section: Astrometry and celestial mechanics
question: What measurement charts the movement of systems across the Milky Way?
article: Astronomy
candidates: One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars. Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth. The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of radial velocity and proper motion plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of dark matter in the galaxy. During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting nearby stars.