Surveyors on Earth use triangulation to determine the distances to distant objects. Astronomers use the same technique to measure the distances to planets and stars. The cosmic distance scale is the family of distance-measurement techniques by which astronomers chart the universe. Parallax is the apparent motion of a foreground object relative to a distant background as the observer's position changes. The larger the baseline, the distance between the two observation points, the greater the parallax.
Astronomers use parallax when measuring the distances to the planets by triangulation.
The same geometric reasoning is used to determine the sizes of objects whose distances are known. Many ancient cultures constructed elaborate structures that served as calendars and astronomical observatories. The study of the universe on the very largest scales is called cosmology. This phenomenon is called retrograde motion. Geocentric models of the universe were based on the assumption that the Sun, the Moon, and the planets all orbit Earth.