Venus on the sun

June 6 will witness a once-in-a-lifetime phenomenon in astronomy: the transit of Venus.

A TRANSIT of Venus over the disc of the sun will occur on Wednesday, June 6, 2012. It is one of the most celebrated phenomena in astronomy. Many astronomers of yore have lamented that they could never hope to see one in their lifetime. The last one took place on June 8, 2004, and was one of the most widely observed astronomical events in history, viewable in its entirety from the United Kingdom, continental Europe, and most of Asia including India. This time, on June 6, the circumstances are almost a reversal of those of 2004. While observers in north-western North America, the Western Pacific, north-eastern Asia, Japan, eastern Australia and New Zealand will have a view of the whole event from first to last contact, observers in the U.K. and India will only be able to see the last part of the transit as the sun rises. The transits of Venus occur in pairs. The next pair of transits will occur in 2117 and 2125. In describing the motions of the planets in relation to the earth, it is convenient to divide the planets into two classes those nearer to the sun than the earth are called inferior and those that are more remote are called superior. In this context, Mercury and Venus are inferior planets. Inferior conjunction of Venus occurs when the planet lies between the sun and the earth. At superior conjunction, Venus is on the opposite side of the sun from us.

The apparent yearly path of the sun against the background stars, passing through the patterns of the zodiac, is called the ecliptic. The ecliptic plane is really the projection of the earth's orbital plane around the sun onto the celestial sphere. Like all planets, Venus orbits the sun in the same sense as the earth counterclockwise, as seen from above the north celestial pole. The orbital plane of Venus is inclined at 3.4 degrees to the plane of the ecliptic.

The two points on Venus' orbit where it crosses the ecliptic plane are known as the nodes of the orbit. The line joining them, which is also the line of intersection of the earth's orbit and Venus' orbital plane, is known as the line of nodes. Since its orbit is slightly inclined to the ecliptic, Venus usually passes north or south of the sun at inferior conjunction (inferior conjunction occurs at an interval of 584 days). But if such conjunction occurs when the inferior planet is near its node, then from the earth it is seen as a small, dark spot moving from east to west across the sun's luminous disc along a path sensibly parallel to the ecliptic. This is known as a transit of Venus. The condition necessary for a transit is similar to the requirement for a solar or lunar eclipse.

An astronomical event where a smaller, darker object passes in front of a larger, brighter one is known as a transit. In other words, a transit occurs when the shadow of one of the inferior planets falls upon the earth. This is also possible for the inferior planet Mercury, and that is why we occasionally witness transits of Mercury. However, solar transits of Venus are exceedingly rare events much rarer than transits of Mercury primarily because Venus is farther from the sun and the proper alignment occurs less frequently. Although transits of Venus are very rare, they occur in pairs, eight years apart, during June or December. Only seven such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882 and 2004). The next event is due on June 6, 2012, but then not again until December 11, 2117 , and December 8, 2125. Venus transits show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years.

Analogous to eclipses

Transits are analogous to annular eclipses of the sun. The planets Venus and Mercury, seen from the earth, are too small to cover the sun completely; their shadow cones fall far short of reaching the surface of the earth. The appearance of a transit is that of a black dot slowly crossing the disc of the sun from east to west. The silhouette of Mercury against the sun is too small to see without a telescope. That of Venus can be observed without optical aid if the sun is properly viewed by projection or through proper filters to protect the eye.

There are four phases during a transit, two at the start (known as ingress) and two at the finish (known as egress): (i) The first exterior contact occurs when the planet first appears to touch the sun's edge or limb; (ii) the first internal contact is the point at which the planet is fully upon the sun's disc but still contiguous with its limb; (iii) the second internal contact occurs when the planet touches the opposite limb of the sun, having crossed its disc; and (iv) the second external contact, the moment when the planet's trailing limb finally clears the sun's disc.

Venus is the third brightest object in the entire sky (after the sun and the moon) and that is why it is very easy to see it even when it is close to the sun. Although Venus is most definitely not a star, it appears more than 10 times brighter than the brightest star, Sirius. If we know just where to look, we can see Venus even in the daytime. On a moonless night, away from city lights, Venus casts a faint shadow. The planet's brightness stems from the fact that, unlike Mercury, Venus is highly reflective with an albedo of over 0.7 that is more than 70 per cent of the sunlight reaching Venus is reflected back into space. Most of the sunlight is reflected from the clouds high in the planet's dense atmosphere.

Venus appears to swing back and forth in the sky, during its synodic period, from one side of the sun to the other. Therefore, we can see Venus from the earth only just before sunrise in the east or just after sunset in the west, and as such the planet is often called the morning star or the evening star, depending on where it happens to be in its orbit. It was named Venus by the Romans, who remained for a considerable period of time at the pinnacle of European civilisation. They were charmed by the beauty of this fair planet in the night sky, so they named it Venus, their goddess of beauty and love.

