Transit : History
History of observation of planets' transit over the Sun disc is relatively short - we cannot find any information about this interesting phenomenon neither by Arabian intellectuals, antique observers, nor by Chinese and other sophisticated nations, which were famous for observation atronomical phenomenons. The reason is simple - the basic tool for these observations is primarily telescope which has been used in astronomy since only the 17th century.
The first person, who expressed the theory of possible observation of planets blocking out the Sun from our view, was the German mathematician, physicist and astronomer Johannes Kepler (1571 - 1630). He deduced this idea on the basis of his three laws of bodies' movement in the solar system (known as the Kepler laws). He dared to express even the exact date of planets' transit in the so-called Rudolph tables. According to Kepler, Mercury's transit was to materialise the 7th November 1631. Venus transit the 6th December 1631, then.
Although he gave notice about this phenomenon to many transoceanic scholars or travellers (according to his calculations he determinated that the observation would not be possible to realize in Europe), any record of Venus transit in that year was not found. As for Mercury, a part of its transit was recorded by Pierre Gassendi. Kepler himself died one year before this significant event.
According to the Rudolph tables, another Venus transit was to materialise only in the year 1761, a bigger approximation to the Sun was to occur just the 4th December 1639. However, the British theologist and astronomer Jeremiah Horrocks corrcted Kepler's calculations. The result of his work sounded promisingly - not only approximation but the transit itself would occur in the mentioned year.
Therefore, along with William Crabtree they realized the first observation of this phenomenon. However, similarly like Gassendi 8 years before, luck was not with them and they did not observe the whole course of the transit. In any case, they were the first, who had the possibility to estimate directly the size of Venus' disc. It is worth mentioning that the value they measured, equaled one angular minute, was more than surprising for many of the astronomers of that time, because the average presumption ranged around ten angular minutes. J. Horrocks summarized his booked observations in the document Venus in sole visa. Shortly after, in his 24 years, he died.
One hundred years later, the astronomers already subjected Venus transit over the Sun disc to careful investigation, to achieve the aim of more exact assessment of absolute distance between the Sun and the planets. According to Kepler's laws, this was possible to realize only with the knowledge of so-called parallax. And just right parallax was necessary to measure. The Father of this idea was the British astronomer Edmund Halley (1656 - 1742), who supposed that from various places on the Earth, Venus would be seen in another side of the Sun disc. That is why it was enough to coact with several observers on the whole Earth. The opportunity occured only 19 years after Halley's death - the 6th June 1761. Many astronomers tried to "catch Venus" to various localities on the Earth, from Siberia to Saint Helena. For all that, the executed observations were almost useless. It was probably the optical fault of the telescopes and to five-minute deviation of time meters. In spite of it, significant discoveries occured. It was mainly the light border part of Venus' disc, which was sighted in the moment of Venus crossing the edge of the Sun disc. Hereby it was proved the existence of Venus' atmosphere.
Only the year 1769 brought the expected results. The 9th November, Mercury transited the Sun and many observers did not hesitate to use this opportunity. In this era, plentiful amounts of astronomical records were created. But they were processed only thirty years later. The credit for their final processing had the German astronomer Johann Franz Encke (among others, he discovered the comet Encke) and so he established the Sun parallax. Later, he established the final distance Sun - Earth as 153 000 000 km (it differs from the real distance just in 4 million km).
A hundred years later, in the years 1874 to 1882, the European observers did not pay much attention to planets' transits over the Sun. The Americans picked up their imaginary baton. For this exceptional occasion, the inhabitants of the New continent prepared sufficiently. Even the United States' government contributed to the ambitious project a huge amount of money.
Despite all technical conditions of those days (eg. invention of photograph) it turned out that new observations did not bring anything revolutionary. Besides, mistakes caused by imprecise measurement occured. That is why the appointed value of the distance between the Sun and the Earth approached just slightly 149 million km.
Technical progress in the 20th century noticeably proved also in astronomical observations. According to radar measurements of Venus it is possible to determine distances in space with accuracy of many decimal places. So, planets' transit over the Sun lost its significance as time moved on. In spite of this, we can still consider observation of this phenomenon to be a very interesting and uncommon experience.
