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Transits of Venus: the history

Garching, March 2004Jean-Eudes Arlot

IMCCE/observatoire de Paris-CNRS

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The transit of June 8, 2004The 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

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Looking at the universeLooking at the universe

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

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The transit of VenusThe 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.

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Measuring distancesMeasuring distances

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

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The parallaxThe 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.

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The parallax of Mars (1672)The parallax of Mars (1672)

Cayenne

Paris

R

D

DR

2sin2

Mars

First measures of the Earth-Sun distance using parallax:

Cassini and Richer 138 millions km Flamsteed 130 millions km

First measures of the Earth-Sun distance using parallax:

Cassini and Richer 138 millions km Flamsteed 130 millions km

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The parallax of Venus : Halley’s methodThe 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.

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The parallax of Venus: Delisle’s methodThe 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

tInstant t Use of the timing of

the contacts instead

of the duration of the

transit

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Kepler’s third lawKepler’s third law

• The semi_major axis a and the period of revolution T are linked by a3/T2=constant for all the planets (1618).

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The astronomical unitThe 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.

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Measuring distances to starsMeasuring 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.

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The historyThe history

• Who has first the idea to observe the transits?

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Who, first, has the idea to observe the transits ?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

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The first observation of a transit: Gassendi in ParisThe 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.

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Visibility of the Mercury transit of 1631Visibility of the Mercury transit of 1631

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First observation of a transit of Venus: J. HorrocksFirst 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

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

• He made three measures in a hurry before the sunset

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Latin text from HorrocksLatin text from Horrocks

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Visibility of the Venus transit of 1639Visibility of the Venus transit of 1639

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Transits during the XVIIIth centuryTransits 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.

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The transit of June 6, 1761The 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 voyages were difficult and the 7-years war (a world war) set ablaze seas and colonies.and colonies.• the coordination of all the astronomers was made by the french astronomer the coordination of all the astronomers was made by the french astronomer Joseph-Nicolas Delisle (1688-1768) who sent his mappemonde to more than Joseph-Nicolas Delisle (1688-1768) who sent his mappemonde to more than 100 astronomers in the world.100 astronomers in the world.•the european population was enthusiasticthe european population was enthusiastic

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The transit of June 6, 1761The transit of June 6, 1761

Projection de HammerProjection de Hammer

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The transit of June 6, 1761The transit of June 6, 1761

•Jean-Batiste Chappe d'AuterocheJean-Batiste Chappe d'Auteroche (1728-1769) went to (1728-1769) went to Tobolsk in SiberiaTobolsk in Siberia (successful observation). (successful observation).

•Alexandre Guy PingréAlexandre Guy Pingré went to went to Rodrigues Island Rodrigues Island (north of (north of Madagascar), (observation partially successful). Madagascar), (observation partially successful).

•Guillaume Joseph Hyacinthe Jean-Batiste Le Gentil de Guillaume Joseph Hyacinthe Jean-Batiste Le Gentil de La GalaisièreLa Galaisière (1725-1792), left by sea in order to observe (1725-1792), left by sea in order to observe the transit the transit in Indies at Pondichéryin Indies at Pondichéry. Unfortunately the city of . Unfortunately the city of Pondichéry was taken by the English and he Pondichéry was taken by the English and he was unable to make a measurementwas unable to make a measurement

Some voyages organized by the FrenchSome voyages organized by the French

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The voyage of Chappe d’AuterocheThe voyage of Chappe d’Auteroche

The travel of Chappe d’Auteroche to Tobol’skThe travel of Chappe d’Auteroche to Tobol’sk

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Relation of the voyage of Le GentilRelation of the voyage of Le Gentil

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The transit of June 6, 1761The transit of June 6, 1761

Some voyages organized by the EnglishSome voyages organized by the English

•Nevil MaskelyneNevil Maskelyne (1732-1811) went to (1732-1811) went to Sainte-HélèneSainte-Hélène where where he was not able to observe because of clouds. he was not able to observe because of clouds.

