Glossary term: Altitude

Description: Altitude has two meanings: it denotes either a specific angle in a certain type of coordinate system ("the altitude of a star") or a vertical distance relative to a given reference level ("5000 m above sea level").

In astronomy (and surveying), altitude is an angle in so-called horizontal coordinate systems. That angle measures how high an object is above the horizon – if you point your finger at the object, and then move that finger straight down to the horizon, the direction of your arm will have changed by the angle that is the altitude. Altitude is measured either in degrees or in radians. An object on the horizon would have an altitude of 0°, and an object that is directly overhead, "at the zenith," would have an altitude of 90°. Negative altitude values are assigned to objects that are currently below the horizon – there, the altitude angle measures how far the object is below the horizon. An object that is straight below your feet, "at the nadir," would have an altitude of –90°.

In other contexts, such as aviation or atmospheric physics, the altitude is a measure of how high a place is above some given reference level. On Earth, altitude is often defined as the height above mean sea level. With this meaning, altitude is measured in a unit of length, such as meters.

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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".