|Fred Espenak's composite image of the Transit of Venus 2004|
After waiting since 2004 for another Transit of Venus, the 2012 Transit of Venus has finally arrived, preparing millions of viewers worldwide to see the spectacular event. Loosely described as the quiet silhouette passing across the luminous disk of the sun, the rare astronomical phenomena of the Transit has stunned and amazed astronomers throughout the ages and now another is upon us. You, who are reading this article now, will never be able to witness another Venus transit again in your lifetime: as the next comes in 2117.
As we already know that Venus transits are rare, coming in couplets distributed over hundred year periods, what exactly is a transit, defined in an astronomical sense? Though eloquent as it may sound, planetary transits are far less common then eclipses, as the planets align much less frequently then the moon does. The Oxford Dictionary of Astronomy states that a transit is "the passage of one object across another of larger apparent diameter, such as Mercury and Venus in front of the Sun, or its shadow across the face of a planet." Correctively speaking, when a shadow crosses a larger object, it is hence called a shadow transit. So, there are two types of transits: shadow transits and regular transits (with no special name except for ‘transit’). Transits of planets across the Sun will not have a shadow cast, but usually transiting moons do.
Io usually casts a shadow when transiting Jupiter’s surface (the Sun’s light help Io cast shadows), while Mercury won’t because it transit’s across the Sun’s disk—nothing is there to cast a shadow. That brings me to an important observation: only two planets may transit the Sun as viewed from earth. They are Mercury and Venus. Because earth is the third planet in planet progression in the solar system, we can only see two planets transit, whereas Saturn can see five; Mercury, Venus, earth, Mars, and Jupiter. Jupiter is probably big enough to blot out the Sun as viewed from Saturn, so it could be considered a planet eclipse. These are rare occurrences; and not much interest has been given to it.
Yet, the history of such a rare astronomical phenomena is quite spectacular. Unlike eclipses that have been viewed from so early on in the books of the past, about one-thousand BC, the first transit (of any celestial body) was viewed on November 7, 1631 by French astronomer Pierre Gassendi. It was a transit of Mercury across the Sun’s disk; predicted by Johannes Kepler just four years before. Mercury also transited in years that followed; on November 7,1677 Sir Edmund Halley (who discovered Halley’s Comet) was the first man ever to witness a complete transit of Mercury, Gassendi obviously did not catch a whole transit, but a partial one. All that you really see is a black spot moving across the Sun’s surface. Mercury takes up 1/194 of the Sun’s disk, so although it may seem like nothing, it is an extremely rare astronomical event.
Venus, because of having a larger orbit, transit much less frequently, making it an extremely rare event. Only seven events have ever been viewed since the making of the telescope (as of 2010). Just one month after Gassendi viewed the transit of Mercury, Venus transited, but when Gassendi tried to view it, he tried in vain, because the transit was not able to be seen in Europe. Later on, astronomers Jeremiah Horrocks and William Crabtree became the first two men ever to witness a transit of Venus, but there has been some controversy to that. On May 24, 1032 AD, Persian polymath Avicenna had claimed to be the first man ever to observe a transit of Venus. He wrote Compendium of the Almagest (a commentary on Ptolemy’s Almagest) in which he concluded that Venus is closer to Earth than the Sun. This was a great step in astronomy at the time, because geocentric views of the universe were taking shape. If the universe was geocentric, that meant the earth was the center of the universe. The heliocentric view (Sun is the center of the universe) was not used at all.
Another boggling concept is transit ‘Saros.’ Just like the eclipse Saros, transits can be grouped into families. The Venus transits of the years 1518, 1761, and 2004 would belong to one family, while transits in 1639, 1882, and 2125 would belong to another. Those groups were determined by a period of 88,756 days (or 243 years) in which this transit ‘Saros’ is grouped. Mercury’s transits can also be grouped, as in one set (separated by 16,802 days or 46 years) separate the years 1957, 2003, and 2049 belong to one group, and 1960, 2006, and 2052 belong to another. Although a little too complex to explain in a short paper, transit ‘Saros’ is a very original idea; for almost all astronomical phenomena can be grouped in some way or another!
As mentioned before, only two planets may ever transit the Sun as viewed from earth. Mercury appears as a small speck on the Sun’s surface, while Venus is a bit larger. Edmund Halley, also used transits as a great help: “Edmund Halley first realized [in 1716] that transits could be used to measure the Sun's distance, thereby establishing the absolute scale of the solar system from Kepler's third law. Unfortunately, his method is somewhat impractical since contact timings of the required accuracy are difficult to make. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance” stated the Transits Page at NASA’s eclipse website. This helped us determine how far the Sun is away from us, and gave Halley the credit for his observation.
In 1631, 1639, 1761, 1769, 1876, 1882, and 2004 Venus was seen transiting. It is much rarer (Mercury transits so much more) because Venus’ orbit is much larger than Mercury’s. The larger the orbit of a planet is; the less likely an astronomical transit is to take place. Only in early June and December can you view one; if there is an eclipse in early June as well, then two spectacular events will occur in one week! On June 5/6 2012 (depending where you live on the globe) Venus will transit the Sun for the last time until 2117. On June 4th (2012), a partial lunar eclipse will occur, so this week will be a treat for all who live around the Pacific Ocean. It turns out the complete visibility for the transit of Venus one/two days later is also the Pacific! (These may also be viewed In North America—but at sunset).
Transits of Venus are special—not only because they are so rare, but because they come in pairs of eight years. That explains why Venus transited in 2004 and will again in 2012. This is because the orbital periods of Venus (224.701 days) and earth are in an eight year (2922 days) resonance within each other. It takes eight years for earth to orbit around the Sun, and Venus thirteen, for both the orbits to exactly line up with each other. The first two times earth and Venus meet with each other, a transit is produced, but, Venus arrives twenty-two hours earlier the third meet, resulting in earth missing Venus completely. That’s why transits are so rare. The next one takes 105.5 or 121.5 years to make another transit. Two Mercury transits, on the other hand, are consecutive between 3.5, 7, 9.5, 10 or 13 years. This pattern is very complex on account of Mercury’s elliptical orbit. From there, a plethora of different year combinations come up, each resulting in a different calculation of years. By adding the years between transits, for example, one used commonly is 10 + 10 + 13 which equals 33, produces a better fit than just 10 or 33. Hundreds of combinations like these can be combined, giving us an irregular pattern of transits.
It is plain to see the history of transits is spectacular. But, will I be able to see the transit of Venus on June 5-6? The answer is yes and no. Yes: everyone on every continent at various times will be able to see the event. No: you need a special filter (to block out dangerous rays from the sun) and a telescope to see the actual planet. DO NOT LOOK DIRECTLY AT THE SUN, as its rays will blind you—many astronomy companies sell special filters for such events.
|Visibility map from eclipse-maps.com|
For more on this amazing celestial events, Sky&Telescope has a plethora of information about viewing times and what you'll see with a telescope (and special filter!).