Transcript Passages Historiques

Transits of Venus:
the history
Garching, March 2004
Jean-Eudes Arlot
IMCCE/observatoire de Paris-CNRS
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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
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Looking 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 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 distances
Parallax or triangulation
or how to measure a distance to somewhere
without going there?
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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.
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The parallax of Mars (1672)
2 R sin
f
2
 Dd
Mars
d
Paris
R
f
Cayenne
D
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 method
c
b
a
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b•
a
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c
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The relative positions of the chords provide the parallax of Venus.
The solar disc is used as a reference frame.
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The measure of the length of the chords is replaced by the measure of the
duration of the transits.
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The measure of a duration is more accurate.
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But remember that the Sun itself has also a parallax.
The parallax of Venus: Delisle’s method
Use of the timing of
Instant t
the contacts instead of
View from surface
the duration of the
View from center of Earth
transit
Dt
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
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Kepler’s third law
•
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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 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 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 history
• Who has first the idea to observe the
transits?
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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 Paris
Transit of Mercury on Nov 7, 1631
• 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
Mercurius in sole visus et venus invisa Parissiis anno 1631.
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"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 "
Visibility of the Mercury transit of 1631
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First observation of a transit of Venus: J. Horrocks
Transit of Venus on Dec 4. 1639
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
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Observation of J. Horrocks (Venus in Sole Visa)
•
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He made three measures in a hurry before the sunset
Latin text from Horrocks
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Visibility of the Venus transit of 1639
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Transits during the XVIIIth century
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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.
The transit of June 6, 1761
• 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.
• 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 enthusiastic
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The transit of June 6, 1761
Projection de Hammer
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The transit of June 6, 1761
Some voyages organized by the French
•Jean-Batiste Chappe d'Auteroche (1728-1769) went to Tobolsk
in Siberia (successful observation).
•Alexandre Guy Pingré went to Rodrigues Island (north of
Madagascar), (observation partially successful).
•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
was unable to make a measurement
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The voyage of Chappe d’Auteroche
The travel of Chappe d’Auteroche to Tobol’sk
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Relation of the voyage of Le Gentil
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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.
•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 (TerreNeuve) where « surrounded by billions of insects " he succeeded
to observe the last contact of the transit.
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Voyages organized for the transit of 1761
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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.
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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.
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Visibility of the transit of 1769
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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.
• 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.
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The transit of 1769
The English: Cook in Tahiti
•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-.
•The observation was made by Charles Green and James Cook in
a place that they named Point Venus, still named Point Vénus
today.
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The voyage of Cook to Tahiti
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The voyage of Cook to Tahiti
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The voyage of Cook to Tahiti
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Voyages organized in 1769
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The results from the transit of 1769
•151 observations, were made from 77 sites.
• Four observations of the complete transit were made : Finland,
Hudson Bay, California and Tahiti.
Author(s)
William Smith
Thomas Hornsby
Pingré et Lalande
Pingré
Lalande
Planmann
Hell
Lexell
Values
8,6045" (1770)
8,78" (1770)
9,2" et 8,88" (1770)
8,80 (1772)
8,55"< P < 8,63" (1771)
8,43 (1772)
8,70" (1773/1774)
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".
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The transits of the XIXth century
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The longitudes are now well determined (telegraph).
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The clocks are good time keepers.
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The travels are faster (steam, Suez channel).
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The travels are still expensive
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The photographs appeared (Daguerréotype)
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The experiences of the XVIIIth century are profitable.
The transit of December 9, 1874
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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
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The voyage to Saint-Paul
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The 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, 1882
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Recapitulation of the measures of the Earth-Sun distance
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Method
date
parallax
"
distance
millions km
Mars
1672
9.5 - 10
130 -140
Venus
Venus
1761
1769
8.3 - 10.6
8.5 - 8.9
125 - 160
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
Eros
1900
1930
8.806
8.790
149.4
149.7
radar
1970
8.79415
149.5978
Viking+radar
2000
149.597870691
Past transits of Venus
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The fac-simile of the reports of the observations and voyages made
during the past centuries are available on a CD-Rom.
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More than 10 000 pages of rare books were scanned.
The recent history of the transits: Mercury
12 arcseconds
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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.
The transit of Mercury on May 7, 2003
Mercury in front of the Sun as seen
With a small telescope and a CCD camera.
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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.
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Art and the transit of Venus
The ceiling
of the council
room of Paris
observatory
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Passage de Vénus (Paris observatory, Prouha, 1878)
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Transit of Mercury (Balla, 1914)
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Comic strip: the transit of Venus
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