Transits of Venus:

1

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

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
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
Looking at the universe

• Look at the sky? Is is possible, just looking at
  the stars, to know their distances to the Earth?

5
Measuring distances

• Parallax or triangulation
• or how to measure a distance to somewhere
• without going there?

6
Measuring a distance with angles the parallax
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

• One measure p and R to calculate a
• R 6400 km and a 150x106 km
• Then p 10" gt difficult to measure

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
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
The parallax of Mars (1672)
First measures of distance using
parallax Cassini and Richer ps 9.5" ( a
138x 106 km) Flamsteed ps 10" ( a 130x
106 km)
11
The parallax of Venus Halleys method

• 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
Keplers first law

• Each planet describes an ellipse of which the
  Sun is at one of the focus (1605).

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
Keplers third law

• The semi_major axis a and the period of
  revolution T are linked by a3/T2constant for all
  the planets (1618).

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
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
The transit of Venus a rare event
Three conditions

• The Earth and Venus should have the same
  heliocentric longitude
• The frequency of this configuration is the
  synodic revolution of
• Venus (RS).

1 F
2 F

• 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
• of Venus (RD).

these two conditions may not be satisfied very
often
3F

• due to the size of the Sun, the Earth and Venus,
  the event occurs even the two conditions are not
  completely satisfied.

18
A transit as seen from space
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

t1, t4 external contacts t2, t3 internal
contacts
t1 - t2 entrance of the planet
t3 - t4 exit of the planet
The external contacts are very difficult to
observe
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
First observation of a transit Gassendi in Paris
Calculation for Paris hour Sun
(true solar time) 2e contact 5h 06 -21 3e
contact 10h28 22

• 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 Keplers predictions
• Three other observations in Europe

22
Visibility of the Mercury transit of 1631
23
First observations of a transit of Venus J.
Horrocks
local time Sun 2e contact 15h15 4 3e
contact 21h30 - 47 sunset 15h50

• 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

24
Observation of J. Horrocks (Venus in Sole Visa)

• He made three measures in a hurry before the
  sunset

25
Latin text from Horrocks
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 !

27
Visibility of the Venus transit of 1639
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 gt no problem with longitude. Method of
Delisle The times of contacts are compared gt
more observations but longitudes have to be
known.
29
The transit of June 6, 1761

• 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.
• 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
30
Le passage du 6 juin 1761
31
The transit of June 6, 1761
The French

• 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 transit with the archduke Joseph (successful
  observation).
• 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
• Madagascar), Thanks to the compagnie des Indes
  (observation partially successful).
• 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

32
The voyage of Chappe dAuteroche
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
Relation of the voyage of Le Gentil
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
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.
• 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.

37
Voyages organized for the transit of 1761
38
Results from the transit of 1761

• 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.

8.5" lt P lt 10.5"
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.
Disappointing results no improvement of the
measures from Mars.
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.
• They took benefit from the observations of the
  transit of 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
40
Visibility of the transit of 1769
41
The transit of 1769
The French

• Pingré studied the best sites of 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.
• 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
• Pingré and the Comte de Fleurieu, observed the
  event from Cape François in Saint-Domingue.

42
The transit of 1769
The English

• 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.
• 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.

43
The voyage of Cook to Tahiti
44
The voyage of Cook to Tahiti
45
The voyage of Cook to Tahiti
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 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.

47
Voyages organized in 1769
48
The results from the transit of 1769

• The English made 69 observations and the French
  34.
• Finally 151 observations, were made from 77
  sites.
• 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"lt P lt 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
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
The transit of December 9, 1874
51
An example the observation at St-Paul
The voyage of Commandant Mouchez at Saint-Paul.

• July 1874 departure from Paris.
• August 9 Suez channel.
• August 30 arrival in Réunion Island
• September 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

52
The voyage to Saint-Paul
53
The observation at Saint-Paul
54
The transit of December 6, 1882
55
The transit of 1882
General circonstances Premie
r 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 au Mexique (Bouquet de la Grye),
• une à la Martinique (Tisserand, Bigourdan,
  Puiseux),
• une en Floride (Colonel Perrier),
• une à Santa-Cruz de Patagonie (Capitaine de
  Frégate Fleuriais),
• une au Chili (Lieutenant de vaisseau de
  Bernardières) ,
• une à Chubut (Hatt),
• une au Rio-Negro (Perrotin, le directeur de
  l'observatoire de Nice),
• une au Cap Horn (Lieutenant de vaisseau
  Courcelle-Seneuil),
• une à Bragado (Lieutenant de vaisseau Perrin).
• Le Naval Observatory envoya huit expéditions à
  travers le monde pour observer le passage.

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
Recapitulation of the measures of the Earth-Sun
distance
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
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
Art and the transit of Venus
The ceiling of the council room of Paris
observatory
61
Passage de Vénus (Paris observatory, Prouha, 1878)
62
Transit of Mercury (Balla, 1914)
63
Comic strip the transit of Venus
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FIN Rendez vous en juin 2004