Atmospheric refraction

Not to be confused with Atmospheric diffraction.
Diagram showing displacement of the Sun's image at sunrise and sunset

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. This refraction is due to the velocity of light through air decreasing (the index of refraction increases) with increased density. Atmospheric refraction near the ground produces mirages and can make distant objects appear to shimmer or ripple, elevated or lowered, stretched or shortened with no mirage involved. The term also applies to the refraction of sound. Atmospheric refraction is considered in measuring the position of both astronomical and terrestrial objects.

Astronomical or celestial refraction causes astronomical objects to appear higher in the sky than they are in reality. Terrestrial refraction usually causes terrestrial objects to appear higher than they really are, although in the afternoon when the air near the ground is heated, the rays can curve upward making objects appear lower than they really are.

Refraction not only affects lightrays but all electromagnetic radiation, although in varying degrees (see dispersion in 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.

Whenever possible, astronomers will 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 a function of the temperature gradient, the temperature, pressure, and humidity (the amount of water vapour is especially important at mid-infrared wavelengths) the amount of effort needed for a successful compensation can be prohibitive. Surveyors, on the other hand, will often schedule their observations in the afternoon when the magnitude of refraction is minimum.

Atmospheric refraction becomes more severe when there are strong temperature gradients, and refraction is not uniform when the atmosphere is inhomogeneous, as when there is turbulence in the air. This is the cause of twinkling of the stars and various deformations of the shape of the sun at sunset and sunrise.

Astronomical refraction

Astronomical refraction deals with the angular position of celestial bodies, their appearance as a point source, and through differential refraction, the shape of extended bodies such as the Sun and Moon.[1]

Values

Atmospheric refraction of the light from a star is zero in the zenith, less than 1′ (one arc-minute) at 45° apparent altitude, and still only 5.3′ at 10° altitude; it quickly increases as altitude decreases, reaching 9.9′ at 5° altitude, 18.4′ at 2° altitude, and 35.4′ at the horizon;[2] all values are for 10 °C and 1013.25 hPa in the visible part of the spectrum.

On the horizon refraction is slightly greater than the apparent diameter of the Sun, so when the bottom of the sun's disc appears to touch the horizon, the sun's true altitude is negative. If the atmosphere suddenly vanished, the sun would too. By convention, sunrise and sunset refer to times at which the Sun’s upper limb appears on or disappears from the horizon and the standard value for the Sun’s true altitude is −50′: −34′ for the refraction and −16′ for the Sun’s semi-diameter. The altitude of a celestial body is normally given for the center of the body’s disc. In the case of the Moon, additional corrections are needed for the Moon’s horizontal parallax and its apparent semi-diameter; both vary with the Earth–Moon distance.

Refraction near the horizon is highly variable, principally because of the variability of the temperature gradient near the Earth's surface and the geometric sensitivity of the nearly horizontal rays to this variability. As early as 1830, Friedrich Bessel had found that even after applying all corrections for temperature and pressure (but not for the temperature gradient) at the observer, highly precise measurements of refraction varied by ±0.19' at two degrees above the horizon and by ±0.50' at a half degree above the horizon.[3] At and below the horizon, values of refraction significantly higher than the nominal value of 35.4′ have been observed in a wide range of climates. Georg Constantin Bouris measured refraction of as much of 4° for stars on the horizon at the Athens Observatory[4] and, during his ill-fated Endurance expedition, Sir Ernest Shackleton recorded refraction of 2°37':[5]

“The sun which had made ‘positively his last appearance’ seven days earlier surprised us by lifting more than half its disk above the horizon on May 8. A glow on the northern horizon resolved itself into the sun at 11 am that day. A quarter of an hour later the unreasonable visitor disappeared again, only to rise again at 11:40 am, set at 1 pm, rise at 1:10 pm and set lingeringly at 1:20 pm. These curious phenomena were due to refraction which amounted to 2° 37' at 1:20 pm. The temperature was 15° below 0° Fahr., and we calculated that the refraction was 2° above normal.”

Day-to-day variations in the weather will affect the exact times of sunrise and sunset[6] as well as moon-rise and moon-set, and for that reason it generally is not meaningful to give rise and set times to greater precision than the nearest minute.[7] More precise calculations can be useful for determining day-to-day changes in rise and set times that would occur with the standard value for refraction[note 1] if it is understood that actual changes may differ because of unpredictable variations in refraction.

Because atmospheric refraction is nominally 34′ on the horizon, but only 29′ at 0.5° above it, the setting or rising sun seems to be flattened by about 5′ (about 1/6 of its apparent diameter).

