Glossary term: Horizon
Description: O horizonte é a linha de limite que separa o céu da superfície da Terra. Em qualquer posição na Terra, vemos apenas uma parte limitada do globo. O limite que divide o que podemos ver do que não podemos ver é comumente chamado de horizonte. Na astronomia, essa definição é refinada da seguinte forma: Nossa própria posição na Terra define um plano horizontal, que é perpendicular à direção para baixo (que pode ser determinada usando um fio de prumo). A interseção desse plano com a esfera celeste define nosso horizonte local astronômico. O sistema de coordenadas horizontais utiliza o plano horizontal para definir posições no céu. O ângulo entre a nossa linha de visão de um objeto e o plano horizontal é chamado de altitude do objeto; o ângulo entre a projeção da linha de visão nesse plano e o norte verdadeiro é chamado de azimute do objeto.
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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher The translation of this term and its definition is still awaiting approval
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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
