Such refraction can also raise or lower, or stretch or shorten, the images of distant objects without involving mirages.

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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. Whenever possible, astronomers will schedule their observations around the times of culmination, when celestial objects are highest in the sky. Atmospheric refraction is considered in measuring the position of both celestial and terrestrial objects.Astronomical refraction [ edit ] Atmospheric refraction distorting the Sun’s disk into an uneven shape as it sets in the lower horizon. 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. 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. 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.

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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. Closer to the horizon, actual measurements of the changes with height of the local temperature gradient need to be employed in the numerical integration. 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. where R is the refraction in radians, n 0 is the index of refraction at the observer (which depends on the temperature, pressure, and humidity), and h a is the apparent altitude angle of the astronomical body.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.