We might expect Venus to appear the brightest when it is full' that is, when we can see the entire sunlit side. Venus is full when it is in superior conjunction that is, when the planet is exactly on the other side of the sun. But we cannot see this phase as it is lost in the sun's glare. We can see an almost full Venus within a few degrees of superior conjunction. When Venus is the closest to the earth, at inferior conjunction, the planet is at the new phase, lying between the earth and the sun. At this time we again cannot see it because the sunlit side faces away from us. As Venus moves away from inferior conjunction, more and more of it becomes visible. But, of course, as this happens its distance from us also increases. Venus' maximum apparent brightness actually occurs about 36 days before or after inferior conjunction. The elongation of the planet at this time is 39 degrees and we see it as a rather fat crescent.

Venus is surrounded by a thick layer of cloud. The same clouds whose reflectivity makes Venus so easy to see in the night sky also make it impossible for us to discern any surface features, at least in visible light. Until the advent of suitable radar techniques in the 1960s, astronomers did not know the rotation period of Venus. Radar observers announced that the Doppler broadening of their returned echoes implied a sluggish 243-day rotation period. Furthermore, Venus' spin was found to be retrograde that is, opposite to that of the earth and most other solar system objects, and in the opposite sense to Venus' orbital motion. NASA's Magellan spacecraft reached Venus in 1990. It carried a synthetic-aperture radar, that is, a radar that allows scientists to combine data from a sequence of positions as the spacecraft flies along the trajectory. Magellan mapped about 99 per cent of Venus' surface with a resolution of about 200 metres.

If we could stand on the surface of Venus and see the sun, it would rise in the west and then set in the east nearly two earth months later, rising again in the west two earth months after that. As Venus' rotation is so slow, the planet's solar day is quite different from its 243-earth-day sidereal rotation period. In fact, one Venus day is a little more than half a Venus year (255 earth days). Backward rotation around its own axis is called retrograde rotation, to distinguish it from forward (direct) rotation. Nearly all the planets in our solar system rotate counterclockwise as seen from the north. Uranus and Pluto are exceptions, and so is Venus. Venus' slow retrograde rotation presents us with a mystery. Why is Venus rotating backwards, and why so slowly? Nobody knows definitely why Venus rotates the wrong way.

Observing the transit of Venus across the luminous disc of the sun offers a way of measuring the most important distance scale in our understanding of the universe the astronomical unit (AU), or the mean distance between the earth and the sun. Expressed as a mean radius of the orbit of the earth, the astronomical unit becomes the standard measure for the universe a celestial meterstick. The observation of the transit of Venus is most significant from this point of view. It was Edmond Halley (16561742), the famous English astronomer, who first realised that the transit of Mercury or Venus could be used to measure the distance of the sun from the earth. As a young man, Halley journeyed to St. Helena, an island in the South Atlantic, where Napoleon Bonaparte was exiled. There Halley mapped the southern stars and in November 1677 observed a transit of Mercury. At once he realised that if two observers were widely separated in latitude, they could see a transiting planet move along different chords as it traversed the sun. If each observer recorded the transit timings from beginning to end, the shift in the planet's position, that is, its parallax, could be calculated and used to determine the earth-sun distance.

But what is parallax? It is the apparent displacement of an observed object because of a change in the point of view. You experience parallax when nearby objects appear to shift their positions against a distant background as you move from one place to another. The solar parallax is an angle formed at the sun by lines drawing from the centre of the earth and from the observer's station on the earth's surface. It is useful to define a special kind of parallax when the sun is on the horizon, and this horizontal parallax is simply the radius of the earth as seen from the sun. The solar parallax is the angle subtended at the centre of the sun by the earth's radius. If this angle is known and the radius of the earth is measured, then the earth-sun distance can be deduced by a simple calculation. The quest to use a transit of Venus to calculate the earth-sun distance became one of the great scientific observations of the 18th century. The most important factors required in this estimation are the four contact timings. Halley's theory inspired astronomers in many countries to mount expeditions to observe the transits of 1761 and 1769. Though Halley successfully predicted the measurement of the earth-sun distance from the observation of a transit of Venus, he could not see the transit of 1761; he died in 1742.

In the history of observation of the transit of Venus, the name of Jeremiah Horrocks is written in golden letters. He was an English clergyman and amateur astronomer who successfully predicted and observed the transit of Venus on November 24, 1639, and at this time, he was only 20 years old. Horrocks calculated the angular diameter and parallax of Venus. He carried out important mathematical work and suggested that they were consistent with a Keplerian orbit, and this work won him posthumous immortality. It is very tragic that such a talented astronomer died at the age of 22.

Mikhail Vasilievitch Lomonosov, the Russian poet and chemist, observed the transit of Venus from St. Petersburg in 1761 and detected a faint halo of light surrounding Venus at ingress and egress. Lomonosov correctly interpreted this as owing to a dense atmosphere around the planet itself and this was the first objective proof of the Venusian atmosphere.

Another interesting effect can be witnessed at ingress or egress. This is the black drop. As Venus passes onto the sun it appears to draw out a dark extension between it and the sun's limb, making the planet look like a black teardrop. The black drop is really an image distortion in the proximity of the solar limb. The primary causes of black drop are atmospheric turbulence and diffraction in the telescope. The black-drop effect caused significant variations in the recorded times of contact during the transit of 1761.

Prof. Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M. P. Birla Planetarium, Kolkata.