General

We live at the bottom of a vast ocean of air, several miles deep, that envelops the surface of the Earth. Though essential for the continuation of all animal and human life it is nothing but a nuisance to the practical astronomer. All telescopic observation has to be through this mass of air which rarely, if ever, is quite still. Apart from the obvious obstructions of thick clouds and fog which prevent observation altogether, telescopic planetary images are often ruined by atmospheric turbulence even when the air is perfectly clear. Air currents and differences of temperature at different levels of the upper atmosphere all conspire to cause irregular refraction of light rays reaching us from the planets. This causes shimmering or ‘boiling’ and telescopic planetary images oscillate and ripple. Fine planetary detail is therefore difficult to see and ‘hold’; if the trembling is bad enough little or nothing of disc markings can be seen. These are what is meant by ‘bad seeing’.

When this happens there is nothing that you can do but to wait until conditions improve. However, there is no need to stop observing in the usually somewhat poor seeing conditions prevailing most of the year at most observing sites. On a night of tremulous seeing occasional steady intervals occur lasting a second or two during which the definition of the telescopic planetary image can be astonishingly good.