Description: Over the course of a year, as the Earth and all astronomical observers on it travel around the Sun, the perspective of those observers changes. Compared with the backdrop of the most distant astronomical objects in the night sky, this change in perspective makes for a change in the apparent position of nearer astronomical objects in the night sky. It makes those objects appear to move on tiny ellipses in the night sky over the course of one year, an effect known as annual parallax. The major axis of that apparent-motion ellipse, expressed as an angle, corresponds to twice the object's so-called (annual) parallax angle. The parallax angle corresponds to a change in observer position by one astronomical unit (AU), that is, by the average Earth–Sun distance, or the Earth's displacement between two astronomical observations performed half a year apart. The distance unit "parsec", short for "parallax second", is defined so that for a star at a distance of one parsec from Earth, the annual parallax angle amounts to one arc second.

Related Terms:

• Parallax

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".