page_title: Horizon
section_title: Zenith angle
hierarchical_section_title: Horizon / Other measures / Zenith angle
caption_attribution_description: English: Maximum possible zenith angle (i.e., grazing) for ray to elevated observer in homogeneous spherical atmosphere.
context_page_description: The horizon or skyline is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not.
  The true horizon is actually a theoretical line, which can only be observed when it lies on the sea surface. At many locations, this line is obscured by land, trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.
  The true horizon is horizontal. It surrounds the observer and it is typically assumed to be a circle, drawn on the surface of a perfectly spherical model of the Earth. Its center is below the observer and below sea level. Its distance from the observer varies from day to day due to atmospheric refraction, which is greatly affected by weather conditions. Also, the higher the observer's eyes are from sea level, the farther away the horizon is from the observer.
context_section_description: When the observer is elevated, the horizon zenith angle can be greater than 90°. The maximum visible zenith angle occurs when the ray is tangent to Earth’s surface; from triangle OCG in the figure at right,
  where  is the observer’s height above the surface and  is the angular dip of the horizon. It is related to the horizon zenith angle  by:
  For a non-negative height , the angle  is always ≥ 90°.
caption: Maximum zenith angle for elevated observer in homogeneous spherical atmosphere
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