Regular readers may be tired of being reminded, but because of the rarity of this event, we will do so, again. The next transit of Venus will occur over June 5-6. The transit (passing) of the planet Venus across the face of the sun is a relatively rare event — occurring in pairs with more than a century separating each pair. There have been all of 53 transits of Venus across the sun between 2000 B.C. and the last one occurred in 2004. On Tuesday, June 5 (Wednesday, June 6 in the Eastern Hemisphere), Earth gets another shot at it. The next pair will not be until December 2117 and December 2125. But beyond this, why has Venus been an object worthy of study for hundreds of centuries?
Venus is a planet of extremes. The surface is hot enough to melt lead due to its runaway greenhouse atmosphere. Yet in many respects, Venus is Earth's twin in general size, gravity and bulk composition.
Venus is the closest planet to Earth of all in the solar system. Its proximity to Earth, combined with its dense and highly reflective clouds, contributed in making it the third brightest object in the sky (the sun and moon are one and two, respectively).
The ancient Babylonians noted the wanderings of Venus in texts as far back as 1600 BC. The Greek mathematician Pythagoras studied the movements of heavenly bodies and eventually became the first to determine that the "morning star" and "evening star" of Greek and Egyptian cultures were in fact one object — Venus.
But for all of the deductions of these ancient astronomers and their medieval contemporaries (including the Aztecs back in the 1500s), no human had seen 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 made several Venetian discoveries, including the fact that Venus changed its illumination phase just like the moon which circled Earth. Galileo's telescope provided strong evidence that Venus moved 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 climates with steady rainfall and lush vegetation.
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 December 14, 1962. All told, Venus 45 missions 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. Even so, Venus holds many lessons for us about the climate and interior of Earth and Earth-like planets in other solar systems.
If you're in the western Pacific, eastern Asia and eastern Australia, you'll get a great view of the entire event. North and Central America, and northern South America get the beginning of the transit on June 5, but the sun will set before the event ends. Conversely, Europeans, as well as those watching in western and central Asia, eastern Africa and western Australia will get a glimpse at the tail end. If you want to know more, check out these URLs.
NASA's web page for all things Venus transit: venustransit.nasa.gov/transitofvenus/ .
2012 Transit of Venus, NASA/Goddard Space Flight Center: eclipse.gsfc.nasa.gov/OH/transit12.html
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