|Click to enlarge (Source: NASA)|
|Map produced by Fred Espaneck for his "2012 Transit of Venus" web
shows that the entire event will be widely visible from the western Pacific,
eastern Asia, and eastern Australia. venustransit.nasa.gov)
Transits of Venus are very rare, coming in pairs separated by more than a hundred years. This June's transit, the bookend of a 2004-2012 pair, won't be repeated until the year 2117. Fortunately, the event is widely visible. Observers on seven continents, even a sliver of Antarctica, will be in position to see it.
The nearly 7-hour transit begins at 3:09 pm Pacific Daylight Time (22:09 UT) on June 5th. The timing favors observers in the mid-Pacific where the sun is high overhead during the crossing. In the USA, the transit will be at its best around sunset. That's good, too. Creative photographers will have a field day imaging the swollen red sun "punctured" by the circular disk of Venus.
|The rare transit of Venus across the face of the Sun in 2004 was one of the|
better-photographed events in sky history. (APOD, NASA)
Observing tip: Do not stare at the sun. Venus covers too little of the solar disk to block the blinding glare. Instead, use some type of projection technique or a solar filter. A #14 welder's glass is a good choice. Many astronomy clubs will have solar telescopes set up to observe the event; contact your local club for details.
Transits of Venus first gained worldwide attention in the 18th century. In those days, the size of the solar system was one of the biggest mysteries of science. The relative spacing of planets was known, but not their absolute distances. How many miles would you have to travel to reach another world? The answer was as mysterious then as the nature of dark energy is now.
Venus was the key, according to astronomer Edmund Halley. He realized that by observing transits from widely-spaced locations on Earth it should be possible to triangulate the distance to Venus using the principles of parallax.
The idea galvanized scientists who set off on expeditions around the world to view a pair of transits in the 1760s. The great explorer James Cook himself was dispatched to observe one from Tahiti, a place as alien to 18th-century Europeans as the Moon or Mars might seem to us now. Some historians have called the international effort the "the Apollo program of the 18th century."
In retrospect, the experiment falls into the category of things that sound better than they actually are. Bad weather, primitive optics, and the natural "fuzziness" of Venus’s atmosphere and other factors prevented those early observers from gathering the data they needed. Proper timing of a transit would have to wait for the invention of photography in the century after Cook’s voyage. In the late 1800s, astronomers armed with cameras finally measured the size of the Solar System as Edmund Halley had suggested.
This year’s transit is the second of an 8-year pair. Anticipation was high in June 2004 as Venus approached the sun. No one alive at the time had seen a Transit of Venus with their own eyes, and the hand-drawn sketches and grainy photos of previous centuries scarcely prepared them for what was about to happen. Modern solar telescopes captured unprecedented view of Venus’s atmosphere backlit by solar fire. They saw Venus transiting the sun’s ghostly corona, and gliding past magnetic filaments big enough to swallow the planet whole.
2012 should be even better as cameras and solar telescopes have improved. Moreover, NASA’s Solar Dynamics Observatory is going to be watching too. SDO will produce Hubble-quality images of this rare event. (Science@NASA)
|Solar Dynamics Observatory will make high resolution photos|
Venus is not only nearby, but its orbit brings it closest to Earth of all the planets. Which along with its bright atmosphere goes a long way toward making it the third brightest object in the sky (the sun and moon are one and two). Along with Smrekar and many other equally intrigued planetary scientists, you can add to the list of those studying the second planet from the sun the ancient Babylonians, who noted its wanderings in texts as far back as 1600 BC. And anyone who has ever sweated out a Pythagorean Theorem in school (A2+B2=C2) might find some solace in knowing that Greek mathematician Pythagoras sweated out the orbits of Venus, eventually becoming the first to determine that what had been believed to be unique and separate evening and morning stars (as believed by the ancient Egyptians and Greeks), was actually just one object – Venus.
But for all that these ancient astronomers and their medieval contemporaries (including the Aztecs back in the 1500s) were able to deduce, no human had ever laid eyes on Venus as more than a bright dot in the sky until Galileo Galilei, who in 1610 was the first human to actually see Venus in various kinds of light. With his telescope, Galileo started cranking out Venetian discoveries, including how the planet changed its illumination phase just like the moon as it circles Earth. Galileo's telescope provided strong evidence that Venus goes around the sun, and not Earth, as most of his contemporaries believed.
After Galileo, Venus came under even more intense scrutiny, both scientific and fanciful. More than one astronomer (and science fiction author) theorized it was home to some type of life form. The thick, impenetrable clouds allowed them to imagine tropical environs with steady rainfall and lush vegetation.
|This is a picture of Venus’s atmosphere, taken by Venus Express|
on 23 July 2007. The view shows the southern hemisphere of the planet. (ESA)
With the dawn of robotic space probes, America's Mariner 2, built by JPL, became history's first interplanetary traveler when it flew past Venus on Dec. 14, 1962. All told, 45 missions targeting Earth's twin have been launched by the United States, Russia (and former Soviet Union), and Japan. All this probing by astronomers and robotic explorers has found Venus to be replete with 900-degree-Fahrenheit (500-degree-Celsius) temperatures in a carbon-dioxide-rich atmosphere with pressures equivalent to being half a mile below the ocean surface. It is not a particularly hospitable environment.