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Transits of Venus: the history Luxembourg, January 2004 Jean-Eudes Arlot IMCCE/observatoire de Paris-CNRS. The transit of June 8, 2004.

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Transits of Venus:

the history

Luxembourg, January 2004

Jean-Eudes Arlot

IMCCE/observatoire de Paris-CNRS

the transit of june 8 2004
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

the transit of venus
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.

looking at the universe
Looking at the universe

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

measuring distances
Measuring distances

Parallax or triangulation

or how to measure a distance to somewhere

without going there?

the parallax of the sun





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" ==> difficult to measure
measuring distances1
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

the parallax
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.

the parallax of mars 1672








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)

the parallax of venus halley s method







The parallax of Venus : Halley’s 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.
kepler s first law

Kepler (1571-1630)

Kepler’s first law
  • Each planet describes an ellipse of which the Sun is at one of the focus (1605).
deuxi me loi de k pler
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);
kepler s third law
Kepler’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).
the astronomical unit
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.

measuring distances2
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.

the transit of venus a rare event
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


  • due to the size of the Sun, the Earth and Venus, the event occurs even the two conditions are not completely satisfied.
a transit as seen from space

Plane of Bessel

Penumbra cone

summit of the cone

of penumbra

summit of the

shadow cone


Axis of the shadow cone

(1) Central transit

Shadow cone



(2) Non-central transit



Below the

shadow cone


(3) Partial transit


(4) No transit

A transit as seen from space
observation of a transit what we see

t1 : 1e contact

t2 : 2e contact


t3 : 3e contact


t4 : 4e contact



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

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
    • 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
first observation of a transit gassendi in paris

Transit of Mercury on Nov 7, 1631

Mercurius in sole visus et venus invisa Parissiis anno 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 "

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 Kepler’s predictions
  • Three other observations in Europe
first observations of a transit of venus j horrocks

Transit of Venus on Dec 4. 1639

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

local time Sun

2e contact 15h15 + 4°

3e contact 21h30 - 47°

sunset 15h50

observation of j horrocks venus in sole visa

t distance (")

3h15 864

3h35 810

3h45 780

3h50 sunset

Diameter of Venus: 1' 16"

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

Painting of F. M. Brown, visible at the City Hall of Manchester

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 !
transits during the xviiith century
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.

the transit of june 6 1761
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

the transit of june 6 17611
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
  • from Luxembourg Palace in Paris.
the transit of june 6 17612
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.
results from the transit of 1761
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" < P < 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.

the transit of venus of june 3 4 1769
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

the transit of 1769
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.
the transit of 17691
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.
the transit of 17692
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.
the results from the transit of 1769
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"< 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".

the transits of the xixth century
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.
an example the observation at st paul
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
reduction of photographs
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.

recapitulation 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 - 160

Venus 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 8.806 149.4

Eros 1930 8.790 149.7

radar 1970 8.79415 149.5978

Viking+radar 2000 149.597870691

Recapitulation of the measures of the Earth-Sun distance
the recent transits of mercury
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.


past transits of venus
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.
art and the transit of venus
Art and the transit of Venus

The ceiling

of the council room of Paris observatory



Rendez vous en juin 2004