Calculating refraction

Young[4][9] distinguished several regions where different methods for calculating astronomical refraction were applicable. In the upper portion of the sky, with a zenith distance of less than 70° (or an altitude over 20°), various simple refraction formulas based on the index of refraction (and hence on the temperature, pressure, and humidity) at the observer are adequate. Between 20° and 5° of the horizon the temperature gradient becomes the dominant factor and numerical integration, using a method such as that of Auer and Standish[10] and employing the temperature gradient of the standard atmosphere and the measured conditions at the observer, is required. Closer to the horizon, actual measurements of the changes with height of the local temperature gradient need to be employed in the numerical integration. Below the astronomical horizon, refraction is so variable that only crude estimates of astronomical refraction can be made; for example, the observed time of sunrise or sunset can vary by several minutes from day to day. As The Nautical Almanac notes, "the actual values of …the refraction at low altitudes may, in extreme atmospheric conditions, differ considerably from the mean values used in the tables."[11]

Plot of refraction vs. altitude using Bennett’s 1982 formula

Many different formulas have been developed for calculating astronomical refraction; they are reasonably consistent, differing among themselves by a few minutes of arc at the horizon and becoming increasingly consistent as they approach the zenith.[12] Bennett[13] developed a simple empirical formula for calculating refraction from the apparent altitude, using the algorithm of Garfinkel[14] as the reference; if ha is the apparent altitude in degrees, refraction R in arcminutes is given by

R = \cot \left ( h_\mathrm{a} + \frac {7.31} {h_\mathrm{a} + 4.4} \right ) \,;

the formula is said to be accurate to within 0.07′ for the altitude range 0°–90°.[7] Sæmundsson[15] developed a formula for determining refraction from true altitude; if h is the true altitude in degrees, refraction R in arcminutes is given by

R = 1.02 \cot \left ( h + \frac {10.3} {h + 5.11} \right ) \,;

the formula is consistent with Bennett’s to within 0.1′. Both formulas assume an atmospheric pressure of 101.0 kPa and a temperature of 10 °C; for different pressure P and temperature T, refraction calculated from these formulas is multiplied by[7]

\frac {P} {101} \, \frac {283} {273 + T}

Refraction increases approximately 1% for every 0.9 kPa increase in pressure, and decreases approximately 1% for every 0.9 kPa decrease in pressure. Similarly, refraction increases approximately 1% for every 3 °C decrease in temperature, and decreases approximately 1% for every 3 °C increase in temperature.

Random refraction effects

Turbulence in the atmosphere scatters the light from stars, making them appear brighter and fainter on a time-scale of milliseconds. The slowest components of these fluctuations are visible 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.

Terrestrial refraction

Terrestrial refraction deals with the apparent angular position and measured distance of terrestrial bodies. It is of special concern for the production of precise maps and surveys.[16]

As a common approximation, terrestrial refraction is considered as a constant bending of the ray of light or line of sight, in which the ray can be considered as describing a circular path. A common measure of refraction is the coefficient of refraction. Unfortunately there are two different definitions of this coefficient. One is the ratio of the radius of the Earth to the radius of the line of sight,[17] the other is the ratio of the angle that the line of sight subtends at the center of the Earth to the angle of refraction measured at the observer.[18] Since the latter definition only measures the bending of the ray at one end of the line of sight, it is one half the value of the former definition.

The coefficient of refraction is directly related to the local vertical temperature gradient and the atmospheric temperature and pressure. The larger version of the coefficient k, measuring the ratio of the radius of the Earth to the radius of the line of sight, is given by:[17]

k = 503 \frac{P} {T^2} \left ( 0.0343 + \frac {dT} {dh} \right ),

where temperature T is given in degrees Kelvin, pressure P in millibars, and height h in meters. The angle of refraction increases with the coefficient of refraction and with the length of the line of sight.

Although the straight line from your eye to a distant mountain might be blocked by a closer hill, the ray may curve enough to make the distant peak visible. A convenient method to analyze the effect of refraction on visibility is to consider a reduced effective radius of the Earth Reff, given by[9]

R_{eff} = \frac {R} {1 - k} ,

where R is the radius of the Earth and k is the coefficient of refraction. Under this model the ray can be considered a straight line on an Earth of increased radius.

The curvature of the refracted ray in arc seconds per meter can be computed using the relationship[19]

\frac {1} {\sigma} = 16.3 \frac{P} {T^2} \left ( 0.0342 + \frac {dT} {dh} \right ) \cos \beta

where 1/σ is the curvature of the ray, P is the pressure in millibars, T is the temperature in degrees Kelvin, and β is the angle of the ray to the horizontal. Multiplying the curvature by the length of the ray path gives the angle of refraction at the observer. For a line of sight near the horizon cos β differs little from unity and can be ignored. This yields

\Omega = 16.3 \frac{L P} {T^2} \left ( 0.0342 + \frac {dT} {dh} \right ),

where L is the length of the line of sight and Ω is the refraction at the observer measured in arc minutes.

