Tracking Venus

Published : Jun 18, 2004 00:00 IST

The transit of Venus, which led to the calculation of the Astronomical Unit, has evoked tremendous interest among the scientific community through the ages.

SINCE we know that the sun rises in the east, it would be a surprise if it did on the west. But if one lived on Venus, this is precisely what would happen since the rotational directions of Venus and the earth on their own axes are opposite to each other. Historically, Venus is important in another sense - the comparison of its motion with that of the earth established that planets move around the sun. On June 8, 2004, Venus will `eclipse' the sun, an event that last occurred 122 years ago. In recorded history, people have seen it only five times. Importantly, this is the first time in our own life-time and the first of the only "pair" of transits in the present century, with the next one being in 2012. Unlike Halley's comet, which returns every 76 years, no one has lived long enough to see the "pair" of Venus transits twice. However, the transits are not periodic and prediction of the dates and times must incorporate the motions of both the earth and Venus around the sun.

The eclipse of the sun by Venus is called a transit because Venus, while moving between the earth and the sun, will not be seen to gobble up the sun. It will be seen as a tiny dot, moving across the sun's disc, that is, it will be seen in transit. The first ever transit of a planet, Mercury, was seen by Pierre Gassendi in Paris in 1631 using the Galilean telescope. Galileo had first used the instrument in 1609 to look into the heavens and a year later to look at Venus. The great astronomer Johannes Kepler had predicted that a transit of Venus was due in 1631, but he did not live to see it; he died in 1630. The rest of the world, too, did not observe it because it occurred after sundown in most of Europe and in other parts of the world people were not aware of Kepler's predictions and even if they were they did not have telescopes to observe the event.

The first recorded observation of the transit of Venus was by British clergyman Jeremiah Horrocks in 1639. The other observer was his compatriot William Crabtree. Horrocks not only observed the event, but also used his observations to measure the distance between the earth and the sun. Called the Astronomical Unit (A.U.), this distance had defied measurement for about 2,000 years at that time. Greek astronomer Aristarchus (circa 310 B.C.-240 B.C.) was the first to try to measure the distance. He observed that if one looks at the sun from two different points on the earth at the same instant of time, the direction of the sun changes. For example, if one moves a distance L on the surface of the earth and finds the direction of the sun to change by an angle , then elementary geometry suggests that the earth to sun distance is given by S=L/. He tried to measure L and the corresponding change, that is, each time he shifted by a certain distance (must be very large) he tried to find the change in the direction of the sun. His results showed that the earth-sun distance was 6,500 kilometres! He also found that the diameter of the sun was about one-tenth of the earth-sun distance. The second result was indeed correct but the first was wrong as Aristarchus took the earth to be a flat disc. About a 100 years after Aristarchus, Erastothenys of Syrene repeated Aristarchus' observations essentially and showed that it was in fact the proper method to find the radius of the earth, considered to be a sphere.

Erastothenys made his observations at two points, Alexandria and Syrene, separated by 5000 stadia. He found that on the solar solstice, while the sun's rays vertically illuminated a well in Syrene, it came at an angle of 70 at Alexandria. The circumference of the earth can be easily calculated to be 2 R = 5000 x 360/7 = 257142 stadia, which finally gives the radius of the earth to be R = 40926 stadia. It is not known what the unit called stadia was, but the above method gave the fundamental principle for determining the radius of the earth, and was one of the greatest experiments in the history of science. It really showed how big the earth really is. We know today that this radius is nearly 6,500 km.

On the appointed day, Horrocks took his 1.5-inch telescope and projected the image of the sun on the wall of a building near his church, magnifying the image to a 6-inch size diameter. The diameter of the image was divided into 30 equal parts, that is, he virtually drew a graph paper with his own hands. Horrocks tried to track the transit from 9 a.m. onwards until 1 p.m. From 1 p.m. to 3-15 p.m., he had to leave for conducting prayers, described by him as "business of highest importance, which for these ornamental pursuits, I could not with propriety neglect". When he returned at 3-15 p.m., he could see the shadow of Venus, as a tiny dot, in transit across the sun. This could not be mistaken for a sunspot, as the size of the dot was larger than that of a typical sunspot. And it moved.

Horrocks wrote: "At this time, an opening in the clouds, which rendered the sun distinctly visible, seemed as if Divine Providence encouraged my aspirations: When, Oh most gratifying spectacle! the object of so many earnest wishes, I perceived a spot of unusual magnitude and perfectly round form, that had just wholly entered upon the left limb of the sun, so that the margin of the sun and spot coincided with each other, forming the angle of contact."

Horrocks could see only three observations, which he recorded, at 3-15 p.m., 3-35 p.m. and 3-45 p.m. He measured the diameter of Venus' shadow and compared it with that of the sun's and found the former to be 4 per cent of the latter. The sun's disc, when viewed from the earth is seen to make an angle of 31'30" (0.5252), while Venus makes 1'16" (0.0210). He found from simple geometry that the method really did not give the earth-sun distance but gave the clue that Venus subtends an angle of 28", that is, 0.0070 at the centre of the sun. He then used an assumption (a remarkably correct one at that) that the earth too subtends an angle of 28 at the centre of the sun. This is called the solar parallax. This automatically gave that the earth-sun distance equals 14,700 times the radius of the earth, which turns out to be 150 million km. And what an astounding number it was - a distance scale no one had ever conceived of.

Man's concept of the universe kept changing. The process that Polish astronomer Nicholas Copernicus began at a Christian seminary was brought to completion nearly 100 years later. Copernicus said that planets moved in orbits and Horrocks explained how big these orbits were. Such outstanding contributions have come once in several centuries. It was indeed the contribution of a lifetime, at least fate ordained it to be so. Jeremiah Horrocks died at the age of 22, after giving mankind the Astronomical Unit, that is, the equivalent of a "metre scale" for the universe - an act reminiscent of Mozart's completion of his masterpiece, prophetically called `The Requiem', during the time he was terminally ill.

The 1761 transit of Venus received great attention from the astronomical community. As early as 1716, Edmund Halley urged astronomers to conduct expeditions worldwide and to follow the transit. Halley died in 1742 but had seen the 1676 transit of Mercury from the island of St. Helena and commanded the astronomical community from the "gulph of the Ganga to the Kingdom of Pegu" to follow the 1761 event in right earnest. His idea was to find an accurate value of the solar parallax, which he said was within human reach, "without any other instruments than the telescopes and good common clocks and without any other qualifications in the observer than fidelity and diligence, with a little skill in astronomy".

The All India People's Science Network (AIPSN), a science popularisation group, has taken up Halley's cause and plans to mobilise school children to view the 2004 transit and repeat Horrocks' observations through simple experiments and calculate the A.U. for themselves - programmes ideally suited for the year of scientific awareness.

The Venus transit has always attracted experts and also evoked interest among the people, for whom mass viewings have been conducted. In India, the 1874 transit evoked a lot of interest. Chintamani Raghunathachary and Ankitam Venkata Narasinga Rao observed the transit from India. Raghunathachary had produced a book on this event, in Urdu, preparatory to the transit, but his observations are not known. Narasinga Rao made observations of the event from his private observatory in Visakhapatnam and published them in the Monthly Notices of the Royal Astronomical Society in 1875.

Expeditions do produce heroes but they also produce tragic heroes. One such person was French astronomer Joseph Hyacynthe Jean-Baptiste Le Gentile de la Galaisiere. The year 1761 was the time of the Seven Years War between England and France. In spite of hostilities, scientists of warring nations were allowed passage through enemy nations. Le Gentile decided to view the event from Pondicherry, but by the time he arrived, Pondicherry had fallen to the British. He decided to board a ship, sailing to Mauritius and the transit occurred during his voyage, making it impossible to conduct any observation. Undaunted, he waited in Madagascar for the next transit, studying the flaura, fauna, and anthropology of the island. He came to Manila in 1766 but got into problems with the local administration. Le Gentile moved to Pondicherry again for the 1769 transit. The British lent him a telescope for the observation. On June 2, 1769, the weather was perfect but the next day, the due date for the transit, the sky became overcast. Half an hour after the transit, the sky cleared up. The next transit being due in 1874, Le Gentile returned to France. There he found that he was thought to be dead and his property was being distributed amongst relatives. Le Gentile fought and won legal battles for the restoration of his rights. The fascinating story of this tragic hero shows how the pursuit of science produces people of dauntless character, whose only aim is to see the triumph of knowledge.

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