions 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 The parallax of Venus: Delisle’s method Advantages –only one contact is necessary –it increases the number of observations Disadvantages - need of an accurate clock - need of a good knowledge of the longitude of the observer View from center of Earth View from surface tt Instant t Use of the timing of the contacts instead of the duration of the transit

13. 13 Kepler’s first law Kepler (1571-1630) Each planet describes an ellipse of which the Sun is at one of the focus (1605).

14. 14 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);

15. 15 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).

16. 16 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. So, just let us measure the distance from Earth to Mars, Venus or … Eros to know the size of the universe.

17. 17 Measuring distances to stars The knowledge of the astronomical unit will allow us to reach the 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.

18. 18 The history Who has first the idea to observe the transits?

19. 19 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 –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

20. 20 The first observation of a transit: Gassendi in Paris First observation of a transit using a dark room Starting the observation from Nov 5, bad weather on 6, Gassendi saw a black spot at the sunrise of Nov. 7 error of 5 hours from Kepler’s predictions Three other observations in Europe Transit of Mercury on Nov 7, 1631 "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.

21. 21 Visibility of the Mercury transit of 1631

22. 22 First observation of a transit of Venus: J. Horrocks First observation of a transit of Venus use of a darkroom with a lens 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

23. 23 Observation of J. Horrocks (Venus in Sole Visa) He made three measures in a hurry before the sunset

24. 24 Latin text from Horrocks

25. 25 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

26. 26 Visibility of the Venus transit of 1639

27. 27 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.

28. 28 The transit of June 6, 1761 for this first transit, all the astronomical community was ready to observe. for this first transit, all the astronomical community 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. the european population was enthusiasticthe european population was enthusiastic

29. 29 The transit of June 6, 1761 Projection de Hammer

30. 30 The transit of June 6, 1761 Jean-Batiste Chappe d'Auteroche (1728-1769) went to Tobolsk in Siberia (successful observation).Jean-Batiste Chappe d'Auteroche (1728-1769) went to Tobolsk in Siberia (successful observation). Alexandre Guy Pingré went to Rodrigues Island (north ofAlexandre Guy Pingré went to Rodrigues Island (north of Madagascar), (observation partially successful). Guillaume Joseph Hyacinthe Jean-Batiste Le Gentil deGuillaume 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 was unable to make a measurement Some voyages organized by the French

31. 31 The voyage of Chappe d’Auteroche The travel of Chappe d’Auteroche to Tobol’sk

32. 32 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.

33. 33 Relation of the voyage of Le Gentil

34. 34 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 …"

35. 35 The transit of June 6, 1761 Some voyages organized by the English 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 not able to observe from Bencoolen (Sumatra), because the French took the city. They came back to Cape town.Charles Mason (1728-1786), James Bradley and Jeremiah Dixon (1733-1779) was not able to observe from Bencoolen (Sumatra), because the French took the city. They came back to Cape town. 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.

36. 36 Voyages organized for the transit of 1761

37. 37 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.

38. 38 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.

39. 39 Visibility of the transit of 1769

40. 40 The transit of 1769 The French: Chappe in California 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 they died from an epidemic of typhus after taking care of the inhabitants, except Pauly who came back to France with the scientific results. they observed also the lunar eclipse on June 18 1769 to measure the longitude. Unfortunately they died from an epidemic of typhus after taking care of the inhabitants, except Pauly who came back to France with the scientific results.

41. 41 The transit of 1769 The English: Cook in Tahiti A group went to the islands of the southern seas as proposed by Thomas Hornsby.A group went to the islands of the southern seas as proposed by Thomas Hornsby. This voyage was conducted by James Cook, and the observation of the transit was made in Tahiti, -an island discovered two years earlier by Samuel Wallis-.This voyage was conducted by James Cook, and the observation of the transit was made in Tahiti, -an island discovered two years earlier by Samuel Wallis-. The observation was made by Charles Green and James Cook in a place that they named Point Venus, still named Point Vénus today.The observation was made by Charles Green and James Cook in a place that they named Point Venus, still named Point Vénus today.

42. 42 The voyage of Cook to Tahiti

43. 43 The voyage of Cook to Tahiti

44. 44 The voyage of Cook to Tahiti

45. 45 Voyages organized in 1769

46. 46 The results from the transit of 1769 151 observations, were made from 77 sites.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".

47. 47 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.

48. 48 The transit of December 9, 1874

49. 49 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.

50. 50 The voyage to Saint-Paul

51. 51 The observation at Saint-Paul Mouchez made the first daguerrotypes at Saint-Paul showing for the first time images of a transit of Venus

52. 52 The transit of December 6, 1882

53. 53 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.

54. 54 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.

55. 55 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

56. 56 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.

57. 57 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

58. 58 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

59. 59 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

60. 60 The recent history of the transits: 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

61. 61 The transit of Mercury on May 7, 2003 Mercury in front of the Sun as seen With a small telescope and a CCD camera.

62. 62 The transit of Mercury on May 7,,2003 The transit of Mercury as seen by TRACE, the solar space observatory. The parallax is easy to see on this image, due to the motion of the satellite around the Earth.

63. 63 Art and the transit of Venus The ceiling of the council room of Paris observatory

64. 64 Passage de Vénus (Paris observatory, Prouha, 1878)

65. 65 Transit of Mercury (Balla, 1914)

66. 66 Comic strip: the transit of Venus

67. 67