The Earth Atmosphere and Space

The Earth’s atmosphere has always acted as a screen between the observer and the rest of the Universe. The pre-Copernicans regarded it as the seat of the volatile elements because of its mobility, separating as it did the sublunar world from the world of the stars. From the time of Galileo, and up until the conquest of space, observations of photons were limited to the narrow window of the visible, and this range was extended only recently by the addition of radio frequencies. Despite the recent development of observation from space, ground-based observation retains considerable advantages in terms of both access and cost. The global strategy of observational astronomy therefore requires an exact knowledge of the properties of the Earth’s atmosphere. With such a knowledge, the potential or the limits of ground-based observation can be defined, and, for each wavelength of the spectrum, the best altitude can be determined and the best sites chosen for new instruments. The choice of site is crucial. Many factors must be taken into account, and we shall describe them here. The Antarctic continent is now accessible for astronomy and will no doubt provide many important opportunities.