Everything about The Horizon totally explained
The
horizon (
Ancient Greek ὁ ὁρίζων, /ho horídzôn/, from
ὁρίζειν, "to limit") is the apparent line that separates
earth from
sky.
More precisely, it's the line that divides all of the directions one can possibly look into two categories: those which intersect the Earth's surface, and those which do not. At many locations, the
true horizon is obscured by nearby trees, buildings, mountains and so forth. The resulting intersection of earth and sky is instead described as the
visible horizon.
Appearance and usage
For observers aboard a ship at sea, the true horizon is strikingly apparent. Historically, the distance to the visible horizon has been extremely important as it represented the
maximum range of communication and vision before the development of the
radio and the
telegraph. Even today, when flying an aircraft under
Visual Flight Rules, a technique called
attitude flying is used to control the aircraft, where the pilot uses the visual relationship between the aircraft's nose and the horizon to control the aircraft. A pilot can also retain their
spatial orientation by referring to the horizon.
In many contexts, especially
perspective drawing, the curvature of the earth is typically disregarded and the horizon is considered the theoretical line to which points on any
horizontal plane converge (when projected onto the picture plane) as their distance from the observer increases. Note that, for observers near the ground, the difference between this
geometrical horizon (which assumes a perfectly flat, infinite ground plane) and the
true horizon (which assumes a spherical Earth surface) is typically imperceptibly small, because of the relative size of the observer. That is, if the Earth were truly flat, there would still be a visible horizon line, and, to ground based viewers, its position and appearance wouldn't be significantly different from what we see on our curved Earth.
In astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the
fundamental plane of the
horizontal coordinate system, the locus of points which have an
altitude of zero degrees. While similar in ways to the geometrical horizon described above, in this context a horizon may be considered to be a plane in space, rather than a line on a picture plane.
Distance to the horizon
The straight line of sight distance
d in kilometers to the true horizon on earth is approximately
The curvature is the reciprocal of the curvature angular radius in
radians. A curvature of 1 appears as a circle of an angular radius of 45° corresponding to an altitude of approximately 2640 km above the Earth's surface. At an altitude of 10 km (33,000 ft, the typical cruising altitude of an airliner) the mathematical curvature of the horizon is about 0.056, the same curvature of the rim of circle with a radius of 10 metres that's viewed from 56 centimetres. However, the apparent curvature is less than that due to refraction of light in the atmosphere and because the horizon is often masked by high cloud layers that reduce the altitude above the visual surface.
Further Information
Get more info on 'Horizon'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://horizon.totallyexplained.com">Horizon Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
