The year 1609 was noteworthy for two astronomical milestones.
That was when Galileo built his first telescopes and began his
meticulous study of the skies. Within months he discovered the
four major satellites of Jupiter, saw that Venus (like our moon)
has illuminated phases and confirmed earlier observations of
sunspots—all evidence that undermined the Aristotelian model of
an unchanging, Earth-centered cosmos. During that same year,
Johannes Kepler published Astronomia Nova, which contained his
detailed calculation of the orbit of Mars. It also established
the first two laws of planetary motion: that planets follow
elliptical orbits, with the sun at one focus, and that planets
sweep through equal areas of their orbits in a given interval.
Small wonder, then, that when the United Nations General
Assembly declared an International Year of Astronomy to promote
the wider appreciation of the science, it selected 2009, the
quadricentennial of those standout accomplishments (among many)
by Galileo and Kepler that