Glossary term: Azimuth
Description: In a horizontal coordinate system, azimuth refers to the direction (angle along the horizon) at which the object is found. It is measured in degrees starting from the north and towards the east. Azimuth values cover a full circle from 0 degrees to 360 degrees. In other words, if you draw an imaginary arc on the celestial sphere from the object to the horizon and perpendicular to the horizon, the azimuth will tell you the location of the point where this arc meets the horizon. An object located directly north would have 0 degrees azimuth, an object directly east would have 90 degrees azimuth and so on. In older textbooks used in multiple countries, the convention was to start measuring the azimuth from the south towards the west. Thus, azimuth values in those textbooks would be shifted by 180 degrees.
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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher
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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
