The outermost tenuous layers of a planetary atmosphere, which merge into the interplanetary medium. In this region, the density is so low that very few collisions occur between the atoms, and atoms moving rapidly are able to escape from the gravitational pull of the planet. The Earth’s exosphere starts at a height of about 400 to 500 kilometres 250 to 300 miles.

A model of the universe in which the fundamental length scale increases with the passage of time. The separation between clusters of galaxies would today be considered an appropriate length scale for this purpose. The discovery that the redshifts of galaxies increase with distance 1929 and the detection of the cosmic background radiation 1964 are generally taken as evidence that the universe is expanding.

The reduction in intensity of light as it passes through an absorbing or scattering medium, such as interstellar material or a planetary atmosphere. See also: atmospheric extinction, interstellar extinction.

Beyond the bounds of our own Galaxy, the Milky Way.

A planet orbiting around a star other than the Sun. Techniques capable of detecting small cyclical changes in the velocities of stars by means of the Doppler effect led to the first substantial evidence for extrasolar planets around normal stars being presented in 1995 and 1996. Stars for which planets have been claimed include 51 Pegasi, 47 Ursae Majoris, Rho1 Cancri, Tau Boötis, Upsilon Andromedae, 70 Virginis, HD 114762 and 16 Cygni B.

Forms of intelligent life, which may or may not exist elsewhere in the universe, other than life as we know it on Earth. Studies in this field focus on the search for radio signals, biologically significant molecules in space and possible planetary systems around stars.

An orbiting spacecraft launched by NASA in 1992 with the object of mapping the entire sky at shorter ultraviolet wavelengths 7-76 nanometres and carrying out high-sensitivity observations over a more restricted sky area. See also: ultraviolet astronomy.

The distance between the surface of the eyelens of an eyepiece and an observer’s eye when the observer is positioned to see clearly the full field of view. In general, the higher the power of the eyepiece, the lower the eye relief.

A combination of small lenses mounted in a tube, used to magnify and focus the image formed by a telescope or other optical instrument. Eyepieces used for visual observation with a telescope are normally interchangeable, pushing or screwing into a draw-tube of standard size. The focal length of the eyepiece fe determines the magnification in combination with the fixed focal length of the telescope, fo: magnification = fo/fe. Different types of eyepiece are used for different applications according to the magnification, image quality and field size required. See also: Huygens eyepiece, Ramsden eyepiece, Kellner eyepiece, orthoscopic eyepiece, Erfle eyepiece.