On June 6, 2012, a fascinatingly rare celestial spectacle, the transit of Venus is going to enthrall avid sky-gazers in an unprecedented manner. It means that the planet Venus slides comfortably between the sun and earth, and its passage becomes vividly visible against the fiery solar disk.

As observed from the earth, Venus seems to be creeping as a petite black dot across the bright burning yellowish-pale face from the northeastern to the western limb of the sun, as this romantic planet awesomely approaches earth from a span of a modest 43 million kilometres. During sunrise (at 05:10 hours), this unique cosmic display will be in full progress and end approximately at 10:24 hours in the morning after extending for about six hours.

Rare phenomenon

A transit could be compared to the solar eclipse, which stems from the moon sailing gracefully across the sun’s luminous visage. This breathtaking once-in-a-lifetime sight can be witnessed entirely from Hawaii, Alaska, Australia, the Pacific and eastern Asia, North America (at its beginning) and Europe (during the concluding phase).

This absolutely rare phenomenon had helped 17th century astronomers roughly calculate the sun-earth distance, known as the astronomical unit (with a mean value established at 150 million km) and could confirm the presence of an atmosphere on Venus (detected by Russian scientist Mikhail Lomonosov from the Petersburg Observatory during the 1761 transit).

Experts have utilised experiences and techniques acquired during the examination of the dimming of sunlight at the Venus transit, while searching for far-away exo or extrasolar planets. Sequences of a strange transit pattern repeat every 243 years, with pairs of transits eight years apart (falling generally in June and December and almost on the same dates) separated by long gaps of 121.5 years and 105.5 years.

Consequently before this upcoming transit in June 2012, the last one had transpired on June 8, 2004. Thereafter, the next Venus transit would be on December 11, 2117 and December 8, 2125, and then on June 11, 2247 and June 9, 2255. Previously the transit pairs were seen on June 6, 1761 and June 3, 1769 and on December 9, 1874 and December 6, 1882.

Such bizarre behaviour results primarily from Venus’ orbit around the sun, which is slightly tilted to the path at a mere 3.4 degrees, on which the earth races around the sun. Thus Venus’ transit does not necessarily happen every time in 1.6 years, when Venus comes between the sun and the earth. Due to this physical configuration and movements of the heavenly entities, Venus usually appears to pass under or over the sun at inferior conjunction (on the same side of the sun as seen from the earth).

A transit occurs when Venus, during the inferior conjunction, is at or near one of its ascending or descending nodes, which are the points in its circuit where the planet crosses the ecliptic (earth’s orbital plane) heading northwards or southwards. Because of the positioning of the nodes relative to the earth’s trajectory (that are exactly 180 degrees apart and show a six-months’ difference in between the two dates that alter slowly over time), Venus transits will take place around June 6 (descending) or December 8 (ascending).

At this transit, Venus is at the eastern realm of the zodiacal constellation Taurus (bull) in the vicinity of the fascinating open star cluster, the Hyades, with the red giant star Aldebaran (Rohini) beaconing enticingly below it. Aldebaran is circa 68 light years away. The ascending node lies in the southern expanse of the broad constellation Ophiuchus (serpent bearer).

Venus is the second terrestrial inferior planet from the sun (first one being Mercury) with a diameter of fairly 12,000 km. It is named after the Roman goddess of love and beauty. It dashes around the Sun every 224.7 earth days from an average distance of simply 108 million km. Also called the earth’s sister planet, it resembles earth in size, gravity and composition. It is covered with opaque reflective clouds of sulfuric acid. Its dense atmosphere contains mostly carbon dioxide (sheer 97 per cent) with atmospheric pressure indicating surprisingly 92 times that of earth.

As the brightest natural object, it is popularly reckoned as the morning or evening star. It is bereft of a carbon cycle to lock carbon back into rocky features or biomass (no organic life). Though Venus had housed oceans long before, the water has probably evaporated after extreme temperature increase due to the runaway greenhouse effect. Venus rotates clockwise (retrograde motion) once every 243 earth days. Interestingly one Venusian day is longer than one year.

A full transit can be relished from some areas of the earth, while from other regions, only a partial transit can be applauded like the one on December 6, 1631. The next such transit will happen on December 13, 2611. Though sometimes partial transits could be appraised from a certain place, they could not be traced from a particular zone as on November 19, 541 BC. Such transit would repeat on December 14, 2854.

The simultaneous occurrence of a Mercury and Venus transit is exceptionally infrequent. On September 22, 373,173 BC such a transit was noticed with the following one to be obvious on July 26, 69,163 and on March 29, 224,508. A solar eclipse during a Venus transit would be unimaginably discerned on April 5, 15,232. The last time such a transit with a solar eclipse was marked on November 1, 15,607 BC. Queerly the day after the Venus transit of June 3, 1769, a total solar eclipse was noticed in Northern America, Europe and Northern Asia.

In 1627, German mathematician and astronomer Johannes Kepler became the first person to predict the Venus transit on December 7, 1631, which nobody could behold in Europe due to his crude calculations. For the first time in modern history, English surveyor and astronomer Jeremiah Horrocks could affirm the Venus transit on December 4, 1639. Relying on English astronomer Edmond Halley’s computations (renowned for discovering Comet Halley), astronomers from the UK, Austria and France travelled to exotic places as Siberia, Norway, Newfoundland, Madagascar and South Africa (Cape of Good Hope) to peer at the Venus transit of 1761.

In 1769, legendary British explorer Captain Cook on his first voyage to Tahiti perceived the Venus transit from a location dubbed Point Venus that exists till now. The unfortunate French astronomer Guillaume Le Gentil spent eight years passionately going as far as Pondicherry (India) and Madagascar to glimpse these 1761 and 1769 transits in vain, thereby losing his wife, possessions and job at the academy and even being declared dead (but fortunately was rehabilitated later).

Protection of the eyes

One should never look at the sun directly with the bare eyes, because gazing at the brilliant disk of the sun (the photosphere) could quickly trigger serious and often permanent eye damage. Since watching the sun with the naked eyes could cause blindness within seconds, one should never stare at the sun through telescopes or binoculars without proper certified optical filters fitted to protect the eyes.

However, transit-lovers can project the image of the sun through telescopes, binoculars or pinhole onto a screen for viewing it. Using exposed black-and-white film as a filter is not safe. So, every precaution should be taken meticulously, when enjoying this extraordinary event to the fullest.