<P> There are further subcategories inside these two main branches of astronomy such as planetary astronomy, galactic astronomy or physical cosmology . </P> <P> 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 Ph. D. 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 . </P> <P> 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 7,000 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 Ph. D. level and beyond . </P> <P> Contrary to the classical image of an old astronomer peering through a telescope through the dark hours of the night, it is far more common to use a charge - coupled device (CCD) camera to record a long, deep exposure, allowing a more sensitive image to be created because the light is added over time . Before CCDs, photographic plates were a common method of observation . Modern astronomers spend relatively little time at telescopes usually just a few weeks per year . Analysis of observed phenomena, along with making predictions as to the causes of what they observe, takes the majority of observational astronomers' time . </P>

How many astronomers are there in the united states