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.
Related Terms:
See this term in other languages
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".
If you notice a factual error in this glossary definition then please get in touch.
Related Diagrams
Horizontal Coordinate System
Caption: This image shows the horizontal coordinate system of an observer on Earth. On the right we see the coordinate system in the local context of the observer. The observer appears here as a point surrounded by the celestial sphere. The ground appears as a plane, while the Earth is round, we can approximate the ground in the area around the observer as a plane. The line where this plane intersects with the celestial sphere is the horizon. The point directly above the observer is the zenith, the point directly below the observer is the nadir which is hidden by the ground.
Two coordinates define this coordinate system, altitude and azimuth, hence the reason this sometimes called an alt-az coordinate system. The altitude is zero at the horizon, maximum (90°) at the zenith and minimum (-90°) at the nadir. Azimuth is the angle around the horizon. Most commonly this is defined to be zero in the direction of north. Any point on the celestial sphere can be defined by these two coordinates. However what objects appear on these position will depend on the time and the location of the observer.
On the left-hand part of the diagram, we can see that when we put the observer and their local horizontal coordinate system in a global context, that the parts of the sky the observer can see depend on their position on the Earth and on the rotation of the Earth. The zenith points perpendicular to the Earth's surface so the position on the sky the zenith and nadir correspond to is dependent on the observer's latitude. The horizon also depends on the observer's position on the Earth.
Credit: Maria Cristina Fortuna/IAU OAE
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
