On a hot day we can look along a surfaced road at a distant object and see that it seems to shake and shiver as we look at it. This happens because ‘bubbles’ of hot air rise from the surface of the road. The fluctuations in air density caused by these bubbles of hot air deflect the light beams passing through the air from the distant object, making the air appear to shiver before our eyes. The bending of light rays is called refraction. Similar but much smaller density variations are found everywhere in the atmosphere. They cause the light from a star, for example, to be slightly deflected from its original path in a random manner. On the timescale of a few seconds, this causes the light from the most compact point object, such as a star, to be smeared over a disc of typically a few arc seconds diameter. When this disc is large, we refer to the observing conditions as BAD SEEING. Images better than one arc second diameter are observed only under conditions of GOOD SEEING. The quality of the seeing changes from hour to hour and from day to day. In addition, atmospheric irregularities cause fluctuations in the brightness of the stars known as SCINTILLATION. On a dark night we observe the random atmospheric refraction as the twinkling of stars. Twinkling tends to be most severe under conditions of poor seeing. The more they twinkle the poorer is the seeing. (Note that planets do not twinkle, because they are ex¬tended objects; the twinkling of each part of a planet’s disc averages out to give a steady image.) There is not very much that the astronomer can do to reduce the effects of seeing. For very bright objects it may be possible to build devices which sense the seeing deflections very quickly and move the images slightly to correct for the deflection. However the light from all but the brightest objects is too faint to allow this to be done. Further, as large apertures average out the seeing deflections, such a technique is only of use with relatively small telescopes. The only other way to reduce the effects of seeing is to choose the best possible observing site where the air is very dry and stable, or else to take the telescope above the Earth’s

In addition to the random refractions that produce seeing visible and infrared waves are also refracted by the layer of atmosphere which surrounds the Earth. We may see the same effect if we look at an object through a slab of glass. As the glass is tilted, the object appears to move by an amount which is greatest when the light from the object to the observer is passing through the glass at a glancing angel. Similarly ATMOSPHERIC REFRACTION causes the apparent positions of stars to be changed as they move closer to the horizon. However, this effect is on a large scale and is highly predictable so it does not degrade the quality of the image near the horizon though it does alter the apparent angular separation of two objects nevertheless, astronomers prefer not to observe objects near the horizon if possible: the seeing in often poorer there, since the light must travel through many more atmospheric density fluctuations, and there is usually more absorption of the light by dust and pollutants near the horizon.

Radio waves longer than 10cm are refracted by the ionosphere as well as by the atmosphere in much the same way that visible and infrared waves are refracted by the atmosphere. As these are large-scale effects and are fairly predictable and repeatable they do not seriously disturb the work of the radio astronomer.

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