A celestial spectacle

The transit of Venus, a rare astronomical event, will occur on June 8 and will be visible all over India.

A TRANSIT of Venus over the disc of the sun will occur on June 8, 2004. Astronomers are eagerly awaiting this rare event, as it will be taking place after a gap of 122 years. No one alive at present has seen a transit of Venus - the last one occurred in 1882.

In describing the motions of the planets relative to the earth, it is convenient to divide them into two classes - those nearer the sun than the earth are called inferior, those that are more remote are called superior. Thus, Mercury and Venus are inferior planets. An inferior conjunction of Venus occurs when the planet comes in between the sun and the earth. During a superior conjunction, Venus is on the opposite side of the sun in relation to the earth. 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 - counter-clockwise, as seen from the north celestial pole. The orbital plane of Venus is inclined at approximately 3 to the plane of the ecliptic. The two points on Venus' orbit where it crosses the ecliptic plane are known as nodes. The line joining them, which is also the line of intersection of the earth's orbit and the orbital plane of Venus, 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 Venus is near its node, then it is seen from the earth 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 the transit of Venus. The condition necessary for a transit is similar to the requirement for a solar or lunar eclipse.

A transit is analogous to an annular eclipse of the sun. It is an astronomical event, where a smaller, dark object passes in front of a larger, bright one. In other words, a transit occurs when the shadow of an inferior planet falls upon the earth. Thus it is possible in the case of the inferior planet Mercury too, and we do witness transits of Mercury occasionally. However, solar transits of Venus are exceedingly rare events - rarer than the transits of Mercury - primarily because Venus is farther from the sun and the proper alignment occurs less frequently. But when they occur, they do so in pairs, eight years apart, in June or December. After the June 2004 event, the next transit is due in 2012, and then only in 2117 and 2125.

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

Venus is the third brightest object in the sky after the sun and the moon and that is why it is very easy to see, even when it is close to the sun or low in the sky. Although Venus is not a star, it appears more than 10 times brighter than the brightest star in the night sky, Sirius. Venus can be seen even during daytime, if we know just where to look for it. On a moonless night, away from city lights, the faint shadow cast by the planet is visible. Its brightness stems from the fact that it 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 clouds that are 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 or just after sunset 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. The early Greek astronomers thought that the morning Venus and the evening Venus were two separate objects. By about the 6th century B.C., the truth had become clear and the planet was named after Aphrodite, the Greek goddess of love; Venus is her Roman name.

One might expect Venus to appear the brightest when it is "full", that is, when the entire sunlit side is visible. Venus is full when it is in superior conjunction, 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 closest to the earth, at inferior conjunction, the planet is at the new phase, lying between the earth and the sun. At this time it cannot be seen because the sunlit side is on the other side. As Venus moves away from inferior conjunction, more and more of it becomes visible. The planet's maximum apparent brightness actually occurs about 36 days before or after inferior conjunction. The elongation of the planet at this time is 39 and it is seen as a rather fat crescent.

Venus is surrounded by a thick cloud, whose reflectivity makes the planet so prominent in the night sky. At the same time it makes it impossible for us to discern any of the surface features of the planet, 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's spin was found to be retrograde - that is, opposite to that of the earth and most objects of the solar system, and in the opposite sense to Venus's orbital motion. The National Aeronautics and Space Administration's (NASA) 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 200m.

If we could stand on the surface of Venus and see the sun, which is not possible, it would rise in the west and set in the east nearly two earth months later, rising again in the west two Earth months after that. As the rotation of Venus 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 (225 earth days). Such backward orbital revolution around the sun is called retrograde rotation - to distinguish it from forward (direct) rotation. Nearly all the planets in the solar system rotate counter-clockwise as seen from the north. Uranus and Pluto are exceptions, and so is Venus. Venus' slow retrograde rotation is mysterious. Why is Venus rotating backwards, and why so slowly? Nobody knows definitely why Venus rotates "the wrong way".

Transits of Venus have been used in attempts to determine the solar parallax. To understand the importance of a transit of Venus to 19th century astronomers, one needs to understand first the concept of parallax. In fact, a number of astronomy books - both old and new - define the measurement of the earth-sun distance as the calculation of the solar parallax. Parallax is defined as the apparent displacement of an object against a background when the object is viewed from different locations. If you hold your thumb at arm's length and alternately close first one eye and then the other, the position of your thumb seems to change against the background objects. This apparent shift is called parallax. In relation to a transit of Venus, we substitute two observers on earth for our two eyes. Venus takes the place of our thumb and the brilliant disc of the sun becomes the background. When Venus is seen by two observers separated by as wide a distance as possible (or practical), the amount of displacement may be measured. From that displacement (parallax), the distance can be calculated. For this procedure to be of value, an accurate measure of timings throughout the transit must be made by all observers. The most important measurements are those of the four contact points. It was Edmond Halley who first realised how to calculate the earth-sun distance by using measurements obtained during a transit of Venus. His theory inspired astronomers in many countries to mount expeditions to observe the transits of 1761 and 1769. Although 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.

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" effect. As Venus passes onto the sun it appears to draw out a dark extension between itself and the sun's limb, making the planet look like a black tear-drop. The black-drop is really an image distortion in the proximity of the solar limb. The primary causes of the black-drop effect 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.

The transit of Venus on June 8 will be visible in Europe, most of Africa and all of Asia except the very Far East. The transit will be visible from all corners of India. The general beginning of the transit is at 10 hours 44 minutes (Indian Standard Time) and the ending is at 6h 56m IST. (For the local circumstances relating to the four metropolitan cities, see table.)

Unlike the transits of Mercury, Venus transits are visible to the naked eye. Since Venus is nearly one arcminute in diameter, it is visible without any optical aid, as a small black dot passing across the disc of the sun. But under no circumstances should an observer look at the sun without using an approved, safe filter. The safest way to observe the transit is by projecting the sun through a small refracting telescope onto a piece of card. Never look along the telescope to line it up. Always have your back to the sun and use the shadow of the tube to home in on the sun. Keep finderscopes capped at both ends for safety (the little plastic ones melt if left uncapped). If you want to observe directly, you must use a genuine solar filter. Make sure it has a proper mounting that securely fastens to the telescope.

Prof. Amalendu Bandyopadhyay is Senior Scientist, M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.