Shadows and Silhouettes:  Looking for Transits
Map shows where on Earth the Mercury Transit will be visible and not visible. Image
Credit: NASA

On November 8, Mercury will transit the Sun. From Earth, we'll see the silhouette of Mercury travel across the disk of the Sun. Transits are uncommon but predictable events. The geometry of Mercury (or Venus), the Earth and the Sun has to be aligned precisely for a transit to occur. Most of the time, these inner planets cross above or below the disk of the Sun.

Historically, transits of Mercury and Venus promised a method to better measure the distance to the Sun. Johannes Kepler predicted and then endeavored to see a transit of Mercury in 1607. Likely, he observed a sunspot with his camera obscura--two years before Galileo discovered sunspots with his primitive telescope in 1609. Later, telescopes aided transit observers in estimating the scale of the solar system. This is a difficult method, fraught with problems of timing, optical effects caused by Earth's atmosphere, poor weather and other complications. Observing transits gave us the first reasonable estimates for the distance scale of the solar system, but other methods have superseded transit observations. Although solar transits are now interesting rather than compelling for scientists, transit observations are far from dead.

NASA's Kepler Mission will be the first space mission to search for Earth-size and smaller planets around other stars in our neighborhood of the galaxy. It's named for Johannes Kepler, the first astronomer to predict a transit. The Kepler spacecraft is a special-purpose telescope that will precisely measure the light variations from distant stars, looking for planetary transits. Searching for transits of distant "Earths" is like looking for a drop in brightness when a moth flies across a searchlight. Measuring repeated transits, all with a regular period, duration and change in brightness, provides a rigorous method for discovering and confirming planets and their orbits--planets the size of Earth and smaller in the habitable zones around other stars. NASA's Kepler Mission launches in 2008, and will search more than 100,000 stars for evidence of other Earths in transit across their stars. We'll learn whether Earth-size planets are common or rare.

Here at home, the transit of Mercury is a good reason for a daylight astronomy event. The transit will last for almost 5 hours, and observers in the Americas, Australia, eastern Asia and the Pacific Ocean will be able to see all or part of Mercury's excursion across the disk of the Sun. (This transit will not be visible from Europe, Africa and western Asia--it will be nighttime there when the transit occurs.) If you will be on the sunny side of planet Earth, now's the time to plan an event with your local astronomy club. [See map of transit visibility here.]

More than 200 astronomy clubs across the US are part of the Night Sky Network (NSN), a network of amateur astronomers who are interested in sharing astronomy with the public. Participating NSN clubs receive kits of materials that support star parties, mini-lessons on astronomy, demonstrations of astronomical concepts. For NSN members, teleconferences with scientists offer further information on current astronomical events, like the transit of Mercury. In early September, a new kit was distributed to NSN clubs: "Shadows and Silhouettes." This new kit was developed by the Astronomical Society of the Pacific in collaboration with the Kepler Mission Education and Public Outreach program, which is conducted by the SETI Institute and Lawrence Hall of Science at University of California Berkeley in collaboration with NASA. So far, 160 clubs have received the new materials, with more applying for their kits each day.

"Shadows and Silhouettes" provides the tools for demonstrations and short lessons on shadows and silhouettes in the astronomical context: how shadows help us observe craters on the Moon, phases of the Moon, eclipses, and--of course--transits. Amateur astronomers across the nation will be reaching out to the public to demonstrate and explain transits and their importance for NASA's Kepler Mission. On November 8, some will be observing the transit of Mercury. You may be able to observe this transit, almost four centuries after Johannes Kepler first predicted that Mercury would transit the Sun in 1607.

For more information, please visit these websites:

-          Night Sky Network. Identify a NSN club in your region, or, learn how your astronomy club can become a member.

-          ASP's "Universe in the Classroom:" Learn how to observe the transit safely.

-          NASA's Kepler Mission: Learn about this mission to find other Earths; build a transit model.

-          SETI Institute: Learn about searching for life beyond Earth.

-          Lawrence Hall of Science: Explore science and math education resources.

More information on Planetary Transits:

  • Images: Venus Transit Gallery
  • Detecting Other Worlds: The Transit or 'Wink' Method
  • Way-Out World: New Technique Finds Most Distant Planet Ever
  • Mercury Transits Sun, Images on Web