June 4, 2012

Venus will make a rare and historic journey across the face of the sun this week, an event that people alive today will almost certainly never have another chance to see. That journey, called a transit, will mark Venus’s passage between Earth and the sun, an astronomical event that comes along only about every 110 years. During the transit, Venus will appear as a dark dot moving across the sun’s disk. The transit can be seen on June 5 in much of the Western Hemisphere and on June 6 in a large part of the Eastern Hemisphere. Transits of Venus happen in pairs about eight years apart. The first of the current pair was in 2004. The previous transit occurred in 1882; the next won’t occur until December 10-11, 2117.

Venus appears as a dark dot moving across the sun's disk during a transit. Venus transits occur in pairs in which one transit happens 8 years after the other. (Jay M. Pasachoff, et al., Williams College Transit of Venus Team with John Seiradakis, et al., Aristotelian University, Thessaloniki, Greece)

A transit of Venus is an amazing astronomical display–and much more. While trying to determine how the planets move around the sun, the great German astronomer and mathematician Johannes Kepler became the first to predict a transit of Venus. But he died soon before the transit and no record of that event exists. However, the English astronomers Jeremiah Horrock and William Crabtree recorded the 1639 event.

In 1716, the English astronomer Edmond Halley proposed that the transit of Venus could be used to determine the distance between Earth and the sun. He suggested that by measuring the duration and time of the transit from many different locations, scientists could triangulate that distance. Unfortunately, measurements made during the next transit, in 1761, were insufficient.

Fortunately, the transit eight years later sparked a worldwide interest in the project. Scientists and other interested people scattered to the far reaches of the globe to gather the needed information. Among the observers was the English explorer James Cook. On orders from the Royal Navy, he traveled to the South Pacific island of Tahiti to make the observations. All the data gathered during the transit enabled scientists to calculate that Earth was 95 million miles (153 million kilometers) from the sun (very close to the actual distance of about 93 million miles [150 million kilometers]).

Scientists plan to use the 2012 transit to help in the search for extrasolar planets. Since 2009, NASA’s Kepler space telescope has been watching for small changes in the brightness of alien stars caused by the transit of an orbiting planet. By analyzing the starlight passing through the extrasolar planet’s atmosphere during the transit, scientists can learn about the chemicals in its atmosphere. During the transit of Venus, scientists will be studying Venus’s atmosphere with the Hubble Space Telescope. Although that planet’s atmosphere is already well understood, the information may help them determine the composition of extrasolar atmospheres.

Here are a few ways to view the transit safely. WARNING: NEVER LOOK DIRECTLY AT THE SUN.
1)    Wear solar eclipse shades, also called “solar filters,” which can be found online and in some science stores. Welders goggles rated “14″ or higher will also protect your eyes.
2)    Make a pinhole projector. Use two pieces of cardboard; punch a small hole in one of them. Hold this piece up to the sun so that sunlight passes through the hole and casts an image on the other piece.
3)    Use a telescope as a projector. You can do this by pointing the telescope at the sun and then projecting the image in the eyepiece onto any nearby flat surface, such as a wall or a piece of cardboard or paper. You can also use a pair of binoculars. Use only one side and cover the other side. (You could use both sides to make two images.)
4)    Watch the live webcast from atop Mauna Kea in Hawaii sponsored by NASA EDGE: http://sunearthday.nasa.gov/2012/transit/webcast.php.

Additional World Book articles:

• Space exploration (Probes to Venus and Mercury)
• Spectrometer
• In Search of Other Worlds (a Special Report)