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Encyclopedia > Atmospheric refraction

Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude. Atmospheric refraction near the ground produces mirages and can make distant objects appear to shimmer or ripple. Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength[1]. The elementary particle that defines light is the photon. ... Electromagnetic radiation is a propagating wave in space with electric and magnetic components. ... This article is about mirage, an optical phenomenon. ...


Atmospheric refraction causes astronomical objects to appear higher in the sky than they are in reality. It affects not only lightrays but all electromagnetic radiation, although in varying degrees (see dispersion (optics)). For example in visible light, blue is more affected than red. This may cause astronomical objects to be spread out into a spectrum in high-resolution images. See also Lists of astronomical objects Category: ... Dispersion of a light beam in a prism. ...


Whenever possible astronomers will always schedule their observations around the time of culmination of an object when it is highest in the sky. Likewise sailors will never shoot a star which is not at least 20° or more above the horizon. If observations close to the horizon cannot be avoided, it is possible to equip a telescope with control systems to compensate for the shift caused by the refraction. If the dispersion is a problem too, (in case of broadband high-resolution observations) atmospheric refraction correctors can be employed as well (made from pairs of rotating glass prisms). But as the amount of atmospheric refraction is function of temperature and pressure as well as humidity (the amount of water vapour especially important at mid-infrared wavelengths) the amount of effort needed for a successful compensation can be prohibitive. An astronomer or astrophysicist is a scientist whose area of research is astronomy or astrophysics. ... In astronomy, the culmination, at a given point, of a planet, star, constellation, etc. ... A telescope (from the Greek tele = far and skopein = to look or see; teleskopos = far-seeing) is an instrument designed for the observation of remote objects. ...


It gets even worse when the atmospheric refraction is not homogenous, when there is turbulence in the air for example. This is the cause of twinkling of the stars and deformation of the shape of the sun at sunset and sunrise. Twinkling consists of changes in the apparent brightness and position of stars caused by atmospheric turbulence and various other irregularities. ...


Values

The atmospheric refraction is zero in the zenith, is less than 1' (one arcminute) at 45° altitude, still only 5' at 10° altitude, but then quickly increases when the horizon is approached. On the horizon itself it is about 34' (according to FW Bessel), just a little bit larger than the apparent size of the sun. Therefore if it appears that the setting sun is just above the horizon, in reality it has already set. Formulae to calculate the times of sunrise and sunset do not calculate the moment that the sun reaches altitude zero, but when its altitude is -50': 16' for the radius of the sun (solar positions are for the centre of the sun-disc, but sunrise and sunset usually refer to the appearance and disappearance of the upperlimb) plus 34' for the refraction. In the case of the Moon one should apply additional corrections for the horizontal parallax of the moon, its apparent diameter and its phase, although the latter is seldom done. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60) of one degree. ... The Rayleigh effect, seconds before sunrise in New Zealand Sunrise, also called sunup in some American English dialects, is the time at which the first part of the Sun appears above the horizon in the east. ... A composite image showing the terminator dividing night from day, running across Europe and Africa. ... Apparent magnitude: up to -12. ... Lunar phase refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. ...


The refraction is also a function of temperature and pressure. The values given above are for 10 °C and 1003 mbar. Add 1% to the refraction for every 3° C colder, subtract if hotter (hot air is less dense, and will therefore have less refraction). Add 1% for every 9 mbar higher pressure, subtract if lower. Evidently day to day variations in the weather will affect the exact times of sunrise and sunset as well as moonrise and moonset, and for that reason are never given more accurate than to the nearest whole minute in the almanacs. An almanac (sometimes spelled almanack) is an annual publication containing tabular information in a particular field or fields often arranged according to the calendar. ...


Finally as the atmospheric refraction is 34' on the horizon, but only 29' half a degree above it, the setting or rising sun seems to be flattend by about 5' or 1/6 of its apparent diameter.


Twinkling of stars at night: when light rays coming from stars travel in atmosphere then they have to pass through different layers of air in atmosphere.as the density of layers changes comtinuosly due to change in temprature so the direction of rays entering the eye changes continuosly.therefore stars appear twinkling.


See also


  Results from FactBites:
 
Atmospheric refraction - Wikipedia, the free encyclopedia (252 words)
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude.
Atmospheric refraction causes astronomical objects to appear higher in the sky (up to half an arcminute at visible wavelengths for objects close to the horizon) than the actual direction from which the light came.
Atmospheric refraction can also distort the shape of the sun during sunrise and sunset and cause chromatic aberrations in the sun's image.
  More results at FactBites »

 
 

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