Rare and spectacular event when Venus crosses in front of the Sun. Lucky who can say that he saw it because generation have no even chance to observe it. The next will happen only in 2117.
The orbital period of Earth and Venus causes a regular pattern in longer time span. Between two Venus transits 8 years, then 105.5 year, then again 8 and 121.5 years have to pass. From the half years it comes, that the events occurs twice in the beginning of December and twice in the beginning of June. From the invention of telescope, or better saying from the time when Galileo Galilei turned it towards the sky, only eight transit of Venus could be observed. This fact also means that before we could find out more from the planet by the interplanetary probes, we got only six chances to investigate its globe in backlight by telescopes to establish its size, its shape, to detect its possible small moonlets, or to use the timing of the transit for other astronomical calculations. Such an astronomical calculation is the exact distance of the Sun and Earth, the so called astronomical unit (AU). Astronomers knew all the orbital period of planets from the 18th century, and using Kepler’s third law, the orbital size ratio of planets could be calculated. If only one of the orbit sizes, its diameter, or better saying the size of its semi mayor axis were known we would get sizes of the orbit of all the planets. Edmond Halley English astronomer suggested in 1716 to use the transit of Venus to measure the Sun-Earth distance, as he thought it to the most appropriate method among the possible ones.
The first time when the measure was possible was the transit of June 7, 1761. The global political situation made impossible both the quantity and the quality of successful measurements. More expeditions started to perform precise observations of transit of 3 June, 1769. Among others the Jesuit astronomer Maximilian Hell with his mate János Sajnovics traveled to island Vardø to observe the event.
At the time of transit of December 9, 1874 the first photographs were made, but that technique was in experimental state, and only few of the were successful. Hungarian observers also attended the event, Laszló Weinek observed from Kerguelen island with a German expedition, Miklós Konkoly Thege observed from Kolozsvár as Venus left the Sun’s disk. French started six expeditions to every part of the world. The prepared commemorative medals for the occasion, roughly 250 bronze and 14 silver pieces. These later were given to the leader of the expeditions (six) and another prominent politicians. The bronze pieces were given to ordinary sailor as well as men of navy, academy and senate. On the obverse an allegoric scene is visible. The Sun God Helios is riding on a quadriga, in front of him Venus as a woman is passing over, another woman is watching them by sitting on the Earth. Legend around: "QVO DISTENT SPATIO SIDERA IVNCTA DOCENT". The meaning of this Latin text was translated in the work of French astronomer Camille Flammarion's Popular astronomy as "Par leur rencontre, les astres nous font connaître les distances qui les séparent". This means “By their meeting, the stars let us reveal the distances between them”. Obviously stars mean the Sun and the Planets at this time. Because of the bad weather conditions only a few measurement were successful, and calculations were inaccurate. In addition to this, observations made of Mars were producing more accurate results for calculating the value of the AU than could be obtained during a transit of Venus. Below the medalist’s name is visible Alphée Dubois.
The reverse of the medal is a four lines legend that informs us on what occasion, when and who, the French Scientific Academy issued it. Into several medals the name of the donated person was engraved.
Hungarian observers watched the transit of December 6, 1882 from United States as members of a German expedition. From Europe and therefore from Hungary only the beginning of the event was visible. From Herény, Hungary (near Szombathely) Jenő Gothard tried to photograph the event and measure contact time. However the were not able to measure contact because of the clouds, and because of low light the pictures were failed also.
On June 8, 2004 whole Hungary got the Venus transit-fever, and in excellent weather condition the whole event was observable, we saw the black-drop effect and other anomalies caused by the atmosphere of Venus. Measured contact timings had no scientific relevance, however scientists of the European Southern Observatory tried to recalculated the AU value using amateur measurement data. They got the average 149608708 ± 11835 km, that is only greater by 10838 km than the current official value.
Thanks to the good weather the transit was observed across all Europe. A German medal remembers the event. The obverse is dominated by the Sun-Earth doubles. On front of the Sun the Venus and its orbit is shown. The astronomical signs of the three actors of the event and their names are placed on the coin. These are the Sun(☉ SONNE), the Earth (♁ ERDE) and the Venus (♀ VENUS). Legend around is the description and time of the event: "DER VENUS-TRANSIT", "8. JUNI 2004".
On the reverse is the so-called "Flammarion engraving" by an unknown artist, actually it appeared for the first time in Flammarion's book published in 1888. Legend around: "EIN MENSCHHEITSTRAUM DIE ENTDECKUNG DES UNIVERSUMS IM MITTELALTER", that is, a man's dream to explore the universe in the Middle Ages.
The visibility of the transit of 2012 was similar to that in 1769. One must travel to east or to beyond the polar circle to see the whole event. From Hungary only end of it was observable. But from China the whole event was visible. They issued a commemorative medal to the event. On the obverse the “western” view of the transit is depicted, Venus as a woman transits in front of the Sun. The medals shows three transit paths for the years 2004, 2012, and the next in 2117. Also displayed the plane of orbits of Venus and Earth that well demonstrates why this phenomenon is so rare, when these three bodies line up into one single line.
On the reverse the “eastern” view of transit displayed. The representation of Sun and Venus are slightly different. Only the transit path of 2012 is on the medal, but also an explanation drawing is visible that demonstrates the parallax. It is well shown why widely separated observers on the Earth see different transit path in front of the Sun. That’s why were expeditions sent to all parts of the world. The wider the separation among observers the better result could be gotten to the astronomical unit.
Only 100 bronze and brass commemorative medals were struck, they are numbered on their rim.