• Jupiter and the four Galilean moons: Callisto, Io, Europa and Ganymede. (Photo by Chanan Greenberg taken on 9/5/09. Used with permission.)

Galileo, perhaps more than any other single person, was responsible for the birth of modern science.

-- Stephen Hawking

Today's issue of the Journal marks the 400th anniversary of the moment our view of the cosmos changed forever. On this night in 1610, an obscure professor at the University of Padua (near Venice, Italy) first saw the moons of Jupiter. For this you need a telescope, and Galileo Galilei had recently built himself one, based on reports of a new design from the Netherlands ("Field Notes," March 9, 2009).

Less than a week after first viewing tiny dots of light next to Jupiter, Galileo realized he was seeing moons orbiting the giant planet. The notion that any body other than the Earth could have satellites flew in the face of the Aristotelian view of the heavens. Aristotle's Earth-centered universe still ruled in early 17th century Europe, despite the publication of Copernicus' sun-centered theory in 1543.

Galileo noted in his journal for Jan. 7, 1610 "... three fixed stars, totally invisible [to the naked eye] by their smallness," close to Jupiter and lying on a straight line through it (see photo). He had discovered three of what we now call the four "Galilean moons" of Jupiter: Io, Europa and Callisto. A week later he discovered the fourth, Ganymede. He guessed, correctly, that when one of them disappeared on the night of January 10, it had been eclipsed by Jupiter.

In addition to the cosmic implications, Galileo recognized the practical importance of Jupiter's moons in giving us a "clock in the sky" which could solve the problem of longitude. Longitude -- essentially the difference between local time and Greenwich time -- was solved by Galileo's discovery, since the eclipse times of Jupiter's moons can be predicted. Today, you can find eclipse tables in, for instance, Astronomy magazine.

Suppose you know from the tables that Io will eclipse at exactly 6:00 a.m. Greenwich time (zero longitude), and your clock is set to local time (your local noon is when the sun is at its zenith). You observe the eclipse at 9:44 p.m., a difference of 8 hours 16 minutes from Greenwich time. Converting to degrees of longitude, that's 124 degrees: the west longitude of Eureka, Calif.

Four hundred years ago tonight was a watershed in the history of science. How will you celebrate?

Using a telescope not that much better than Galileo's, Barry Evans (barryevans9@yahoo.com) watches Jupiter's moons dance around the planet from his back yard in Old Town Eureka.