Transcript Passages Historiques

Transits of Venus:
the history
Luxembourg, January 2004
Jean-Eudes Arlot
IMCCE/observatoire de Paris-CNRS
1
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
2
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.
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
Measuring distances
Parallax or triangulation
or how to measure a distance to somewhere
without going there?
5
Measuring a distance with angles : the parallax
the triangulation
c?
B?
a’
a
A?
C
Base
6
b c
c  180  a  b 
c  a  b
sin c sin b sin a


C
B
A
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
R
Terre
•
•
•
7
p
a
R
p
a
 parallaxe solaire
One measure p and R to calculate a
R = 6400 km and a ~ 150x106 km
Then p ~ 10" ==> difficult to measure
Measuring distances
Measure of the distance Earth-Sun
or measure of the radius of the Earth?
8
This shows the necessity to have a good model before measuring
anything
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.
9
The parallax of Mars (1672)
2 R sin
f
2
 Dd
Mars
d
Paris
R
f
Cayenne
D
First measures of distance using parallax:
Cassini and Richer ps = 9.5" ( a = 138x 106 km)
Flamsteed
10
ps = 10"
( a = 130x 106 km)
The parallax of Venus : Halley’s method
c
b
a
•
b•
a
•
c
•
•
11
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
• Each planet describes an ellipse of which the Sun is at one of the
focus (1605).
12
Kepler (1571-1630)
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);
13
Kepler’s third law
•
14
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.
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.
15
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.
16
The transit of Venus: a rare event
Three conditions :
1F
2F
• The Earth and Venus should have the same heliocentric longitude
•The 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
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
17
• 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
planet
Axis of the shadow cone
Sun
(2)
Shadow cone
(3)
(1)
Sun
18
(4)
(1) Central
transit
(2) Noncentral
transit
Below the
shadow cone
(3) Partial
transit
(4) No
transit
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
t4
t3
t1
t2
t1 :
1e contact
t2 :
2e contact
t3 :
3e contact
t4 :
4e contact
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
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
– 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
20
First observation of a transit: Gassendi in Paris
Calculation for Paris
hour
Sun
(true solar time)
2e contact
3e contact
5h 06
10h28
-21°
+22°
Transit of Mercury on Nov 7, 1631
•
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
Mercurius in sole visus et venus invisa Parissiis anno 1631.
21
"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
22
First observations of a transit of Venus: J. Horrocks
2e contact
3e contact
sunset
local time
15h15
21h30
15h50
Sun
+ 4°
- 47°
Transit of Venus on Dec 4. 1639
23
•
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
Observation of J. Horrocks (Venus in Sole Visa)
•
He made three measures in a hurry before the sunset
t
distance (")
3h15
864
3h35
810
3h45
780
3h50
sunset
Diameter of Venus: 1' 16"
24
Latin text from Horrocks
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
Visibility of the Venus transit of 1639
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.
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.
28
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
29
Le passage du 6 juin 1761
Projection de Hammer
30
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.
31
The voyage of Chappe d’Auteroche
The travel of Chappe d’Auteroche to Tobol’sk
32
Relation of the voyage of Le Gentil
34
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 (17331779) 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.
36
Voyages organized for the transit of 1761
37
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.
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.
• 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
39
Visibility of the transit of 1769
40
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.
41
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.
42
The voyage of Cook to Tahiti
43
The voyage of Cook to Tahiti
44
The voyage of Cook to Tahiti
45
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.
46
Voyages organized in 1769
47
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)
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".
48
The transits of the XIXth century
49
•
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.
The transit of December 9, 1874
50
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
51
The voyage to Saint-Paul
52
The observation at Saint-Paul
53
The transit of December 6, 1882
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.
56
Recapitulation of the measures of the Earth-Sun distance
57
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
The recent transits of Mercury
12 arcseconds
58
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
59
•
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
The ceiling
of the council room
of Paris observatory
60
Passage de Vénus (Paris observatory, Prouha, 1878)
61
Transit of Mercury (Balla, 1914)
62
Comic strip: the transit of Venus
63
64