Stargazers take note: Today marks the beginning of a four-day celestial celebration called 100 Hours of Astronomy, part of the International Astronomical Union's International Year of Astronomy (IYA2009). The IYA2009 marks the 400th anniversary of Italian astronomer Galileo Galilei turning his telescopes to the skies and beginning a new era of astronomical observation.

A kickoff event at the Franklin Institute in Philadelphia today showcased one of Galileo's surviving telescopes. According to the institute, this marks the first time one of the two remaining instruments has left Italy.

An international "star party" is scheduled to take place during which telescopes will be made available to the public at different sites around the globe. Many are amateur telescopes set up in parks or on sidewalks; the 100 Hours of Astronomy Web site has details on many of the planned activities. Most star parties are scheduled to take place on Saturday, but some are planned for other times, such as one beginning this evening in New York City, where Columbia University will set up telescopes in Harlem's Powell Plaza for viewing the moon and Saturn.

For those who want to participate but find themselves stifled by bad weather or a lack of local events, the Galaxy Zoo project is challenging armchair astronomers to get involved from their computers. Galaxy Zoo, an online endeavor that throws telescopic images of galaxies to the public for classification (spiral versus elliptical, elongated versus round, and so on), is aiming to record a million classification clicks by the end of the 100-hour span.

And Twitterers can follow the proceedings at

Credit: IYA2009/100 Hours of Astronomy