Atmospheric refractionRefraction is the change in direction of a wave due to a change in its velocity. This is most commonly observed when a wave passes from one medium to another...
is the deviation of
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
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
mirageA mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at". This is the same root as for "mirror" and "to...
s and can make distant objects appear to shimmer or ripple. The term also applies to the refraction of
soundSound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.- Perception of sound...
.
Atmospheric refraction causes
astronomical objectAstronomical objects are significant physical entities, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence. Some astronomical objects, such as Themis and Neith are, in...
s to appear higher in the sky than they are in reality.
Atmospheric refractionRefraction is the change in direction of a wave due to a change in its velocity. This is most commonly observed when a wave passes from one medium to another...
is the deviation of
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
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
mirageA mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at". This is the same root as for "mirror" and "to...
s and can make distant objects appear to shimmer or ripple. The term also applies to the refraction of
soundSound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.- Perception of sound...
.
Atmospheric refraction causes
astronomical objectAstronomical objects are significant physical entities, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence. Some astronomical objects, such as Themis and Neith are, in...
s 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)In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
). 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.
Whenever possible
astronomerAn astronomer is a scientist who studies celestial bodies such as planets, stars, and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s will always schedule their observations around the time of
culminationIn astronomy, the culmination, at a given point, of a planet, star, constellation, etc. is the time within the diurnal motion when it appears on an observer's meridian....
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
telescopeA telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century...
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.
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
twinklingScintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through a medium, most commonly the atmosphere ....
of the stars and deformation of the shape of the sun at sunset and sunrise.
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
sunriseSunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...
and
sunsetSunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".In astronomy the time of sunset is...
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
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
one should apply additional corrections for the horizontal parallax of the moon, its apparent diameter and its
phaseA lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
, although the latter is seldom done.
The refraction is also a function of temperature and pressure. The values given above are for 10 °C and 100.3
kPaThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
. 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 0.9 kPa 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 accurately than to the nearest whole minute in the
almanacAn almanac is an annual publication containing tabular information in a particular field or fields often arranged according to the calendar...
s.
As the atmospheric refraction is 34′ on the horizon itself, but only 29′ above it, the setting or rising sun seems to be flattened by about 5′ (about 1/6 of its apparent diameter).
Random refraction effects
TurbulenceIn fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time. Flow that is not turbulent is called laminar flow...
in the atmosphere magnifies and de-magnifies star images, making them appear brighter and fainter on a time-scale of milliseconds. The slowest components of these fluctuations are visible to the eye as twinkling (also called “scintillation”).
Turbulence also causes small random motions of the star image, and produces rapid changes in its structure. These effects are not visible to the naked eye, but are easily seen even in small telescopes. They are called “seeing” by astronomers.
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