Rare Transit of Venus Today

Today Venus will pass between Earth and the sun, appearing as a small black dot moving across the sun's face. The transit of Venus is a rare event, and this afternoon will likely be your last chance to experience it—the next one won't happen until 2117.

From Pasadena, Venus will begin creeping across the sun at 3:06 p.m., and the transit will be visible until the sun sets at 8:02 p.m.

Caltech's Astronomy Outreach group will host a viewing event from 3:00 p.m. to 7:30 p.m. at the Cahill Center for Astronomy and Astrophysics on campus. Protective glasses will be available for spectators who want to watch the transit directly. The group will also set up three telescopes for a close-up view; two of them are reserved for guests who have signed up through the outreach group's website. Whatever you do, however, do not look directly at the sun, or your eyes will be severely damaged.

Live feeds from the solar telescope in Caltech's Linde + Robinson Laboratory, from telescopes atop Mauna Kea, Hawaii, and from NASA headquarters in Washington, D.C., will be shown on screens in the lobby of Cahill.

Starting at 3:15 inside Cahill's Hameetman Auditorium, Caltech astronomers will give a series of short talks. Gregg Hallinan, assistant professor of astronomy, will talk about his research on the sun and the history of the Venus transit. John Johnson, assistant professor of astronomy, and graduate student Tim Morton will discuss how planetary transits can help astronomers learn more about planets around other stars. For example, by giving astronomers a chance to see a planetary transit up close, the Venus transit will help astronomers better understand data from NASA's Kepler space telescope, which is searching for exoplanets that pass in front of their stars. Kepler has already found more than 1,000 possible planets.

When Venus crosses in front of the sun, its atmosphere will be visible as a thin ring of haze around the black disk. By studying how the Venusian atmosphere breaks up sunlight into its component wavelengths, astronomers may glean information that could help them probe the atmospheres of other planets.

Such a transit is a rare event because the plane of Venus's orbit is tilted with respect to Earth's orbital plane. Venus's last transit occurred in 2004, and they happen in pairs with intervals of eight years. The period between each pair, in turn, alternates between 105.5 and 121.5 years.

In addition to the event at Cahill, there will be a symposium in Robert P. Sharp Lecture Hall in 155 Arms from 3:30 p.m. to 5:00 p.m. Noel Swerdlow, a visiting associate in history, will talk about how scientists used the Venus transits in 1761 and 1769 to measure the size of the solar system. Andrew Ingersoll, professor of planetary science, will discuss the scorching hot Venusian climate. Finally, Heather Knutson, assistant professor of planetary science, will give an overview of what astronomers currently know about exoplanets and discuss some of the discoveries astronomers anticipate in the ongoing hunt for planets.

For more information about the event at Cahill, click here.