Exploratorium: Transit of Venus

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Except for the Sun and Moon, Venus is the brightest natural object in Earth’s sky.



These images of Venus were created from radar images taken by the Magellan spacecraft, with additional data from Pioneer and Venera. Copyright 1998 by Calvin J. Hamilton

Thousands of years ago, early Greek, Egyptian, Babylonian, and Chinese observers knew of Venus and recorded the planet’s motions. At Chichen Itza in Central America, the Maya built an observatory that focused on the points where Venus rose and set on the horizon. To some, Venus was known as the Morning and Evening Star, and was commonly thought to be two different heavenly bodies.

With the development of the telescope in the 1600s, astronomers began to study the planet more closely. By the 1700s, most considered Venus to be Earth’s "twin," believing that the planet, which is almost the same size as Earth, was inhabited by humans. A century later, scientists decided instead that Venus must be warm and wet; more like the early Earth— swamp-dwelling dinosaurs and all.

Named for the Roman goddess of love and beauty, Venus remained a mystery until the 1900s, when radar imaging and space probes began revealing a planet as hellish as it was handsome.

Venus is the second planet from the sun, but the hottest in the solar system. Veiled in clouds of sulfuric acid, its heavy atmosphere traps solar energy, so temperatures on the planet can get hot enough to melt lead (900 degrees F/480 degrees C). At the surface, flattened craters mark two desert continents, and volcanoes rise from giant plains smoothed by lava flows. Scientists think bright, reflective areas may be covered in glittering iron pyrite—"fool’s gold."

In Venus’s carbon-dioxide-rich-atmosphere (deadly to humans), lightning storms are common, and fierce winds whip around the planet at high elevations. Gravity is about the same as on Earth, but atmospheric pressure on Venus is crushing: about 90 times that of Earth’s.

Because Venus rotates so slowly on its axis, a single Venus day is 243 Earth days long. A patient observer would see the Sun rise in the west and set in the east, since Venus’s direction of spin is opposite that of Earth’s. Stranger still, in that time, more than a year would have passed: A year on Venus is 224.7 Earth days.

Inhospitable as Venus may seem, scientists think the planet might once have supported life. In fact, some researchers have recently suggested we could find life there today—microbes hiding high up in the planet’s acidic cloud cover, where pressure is closer to Earth’s and water vapor is suspended.

Fortunately, two missions to Venus are now in the works, both designed to test the planet’s atmosphere. If all goes well, by 2010 we may have tantalizing new information about our forbidding twin.