David H. Levy's Evening Stars appears in each issue of Astronomy magazine. This article appeared in August 2009. Registered subscribers have access to the complete online archive of "David H. Levy's Evening Stars." Subscribe today!
The International Year of Astronomy is really about Galileo. It's not because he was the first to look through a telescope — remember, last month we discussed the English perspective glass that was in use decades earlier. It's because he was the first to carefully record what he saw, promptly publish his work, and accept the consequences of the conclusions he drew.
Galileo was born in 1564, the same year as William Shakespeare. His first name derives from a Tuscan custom of the era that the first name of the firstborn child would be a repeat of the surname. Although Galileo lived to age 78, extraordinarily long for his time, he was often ill with arthritis or rheumatism.
Galileo was well past mid-career when he looked at Jupiter January 7, 1610. He wrote, "I perceived … three starlets, small indeed, but very bright." Certain that these were "fixed" stars, he enjoyed his view enough that on the next night he sought them out again. The starlets had moved with respect to Jupiter, and over the next few evenings they moved enough to convince Galileo that they were moons circling their planet.
Later in 1610, Galileo used his telescope to record the positions of sunspots crossing the full diameter of the Sun. Although he did not use a filter, he likely performed his observations near sunrise or sunset, when the natural atmospheric haze dims the Sun slightly. The poor quality of his objective lens probably also acted as a sort of filter.