According to its official web site, “The International Year of Astronomy 2009 (IYA2009) is a global celebration of astronomy and its contributions to society and culture and marks the 400th anniversary of the first use of an astronomical telescope by Galileo Galilei.”
General works about astronomy are classed in 520 Astronomy and allied sciences, e.g., Astronomy: Principles and Practice and Out-of-this-World Astronomy: 50 Amazing Activities & Projects.
Comprehensive biographies of Galileo Galilei are classed in 520.92 Astronomers, built with 52 from 520 (the final placeholder zero is deleted before Table 1 notation is added) plus T1—092 Persons. The final placeholder zero must be deleted because, as the “Introduction to the Dewey Decimal Classification” (section 8.5) instructs: “The classifier should never use more than one zero in applying a standard subdivision unless instructed to do so. If more than one zero is needed, the number of zeros is always indicated in the schedules.” The built number 520.92 can be found in the print Relative Index or in WebDewey.
In the section entitled “Comprehensive biography” in the Manual Entry T1—092 Persons is the instruction: “Use the number for the subject of the person's most noted contribution for the comprehensive biography of the person.” That is why the comprehensive number for Galileo is in the field of astronomy, even though he also made important contributions in other fields. If his contributions in physics are considered of equal importance, then 520 Astronomy and allied sciences is preferred over 530 Physics by the first-of-two rule. Examples of works classed in 520.92 Astronomers are Galileo: Astronomer and Physicist and Galileo: Renaissance Scientist and Astronomer.
General works about the history of telescopes are classed in 522.209 History of telescopes (built with 522.2 Astronomical instruments Telescopes plus T1—09 Historical, geographic, persons treatment), e.g., Eyes on the Skies: 400 years of Telescopic Discovery and Stargazer: The Life and Times of the Telescope. The heading for 522.2 Astronomical instruments Telescopes is an example of a dual heading; standard subdivisions may be added for astronomical instruments, for telescopes, or for both.