A simple approximation is to consider that a mountain's apparent altitude at your eye (in degrees) will exceed its true altitude by its distance in kilometers divided by 1500. This assumes a fairly horizontal line of sight and ordinary air density; if the mountain is very high (so much of the sightline is in thinner air) divide by 1600 instead.

See also

Notes

  1. ↑ For an example see Meeus 2002[8]

References

  1. ↑ Bomford, Guy (1980), Geodesy (4 ed.), Oxford: Oxford University Press, pp. 282–284, ISBN 0-19-851946-X
  2. ↑ Allen, C.W. (1976). Astrophysical quantities (3rd ed. 1973, Repr. with corrections 1976. ed.). London: Athelone Press. p. 125. ISBN 0-485-11150-0.
  3. ↑ Fletcher, Alan (1952), "Astronomical Refraction at Low Altitudes in Marine Navigation", Navigation (London: The Institute of Navigation) 5 (4): 314–315
  4. 1 2 Young, Andrew T. (2004). "Sunset Science. IV. Low-Altitude Refraction". The Astronomical Journal 127: 3622–3637. Bibcode:2004AJ....127.3622Y. doi:10.1086/420806.
  5. ↑ Shackleton, Sir Ernest (1919). South: the story of Shackleton's last expedition. London: Century Publishing. p. 49. ISBN 0-7126-0111-2.
  6. ↑ Schaefer, Bradley E.; Liller, William (1990). "Refraction near the horizon". Publications of the Astronomical Society of the Pacific 102: 796–805. Bibcode:1990PASP..102..796S. doi:10.1086/132705.
  7. 1 2 3 Meeus, Jean (1991). Astronomical algorithms (1st English ed.). Richmond, Va.: Willmann-Bell. pp. 102–103. ISBN 0-943396-35-2.
  8. ↑ Meeus, Jean (2002). [Mathematical astronomy morsels] (1st English ed.). Richmond, Va.: Willmann-Bell. p. 315. ISBN 0-943396-74-3.
  9. 1 2 Young, Andrew T. (2006). "Understanding Astronomical Refraction". The Observatory 126: 82–115. Bibcode:2006obs...126...82y.
  10. ↑ Auer, Lawrence H.; Standish, E. Myles (2000). "Astronomical Refraction: Computation for All Zenith Angles". Astronomical Journal 119 (5): 2472–2474. Bibcode:2000AJ....119.2472A. doi:10.1086/301325. | quote = This paper and the method presented in it were submitted for publication in 1970 July. Unfortunately, the referee did not understand the utility of our new approach, and for personal reasons we did not have the time to argue the point sufficiently. We did distribute preprints, and the method has become, with improved atmospheric models, the technique of choice for the computation of refraction (see, e.g., Seidelmann [Explanatory Supplement to the Astronomical Almanac,] 1992).
  11. ↑ The nautical almanac for the year 1988, Washington / London: United States Naval Observatory / Her Majesty's Stationery Office, 1986, p. 261
  12. ↑ Wittmann, A. D. (1997), "Astronomical refraction: formulas for all zenith distances", Astronomische Nachrichten 318 (5): 305–312, Bibcode:1997AN....318..305W, doi:10.1002/asna.2113180507
  13. ↑ Bennett, G.G. (1982). "The Calculation of Astronomical Refraction in Marine Navigation". Journal of Navigation 35: 255–259. Bibcode:1982JNav...35..255B. doi:10.1017/S0373463300022037.
  14. ↑ Garfinkel, B. (1967). "Astronomical Refraction in a Polytropic Atmosphere". Astronomical Journal 72: 235–254. Bibcode:1967AJ.....72..235G. doi:10.1086/110225.
  15. ↑ Sæmundsson, Þorsteinn (1986). "Astronomical Refraction". Sky and Telescope 72: 70. Bibcode:1986S&T....72...70S.
  16. ↑ Bomford, Guy (1980), Geodesy (4 ed.), Oxford: Oxford University Press, pp. 42–48, 233–243, ISBN 0-19-851946-X
  17. 1 2 Hirt, Christian; Guillaume, Sebastian; Wisbar, Annemarie; Bürki, Beat; Sternberg, Harald (2010), "Monitoring of the refraction coefficient in the lower atmosphere using a controlled setup of simultaneous reciprocal vertical angle measurements", Journal of Geophysical Research (American Geophysical Union) 115: D21102, Bibcode:2010JGRD..11521102H, doi:10.1029/2010JD014067
  18. ↑ Bomford, Guy (1980), Geodesy (4 ed.), Oxford: Oxford University Press, p. 236, ISBN 0-19-851946-X
  19. ↑ Bomford, Guy (1980), Geodesy (4 ed.), Oxford: Oxford University Press, p. 235, ISBN 0-19-851946-X

Further reading

External links

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