Outside the radio and optical windows, the main source of night-sky emission is the re-radiation which accompanies absorption. In the radio window the sky is very dark, by day and by night, since it radiates very little indeed. Radio observations at wavelengths longer than a few centimetres are therefore just as easy by day as by night and are unaffected by clouds or rain, though they are affected by man-made and natural terrestrial interference. Radio radiation from the Sun can interfere with observations of faint objects in directions close to that of the Sun. Our Galaxy produces a faint band of radio radiation, concentrated along the Milky Way? which can hamper observations of the faintest sources at wave¬lengths of 0.5m or longer. However, both these effects are fairly small and much less serious than the NIGHT-SKY RADIATION.

There are a number of mechanisms that produce diffuse optical background radiation, which brightens the night sky and makes it more difficult to observe the faintest objects. Our planet is continually sheathed in a faint nocturnal glow (NIGHTGLOW or AIRGLOW) that is seldom noticed. It is caused by chemical reactions in the upper atmosphere that result in the emission of light.

Another source of unwanted radiation is the stream of energetic charged particles repeatedly shot out from the Sun. Many of these particles become tangled with the Earth’s magnetic field. They then radiate their energy as the beautiful polar aurorae, mostly in the zones 20° to 25° from the geomagnetic poles.

A minor component of the Solar System is a diffuse distribution of dust grains in its plane. These dust grains scatter sunlight to produce the ZODIACAL LIGHT. Apart from moonlight and the aurora, the zodiacal light is the brightest component of the night-sky radiation. It can be an impressive spectacle, especially in the tropics where the plane of the Solar System (the ECLIPTIC) is almost vertical to the horizon.

Sunlight and moonlight are the main contributors to the bright¬ness of the night sky. Therefore observations of the faintest optical objects must wait until the Sun is well down below the horizon and until the Moon has either set or waned so that it produces only a little light. The contributions of sunlight and moonlight are much less important in the infrared than in the visible region of the spectrum. Because of this, it is often possible for infrared astronomers to work during twilight and on moonlit nights when many optical astronomers find the sky too bright for useful observations of faint objects. Some far-infrared observations can even be made throughout the day.

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