Dear Readers, be ready to witness one more celestial event this year, i.e. 'Transit of Venus' on June 6, 2012. Though visually the phenomenon is not as spectacular as total solar eclipse, but is of great significance for the astronomers, as it is one of the rare celestial phenomena. It is interesting to note that today there is no living person who has seen both the transits, 1874 and 1882. If you have seen the Transit of Venus on 8th June, 2004, then you are very fortunate to witness the transit of Venus again on June 6, 2012.

Transits: What are they?
When moon comes in between the Earth and the Sun, it is called eclipse. However, when either Mercury or Venus (one of the interior planets) – comes between the Sun and the Earth, it is called transit. On June 6, 2012, we will have an opportunity of watching Venus move across the disc of the Sun. Unfortunately; unlike the 2004 this time we will not be able to see the full sequence of the transit. From India as well as from our valley, when the Sun will rise, the Venus will already be in transit state.
The discs of the planet Mercury and Venus as seen from Earth are much smaller than that of Moon. Therefore, they make no more than a small black dot when they move in front of Sun. With every transit, depending upon the geometry involved, this dot may traverse a different path across the face of Sun. The transits do not take place frequently because the orbit of both Mercury and Venus are tilted at small angles to the ecliptic (the average plane of Earth’s orbit around the sun–the planets are always seen close to it). Hence, they will usually be either above (north) or below (south) the ecliptic. The Transit of Mercury can be seen 13 to 14 times in a century. The first transit of Mercury of twenty first century took place on May 7, 2003, which was visible in the entire country. However, transit of Venus is rarest of planetary alignment. Indeed only seven such events have occurred since the invention of the telescope (1631, 1639, 1874, 1882, 1961, 1969 &, Dec 8, 2004)

What is a transit of Venus?
A transit of Venus is the passage of the planet Venus across the disk of the sun. At this time, the planet can be seen as a small black disk slowly moving in front of the sun. The orbits of Mercury and Venus lie inside Earth’s orbit, so they are the only planets which can pass between Earth and Sun to produce a transit. Transits are very rare astronomical events. Transits of Venus are much rarer than transits of Mercury. In the Case of Mercury, there are on average thirteen transits each century. However, Venus Transits occur approximately 4 times in 243 years. They occur in pairs of transit events separated by about 8 years and these pairs are separated by over a century.

When is the transit of Venus?
Such a rare event of transit of Venus will occur on 6th June 2012. Following the event in June 2012, the next Venus Transit will thereafter be seen in December 2117 and December 2125. Earlier, in the year 1882 a transit of Venus occurred. The first recorded observation of a transit was by the English Astronomer Jeremiah Horrocks in 1639, about three decades after the invention of the telescope. Since then, transits have been witnessed in 1761, 1769, 1874, and 1882. The transits of 1761 and 1874 were fully visible in India. The transit of 8th June 2004 was witnessed entirely from all over India, unlike the forthcoming one on June 6, 2012, in which we can not witness the first and the second contact. The Venus will already be in transit phase at the time of sunrise in India on June 6, 2012. Again only in the year 2255, June 9, the entire Venus transit will be visible in India.

Why is a transit of Venus so rare?
Transits of Venus have a strange pattern of frequency. The last transit happened in 1880 and for about 1211/2 years there occurred no transit. Then there was a transit in 2004 which will be followed by another transit of Venus eight years later in the year 2012. Then there will be a span of about 1051/2 years before the next pair of transits occur, again separated by eight years. The reason for these long intervals is because of the difference in the orbital planes of the Earth and the Venus. If Venus and the Earth orbited the sun in the same plane as the sun, transits can happen frequently. However, the orbit of Venus is inclined to the orbit of Earth, by about 3.4 degrees, so when Venus passes between the sun and the Earth, Venus usually is a little bit above or a little bit below the sun, invisible in the sun’s glare. However, if the Venus passes between the Sun and the Earth near the nodes, a transit occurs. The points where the orbit of Venus crosses the Ecliptic are called the ‘nodes’. The ascending node is where the planet crosses from south to north; the descending node is where it crosses from north and south. It takes Venus about 225 days and Earth about 365 days to circle the sun. This means that when Venus returns to one of the nodes, Earth is not yet there, and vice versa. In addition, a few more effects lead to the observed long and slightly irregular intervals. The transit of Venus in the year 2004  occurred near the descending node. The upcoming transit in 2012 will also occur near this node.
What is the significance of a transit?
The Venus Transit brings our attention to the complex motion of the planets around the Sun, the Kepler’s laws, dynamics, gravitation. The scarcity and the periodicity of the passages show us also the difficulty of astronomical calculations for the prediction of celestial phenomena. The Transit of 1761 allowed a scientist named Mikhail Lomonosov to prove that Venus had an atmosphere, which could be seen as a blurry halo around the planet. Observations of the transits of Venus across the sun also yielded information on Venus’ size. Most importantly, the geometry of transit of Venus allows us to estimate the Sun-Earth distance, known as Astronomical Unit.

Is it safe to view the transit of Venus?
One should never look at the Sun directly with naked eye or with any other optical device such as telescope or binoculars. Severe eye damage or blindness can result if you view the sun without proper eye protection. The safest and most inexpensive method for observing the Sun is by projection. A pinhole or small opening is used to form an image of the Sun on a screen placed about a meter behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card, but care must be taken to ensure that no one looks through the device. The main advantage of the projection methods is that nobody is looking directly at the Sun.

Do's
* View only the projection of the solar disk.
* Project the image of Sun on a shaded wall through a pinhole, telescope or a pair of binoculars.
* Direct viewing should be done using only a scientifically tested/certified solar filter. Note that only people with keen eye sight can see Venus as small dark spot on the disk of the Sun. During six hour transit, the dark spot will appear to cross the northern part of the solar disk from East to West.

Don'ts
* Don't attempt to observe any phase of the transit with the naked eye. (Use a safe solar filter)
* Never look directly through a telescope or a binocular.
*Don't use smoked glass, colour film, sun glasses, exposed black and white film, photographic neutral density filters and polarizing filters. They are all unsafe.
*Don't look at the Sun continuously even with your solar filters, view it intermittently for few seconds

Author is Lecturer physics, Dept. of school Education. Reach him at ishtiyaqsyed@gmail.com

Lastupdate on : Sat, 2 Jun 2012 21:30:00 Makkah time
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