Venus transit

One calls transit of Venus the passage of the planet Venus exactly between the Ground and the Sun, occulting a small portion of the solar disk. During the transit, Venus can be observed since the Earth in the shape of a small black disc moving in front of the Sun. The duration of such transits is in general of a few hours (that of 2004 lasted 6 hours). A transit is similar to a solar eclipse by the the Moon, but although Venus makes almost four times the size of the Moon, it appears much smaller because of more important distance which separates it from the Earth. Before the space era, the observation of transits of Venus was used for the scientists to calculate the distance Ground-Sun by the method of the Parallaxe S.

The Venus transits belong to the foreseeable astronomical phenomena least frequent and currently occur according to a sequence which is repeated every 243 years, with pairs of transits separated by 8 years followed by long intervals of 121,5 and 105,5 years. Before 2004, the pair of transit preceding date of December 1874 and December 1882. The first of the pair of transits of the beginning of the 21e century took place on June 8th 2004 (see Transit of Venus of 2004) and the following will take place on June 6th 2012 (see Transit of Venus of 2012). After 2012, the next transits will take place into 2117 and 2125.

A Venus transit can be observed in full safety with the same precautions as for the observation of the phases partial of a solar eclipse. To fix the solar disk without protection quickly involves serious ocular damage and sometimes of the permanent lesions.

Conjunctions

Normally, when Venus and the Earth is in conjunction, they are not aligned with the Sun. The Venus orbit is tilted of 3,4° compared to that of the Earth and thus passes in lower part (or to the top) of the Sun in the sky. Observed since the Earth, Venus in lower conjunction can be isolated until 9,6° Sun although the slope is only of 3,4°. As the angular Diamètre of the Sun is from approximately 1/2 degree, Venus passes then to the top or in lower part of the Sun to more than 18 solar diameters.

Sequence 105,5/8/121,5/8 is not the only possible one during the 243 years time because of the light shift between the conjunction and the passage to the line of the nodes. Before 1518, there were only three transits every 243 years following sequence 8/113,5/121,5, and the eight transits preceding that by year 546 were spaced 121,5 years. The current sequence will continue until 2846 and will then be replaced by sequence 105,5/129,5/8. Thus, the 243 years period is relatively stable but the number of transits and their spacing for this period change during the ages.

Old story

In antiquity, the Greek astronomers , Egyptian, Babylonian and Chinese knew Venus and noted its movements. The old Greeks thought that the morning and evening appearances of Venus corresponded to two different objects, Hesperus the evening star and Phosphorus the star of the morning. One allots to Pythagore the discovery which it was about same planet. With, Héraclide of the Bridge emitted the assumption that Venus and Mercure orbited around the Sun and not of the Earth. No element makes it possible to affirm that these cultures knew the transits.

Venus was important for the civilizations précolombiennes, in particular for the Mayas which named it Chak ek , “the large star”.

Modern observations

In spite of its scarcity, the interest of the observation of a Venus transit is that it makes it possible to calculate the size of the Solar system by employing the method of the Parallaxe S. the technique consists in measuring the light difference of the hour of beginning (or end) of the transit observed since of the points very far away from terrestrial surface. The difference between the places of observation makes it possible to calculate the Sun-Venus distance by Triangulation.

Although at the 17th century the astronomers could calculate the relative distances from each planet compared to the Sun in term of distance Ground-Sun (i.e. in astronomical Unité), this basic unit never had been precisely measured.

Johannes Kepler was the first to predict a Venus transit for 1631, but it was not observed because the prediction of Kepler was not enough precise to determine that the transit would not be visible since the majority of Europe.