•Charles MasonCharles Mason (1728-1786), (1728-1786), James BradleyJames Bradley and and Jeremiah Jeremiah DixonDixon (1733-1779) was not able to observe from (1733-1779) was not able to observe from BencoolenBencoolen (Sumatra), because the French took the city. They came (Sumatra), because the French took the city. They came back to Cape town.back to Cape town.

•John WinthropJohn Winthrop, professor in Harvard went to , professor in Harvard went to St-John (Terre-St-John (Terre-Neuve)Neuve) where « surrounded by billions of insects " he where « surrounded by billions of insects " he succeeded to observe the last contact of the transit.succeeded to observe the last contact of the transit.

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Voyages organized for the transit of 1761Voyages organized for the transit of 1761

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Results from the transit of 1761Results from the transit of 1761

• The number of observers was The number of observers was 120120, on , on 62 sites62 sites (S. Newcomb, 1959). (S. Newcomb, 1959).

• Note that some sites of observations were previously selected Note that some sites of observations were previously selected (Bencoolen, Pondichéry, Batavia) by Halley in 1716.(Bencoolen, Pondichéry, Batavia) by Halley in 1716.

The large error is due to:The large error is due to:- a bad knowledge of the longitudes- a bad knowledge of the longitudes of the sites of observation of the sites of observation - the black drop effect- the black drop effect which decreases the precision of the measurement which decreases the precision of the measurement of the time of the contacts.of the time of the contacts.

8.5" < 8.5" < < 10.5" < 10.5"

Disappointing results : no improvement of the measures Disappointing results : no improvement of the measures from Mars.from Mars.

Disappointing results : no improvement of the measures Disappointing results : no improvement of the measures from Mars.from Mars.

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The transit of Venus of June 3-4, 1769The transit of Venus of June 3-4, 1769

• The organization of the observations for 1769 were made The organization of the observations for 1769 were made by Lalande in France and Thomas Hornsby in England. by Lalande in France and Thomas Hornsby in England. • They took benefit from the observations of the transit of They took benefit from the observations of the transit of 1761.1761.

•27 refractors were used, only 3 were used in 1761.27 refractors were used, only 3 were used in 1761.

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Visibility of the transit of 1769Visibility of the transit of 1769

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The transit of 1769The transit of 1769

The French: Chappe in CaliforniaThe French: Chappe in California

• ChappeChappe accompanied by Pauly, Noël and Dubois and by accompanied by Pauly, Noël and Dubois and by two spanish astronomers Vicente de Doz et Salvador de two spanish astronomers Vicente de Doz et Salvador de Medina went to California on the Medina went to California on the west coast of Mexicowest coast of Mexico, , near Cape Lucas today named San José del Cabo. 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 they observed also the lunar eclipse on June 18 1769 to measure the longitude. Unfortunately they died from an measure the longitude. Unfortunately they died from an epidemic of typhus after taking care of the inhabitants, epidemic of typhus after taking care of the inhabitants, except Pauly who came back to France with the scientific except Pauly who came back to France with the scientific results.results.

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The transit of 1769The transit of 1769

The English: Cook in TahitiThe English: Cook in Tahiti

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

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The voyage of Cook to TahitiThe voyage of Cook to Tahiti

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The voyage of Cook to TahitiThe voyage of Cook to Tahiti

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The voyage of Cook to TahitiThe voyage of Cook to Tahiti

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Voyages organized in 1769Voyages organized in 1769

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The results from the transit of 1769The results from the transit of 1769

•151 observations151 observations, were made from , were made from 77 sites77 sites. .

• Four observations of the complete transit were made : Four observations of the complete transit were made : Finland, Hudson Bay, California and Tahiti.Finland, Hudson Bay, California and Tahiti.

Author(s) ValuesAuthor(s) ValuesWilliam Smith 8,6045" (1770) William Smith 8,6045" (1770) Thomas Hornsby 8,78" (1770) Thomas Hornsby 8,78" (1770) Pingré et Lalande 9,2" et 8,88" (1770) Pingré et Lalande 9,2" et 8,88" (1770) Pingré 8,80 (1772) Pingré 8,80 (1772) Lalande 8,55"< P < 8,63" (1771) Lalande 8,55"< P < 8,63" (1771) Planmann 8,43 (1772) Planmann 8,43 (1772) Hell 8,70" (1773/1774) Hell 8,70" (1773/1774) Lexell 8.68" (1771) et 8,63" Lexell 8.68" (1771) et 8,63" (1772) (1772)

Author(s) ValuesAuthor(s) ValuesWilliam Smith 8,6045" (1770) William Smith 8,6045" (1770) Thomas Hornsby 8,78" (1770) Thomas Hornsby 8,78" (1770) Pingré et Lalande 9,2" et 8,88" (1770) Pingré et Lalande 9,2" et 8,88" (1770) Pingré 8,80 (1772) Pingré 8,80 (1772) Lalande 8,55"< P < 8,63" (1771) Lalande 8,55"< P < 8,63" (1771) Planmann 8,43 (1772) Planmann 8,43 (1772) Hell 8,70" (1773/1774) Hell 8,70" (1773/1774) Lexell 8.68" (1771) et 8,63" Lexell 8.68" (1771) et 8,63" (1772) (1772)

The conclusion was that the parallax was from 8,43" to The conclusion was that the parallax was from 8,43" to 8,80 8,80 "" . This was a real improvement regarding the . This was a real improvement regarding the result of 1761 providing a parallax from 8,28 to 10,60".result of 1761 providing a parallax from 8,28 to 10,60".

The conclusion was that the parallax was from 8,43" to The conclusion was that the parallax was from 8,43" to 8,80 8,80 "" . This was a real improvement regarding the . This was a real improvement regarding the result of 1761 providing a parallax from 8,28 to 10,60".result of 1761 providing a parallax from 8,28 to 10,60".

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The transits of the XIXth centuryThe 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.

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The transit of December 9, 1874The transit of December 9, 1874

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An example: the observation at St-PaulAn 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%The probability of fair weather was only 8 to 10%

In spite of tempest and bad weather, the observation was a In spite of tempest and bad weather, the observation was a success: 500 exposures of the transit were madesuccess: 500 exposures of the transit were made

The voyage of Commandant Mouchez at Saint-Paul.The voyage of Commandant Mouchez at Saint-Paul.

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The voyage to Saint-PaulThe voyage to Saint-Paul

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The observation at Saint-PaulThe observation at Saint-Paul

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

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The transit of December 6, 1882The transit of December 6, 1882

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Recapitulation of the measures of the Earth-Sun distanceRecapitulation of the measures of the Earth-Sun distance

Method date parallax distance " millions km

Mars 1672 9.5 - 10 130 -140

Venus 1761 8.3 - 10.6 125 - 160Venus 1769 8.5 - 8.9 145 - 155

Mars 1862 8.84 149

Flora 1875 8.87 148

Mars 1885 8.78 150

Venus 1874 - 82 8.790-8.880 148.1 - 149.7

Eros 1900 8.806 149.4Eros 1930 8.790 149.7

radar 1970 8.79415 149.5978

Viking+radar 2000 149.597870691

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Past transits of VenusPast 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.

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The recent history of the transits: MercuryThe 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.

12arcseconds

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The transit of Mercury on May 7, 2003The transit of Mercury on May 7, 2003

Mercury in front of the Sun as seenWith a small telescope and a CCD camera.

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The transit of Mercury on May 7, ,2003The 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.

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Art and the transit of VenusArt and the transit of Venus

The ceiling The ceiling of the council of the council room of Paris room of Paris observatoryobservatory

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

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Transit of Mercury (Balla, 1914)Transit of Mercury (Balla, 1914)

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Comic strip: the transit of VenusComic strip: the transit of Venus

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