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Geometrical basis for calculating the distance to the horizon, tangent-secant theorem Geometrical distance to the horizon, Pythagorean theorem Three types of horizon. If the Earth is assumed to be a featureless sphere (rather than an oblate spheroid) with no atmospheric refraction, then the distance to the horizon can easily be calculated. [6]
The particle horizon, also called the cosmological horizon, the comoving horizon, or the cosmic light horizon, is the maximum distance from which light from particles could have traveled to the observer in the age of the universe. It represents the boundary between the observable and the unobservable regions of the universe, so its distance at ...
In standard cosmology, comoving distance and proper distance (or physical distance) are two closely related distance measures used by cosmologists to define distances between objects. Comoving distance factors out the expansion of the universe , giving a distance that does not change in time except due to local factors, such as the motion of a ...
Graphs of distances to the true horizon on Earth for a given height h. s is along the surface of Earth, d is the straight line distance, and ~d is the approximate straight line distance assuming h << the radius of Earth, 6371 km. In the SVG image, hover over a graph to highlight it.
For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away. [9]
Posidonius calculated the Earth's circumference by reference to the position of the star Canopus.As explained by Cleomedes, Posidonius observed Canopus on but never above the horizon at Rhodes, while at Alexandria he saw it ascend as far as 7 + 1 ⁄ 2 degrees above the horizon (the meridian arc between the latitude of the two locales is actually 5 degrees 14 minutes).
Altitude (alt.), sometimes referred to as elevation (el.) or apparent height, is the angle between the object and the observer's local horizon. For visible objects, it is an angle between 0° and 90°. [b] Azimuth (az.) is the angle of the object around the horizon, usually measured from true north and increasing eastward.
Diagram showing the relationship between the zenith, the nadir, and different types of horizon. The zenith (UK: / ˈ z ɛ n ɪ θ /, US: / ˈ z iː n ɪ θ /) [1] is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction opposite to the gravity direction at that location . The ...