The surface of the Earth is covered in a thin layer of gas called the ATMOSPHERE of our planet. It is made up of a large number of constituent gases of which nitrogen, oxygen, argon, carbon dioxide and water vapour are the most abundant, with ozone being important at high altitudes. Much of the radiation entering the top of the atmosphere is completely absorbed or reflected before it reaches the ground. Life on the surface of Earth is only possible because the atmosphere protects us so efficiently from the intense ultraviolet radiation produced by the Sun. Unfortunately for astronomers, this means that much of the radiation from distant stars and galaxies gets no nearer the surface of the Earth than the top of the atmosphere, and thus it restricts substantially our view of the Universe.

We see what happens to the various forms of electromagnetic radiation as they pass into the Earth’s atmosphere. The outermost layer of the atmosphere that affects the passage of electromagnetic radiation is called the IONOSPHERE. It consists of a layer of hot, ionized gas (called a PLASMA) which reflects long-wavelength radio waves back into space. Long medium and short waveband radio stations on Earth make use of this ability of the ionosphere to reflect radio waves. Radio waves are reflected from the underside of the ionosphere, so that signals may be received clearly from broadcast stations far beyond the horizon of the transmitter. However, radio waves with wave¬lengths from about 1mm to 10m pass through the atmosphere almost without absorption. From 1 nm to about 1 mm (from the near infrared to the millimetre wavebands), electromagnetic waves are strongly absorbed by molecules in the upper atmosphere, notably water and carbon dioxide. In the visible part of the spectrum (350nm-700nm), the atmosphere is transparent and most of this radiation reaches the surface of the Earth. Waves of shorter wavelength are strongly absorbed: the ultraviolet waves by molecules, X-rays by individual atoms and gamma rays by atomic nuclei. This means that the ground-based astronomer can only work satisfactorily in two spectral regions: from 1mm to 30m (the RADIO WINDOW) and from 350nm to 700nm (the OPTICAL WINDOW). With care it is also possible to work in parts of the infrared region of the spectrum, provided the astronomer chooses a site which is as high and dry as possible. An example of such a site is Mauna Kea on Hawaii at an altitude of 4500m. Such a site minimizes the amount of water vapour between the telescope and the top of the atmosphere.

One might suppose that it should be possible to observe relatively bright objects in other parts of the spectrum, even if only a few per cent of the radiation from the object penetrates to the Earth’s surface. Unfortunately this is not usually possible. The temperature of the Earth’s atmosphere varies by only a fairly small amount, in the range 240K-310K. Although it absorbs vast amounts of energy from the Sun it maintains its temperature in this narrow range byre-radiating nearly all of the energy it absorbs.

We cannot see the stars in the daytime because the sky is so bright, due to the scattering of sunlight. As soon as the Sun sets, the sky darkens quickly. Note, however, that in the infrared part of the spectrum the atmosphere is radiating because it is hot; it cools only slowly so that the infrared sky remains bright all night long. It is often easier, therefore, to observe in wavebands where the absorption and hence re-radiation of the atmosphere is small.

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