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Whenever possible, astronomers will schedule their observations around the times of culmination, when celestial objects are highest in the sky. This causes suboptimal seeing conditions, such as the twinkling of stars and various deformations of the Sun's apparent shape soon before sunset or after sunrise. 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. 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 [12] and employing the temperature gradient of the standard atmosphere and the measured conditions at the observer, is required.
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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. 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. Young [6] [11] distinguished several regions where different methods for calculating astronomical refraction were applicable.above it, the setting or rising sun seems to be flattened by about 5′ (about 1/6 of its apparent diameter). A further expansion in terms of the third power of the cotangent of the apparent altitude incorporates H 0, the height of the homogeneous atmosphere, in addition to the usual conditions at the observer: [17] R = ( n 0 − 1 ) ( 1 − H 0 ) cot h a − ( n 0 − 1 ) [ H 0 − 1 2 ( n 0 − 1 ) ] cot 3 h a .
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Such refraction can also raise or lower, or stretch or shorten, the images of distant objects without involving mirages. where R is the refraction in seconds of arc, b is the atmospheric pressure in millimeters of mercury, and t is the temperature in Celsius. Astronomical or celestial refraction causes astronomical objects to appear higher above the horizon than they actually are. If the atmosphere suddenly vanished at this moment, one couldn't see the sun, as it would be entirely below the horizon. Astronomical refraction [ edit ] Atmospheric refraction distorting the Sun’s disk into an uneven shape as it sets in the lower horizon.
If observations of objects near the horizon cannot be avoided, it is possible to equip an optical telescope with control systems to compensate for the shift caused by the refraction. displaystyle R=(n_{0}-1)(1-H_{0})\cot h_{\mathrm {a} }-(n_{0}-1)[H_{0}-{\frac {1}{2}}(n_{0}-1)]\cot Terrestrial refraction usually causes terrestrial objects to appear higher than they actually are, although in the afternoon when the air near the ground is heated, the rays can curve upward making objects appear lower than they actually are.
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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. Surveyors, on the other hand, will often schedule their observations in the afternoon, when the magnitude of refraction is minimum. If the dispersion is also a problem (in case of broadband high-resolution observations), atmospheric refraction correctors (made from pairs of rotating glass prisms) can be employed as well. 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. Day-to-day variations in the weather will affect the exact times of sunrise and sunset [8] 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.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. Since the amount of atmospheric refraction is a function of the temperature gradient, temperature, pressure, and humidity (the amount of water vapor, which is especially important at mid- infrared wavelengths), the amount of effort needed for a successful compensation can be prohibitive. 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. 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.
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Closer to the horizon, actual measurements of the changes with height of the local temperature gradient need to be employed in the numerical integration. Atmospheric refraction becomes more severe when temperature gradients are strong, and refraction is not uniform when the atmosphere is heterogeneous, as when turbulence occurs in the air. Refraction not only affects visible light rays, but all electromagnetic radiation, although in varying degrees.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. 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.
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