Historically, astronomy was more concerne 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. 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 6500 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 10,000 members from 87 different countries who are involved in astronomical research at the PhD level and beyond.