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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.
Luxembourg, January 2004
IMCCE/observatoire de Paris-CNRS
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?
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.
Look at the sky? Is is possible, just looking at the stars, to know their distances to the Earth?
Parallax or triangulation
or how to measure a distance to somewhere
without going there?
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 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.
cThe parallax of Venus : Halley’s method
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.
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.
Three conditions :
+ these two conditions may not be satisfied very often
t2 : 2e contact
t3 : 3e contact
t4 : 4e contact
t1Observation of a transit: what we see
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
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
(true solar time)
2e contact 5h 06 -21°
3e contact 10h28 +22°
local time Sun
2e contact 15h15 + 4°
3e contact 21h30 - 47°
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.
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
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.
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
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 voyage of Commandant Mouchez at Saint-Paul.
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.
" 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 149.597870691Recapitulation of the measures of the Earth-Sun distance
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.
of the council room of Paris observatory
Rendez vous en juin 2004