An astronomer is a scientist who studies celestial bodies such as moons, planets, stars, nebulae, and galaxies); the physics, chemistry, and evolution of such objects; and phenomena that originate outside the atmosphere of Earth (such as supernovae explosions, gamma ray bursts, and cosmic background radiation). A related but distinct subject, cosmology, is concerned with studying the universe as a whole.
Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using physical laws. Today, that distinction has mostly disappeared and the terms "astronomer" and "astrophysicist" are interchangeable. Professional astronomers are highly educated individuals who typically have a PhD in physics or astronomy and are employed by research institutions or universities. They spend the majority of their time working on research, although they quite often have other duties such as teaching, building instruments, or aiding in the operation of an observatory. The number of professional astronomers in the United States is actually quite small. The American Astronomical Society, which is the major organization of professional astronomers in North America, has approximately 7,700 members. This number includes scientists from other fields such as physics, geology, and engineering, whose research interests are closely related to astronomy. The International Astronomical Union comprises almost 10,145 members from 70 different countries who are involved in astronomical research at the PhD level and beyond.
The Nicolaus Copernicus Monument in Warsaw is one of the Polish capital's notable landmarks. It stands before the Staszic Palace, the seat of the Polish Academy of Sciences on Krakowskie Przedmieście. Designed by Bertel Thorvaldsen in 1822, it was completed in 1830.