The first observation of a Venus transit was made by Jeremiah Horrocks since its residence of Much Hoole close to Preston in England, the December 4th 1639 (on November 24th according to the Calendrier Julien then into force in this country). His/her friend William Crabtree observed the transit since Salford close to Manchester. Kepler had predicted the transits of 1631 and 1761 and one rubbing in 1639. Horrocks corrected the orbital Venus parameters establish by Kepler and noticed that the Venus transits would take place per 8 years separate pair and could thus predict that of 1639. Although it was not certain exact hour, it calculated that the transit would start roughly with 15:00. Horrocks focused the image of the Sun on a piece of paperboard using simple a Télescope to observe it in full safety. After having waited all the day, it was likely to see the transit whereas the clouds which masked the Sun released with 15:15, just an hour before the sunset. Its measurements enabled him to as well make bearable estimates on the Venus size as at the distance Ground-Sun. Its estimate of the distance Ground-Sun was of 95,6  Gm (59,4 billion thousand S, is 0,639  ua) - about two thirds of the real distance, but the most precise measurement of the time. However, the observations of Horrocks were published only in 1661, well after its death.

While being based on the observation of the Venus transit of 1761 from the observatory of Saint-Pétersbourg, Mikhaïl Lomonossov predicts the existence of a atmosphere on this planet. Lomonossov detected the Réfraction solar rays and deduced from it that only the presence of an atmosphere could explain the appearance of a ring of light around the part of Venus which was not yet in contact with the solar disk at the beginning of the transit.

The pair of transits of 1761 and 1769 was used to precisely calculate the value of the astronomical unit by the method of the parallaxes described by James Gregory in Optica Promota in 1663. According to the proposal made by Edmond Halley (then deceased since nearly twenty years). The majority succeeded in observing at least part of the transit, but the best result was obtained by Jeremiah Dixon and Charles Mason with the Cape of Good Hope. For the transit of 1769, the scientists went in the Hudson Bay, in Low-California (then controlled by Spain) and in Norway in addition to the first voyage of the captain Cook intended to carry out this observation since Tahiti. The Czech astronomer Christian Mayer was invited by Catherine II of Russia to observe the transit since Saint-Pétersbourg, but its observations were especially obstructed by the clouds. Unfortunate the Guillaume Nice the spent eight years to travel to try to observe the two transits; its failure made him lose woman and possessions and he was even declared died (its history became the screen of the part the Transit of Venus of Maureen Hunter).

In 1771, by recutting the data of the transits of 1761 and 1769, the French astronomer Jerome Lalande establishes the value of the astronomical unit to 153 million kilometers (±1 million). The precision less good than was discounted because of the phenomenon of the black drop, but constituted a considerable improvement compared to calculations of Horrocks. The original methods of detection concentrate on the very massive exoplanètes (more similar to Jupiter than with the Earth), whose gravity is sufficient to make oscillate its star in a measurable way on the level of sound own Mouvement, of its radial Speed or the Effect Doppler. To measure the fall of luminous intensity during a transit is potentially more sensitive and would make it possible to detect smaller planets.

Last and future transits

The transits currently proceed in June or December (see the table). These dates advance slowly in the seasons; before 1631 they occurred in May and November.

The simultaneous occurrence of a Transit of Mercury and of a Venus transit is possible but in a very remote future. The next one is planned for on July 26th: 69163, then in the year: 224508.

The simultaneous occurrence of a solar eclipse with a Venus transit is possible but very rare, the next simultaneous eclipse with a Venus transit is envisaged on April 5th: 15232.

Observation

The surest observation aid is to project the image of the Sun using a Télescope, of Jumelles or a bored paperboard of a hole. But the phenomenon can also be looked at directly thanks to suitable filters such as a solar filter of astronomy covered of a layer of chromium, glasses of solar observation of eclipse or glasses of welding of index 14.

The old method consisting in using a negative photograph black & exposed white is not regarded any more as sure: the small imperfections or perforations of film let pass UV harmful. In the same way, negative a color does not contain money and is thus transparent with the infra-red raies which can burn the retina. To look at the Sun without protection directly can cause a temporary or permanent loss visual functions by damaging or destroying the retinal cells. For the astronomer amateur, four interesting moments ago during a transit, when the Venus circumference is tangent with that of the solar disk: