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| Tuesday, 6 May 2003 |
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Observing the Transit of Mercury across the sun by Fr. Mervyn Fernando A noteworthy celestial event will occur on Wednesday 7th May when the planet nearest the sun, Mercury, could be seen going across its face. This phenomenon is naturally applicable only to the two inner planets, namely, those between the Earth and the Sun, Mercury and Venus. If the earth, sun and the two planets were exactly on the same plane, this phenomenon - transit across the face of the sun - would occur with every inferior conjunction of the two planets. But because the planets' orbital planes vary, the case of Mercury by 7 degrees C to that of the Earth, transits occur either 7 or 13 years apart, or combinations of them. This is the first of only 14 transits of Mercury in the 21st century. Because the planet is almost at aphelion (point farthest from the Sun, and therefore closest to Earth) on May 7th, it will show a relatively large disc (about 12.0" across), larger than that of Mars and Venus on the same day. We in Sri Lanka are fortunate that we will be able to observe this transit almost through its entire period of over five hours, commencing around 11.15 a.m. and terminating about 4.30 p.m. It will be seen as a small black dot moving slowly across the face of the sun (as shown in the diagram), through a small telescope, equipped with a sun filter. Looking at the sun with the naked eye or through any instrument without a sun filter will cause irreparable eye damage. Mercury is a very elusive planet for two reasons; on account of its small size,a little over 1/3 the size of the Earth, and because of its proximity to the Sun. It can be seen very low in the sky, even by the naked eye under good seeing conditions, only at sunrise and at sunset, like its much brighter elder sister, Venus. Both are evening and morning "stars". In Greek and Roman mythology. Hermes, the god of magic, medicine, commerce and the occult, who carried messages between gods and mortals, was known as Mercury. For the same (above mentioned) reasons, of the classical planets. Mercury had proved to be the most difficult to study with earth-based optical telescopes. It was only in 1965, with the help of the Arecibo radio telescope in Puerto Rico, that the length of Mercury's day was definitely established, an extraordinarily long 58.65 earth-days. Earlier it was believed that Mercury's rotation was synchronous with its revolution, keeping the same face towards the sun, as moon does to the earth. Most of the information we have about mercury has come from the spacecraft Mariner 10 which made three close approaches to the planet in 1974 and 1975, and mapped about 40 of the planets' surface. As a result of its very long day (and night) the planet is subject to extremes of temperature; day temperatures can go up to 450 degrees C, at perihelion (nearest distance to Sun), and - 180 degrees C at night. The maps of Mercury drawn from the data of Mariner 10 reveal a surface similar to the Moon, with craters and "seas" (maria). The largest impact crater, Caloris Basin, is huge, about 1300 Km in diameter. Many of the large craters (excluding Caloris Basin) have been given the names of famous musicians, artists and writers: Beethoven, Tolstoi, Goethe, Matisse, Mozart, Homer, Rodin, Rapahael etc. Mercury is the only planet that has the distinction of being associated with Einstein and his theory of relativity. The observed advance of the perihelion of Mercury, of about 43 are seconds per century could not be accounted for by Newton's theory of gravitation; but Einstein's theory of the curvature of space time was able to predict this small relativistic contribution to the advance of the perihelion. This is one of the two "classic" proofs of Einstein's theory (the other being the prediction of the bending of light passing close to the sun, experimentally verified at the eclipse of the sun in 1918). Watching this transit will serve as a warm up to the much grander spectacle of the transit of Venus on June 8, 2004. (The writer is Director of SUBODHI Institute and President of the SL Astronomical Association). |
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