corona.

A rare type of asteroid with high albedo. The composition may be similar to that of the meteorites known as enstatite achondrites.

English name for the constellation Aquila.

An emission nebula surrounding a brilliant cluster of young stars in the constellation Serpens. The nebula glows with the characteristic red colour of hydrogen gas ionized by the radiation from the stars, which are estimated to be only 2 million years old. Detailed observations show it to be a region of ongoing star formation. The nebula lies at a distance of 7,000 light years and is half a degree in diameter.

Originally 1926, a term used by Edwin Hubble for tightly wound spiral galaxies types Sa and SBa, which he considered to be the first stage in a progression through intermediate Sb, SBb to late Sc, SBc. It is now thought very unlikely that spiral galaxies evolve in this manner and, in this sense, the nomenclature has generally fallen into disuse, though it is encountered occasionally. In modern astrophysics the term is increasingly applied indiscriminately to any type of galaxy that is relatively young as judged from its observed properties. Galaxies at very high redshifts, or galaxies containing large amounts of dust and exhibiting rapid star formation may be loosely described by the adjective early, used essentially as a synonym for young. See also: Hubble classification.

An obsolete term applied to the hottest and most massive types of stars, usually those of spectral types O, B and A. The expression reflects the belief once held, but now known to be completely wrong, that the sequence of spectral classes - from hottest to coolest - represents an evolutionary progression. Despite its misleading nature, the term is still frequently used.

The third planet from the Sun. From the astronomical perspective, Earth belongs to the group of terrestrial planets, which also includes Mercury, Venus and Mars. It is with this group, and also the Moon, that its origin, structure and evolution are often compared. Earth has an atmosphere intermediate in density between those of Venus and Mars. It is unique in possessing vast oceans of liquid water. The complex interaction between ocean, atmosphere and planetary surface determines the energy balance and the temperature regime. Cloud cover is typically 50 per cent, and heat trapped within the atmosphere the greenhouse effect raises the average temperature by more than 30 degrees. The present composition of the atmosphere is 77 per cent molecular nitrogen, 21 per cent molecular oxygen, 1 per cent water vapour and 0.9 per cent argon. Carbon dioxide is the most important trace constituent. The high concentration of oxygen, which dates from 2,000 million years ago, is a direct result of the existence of plants. The presence of oxygen allowed the formation of the high-level ozone layer, which shields the surface from solar ultraviolet radiation damaging to life. Earth is the only major planet known for certain to be geologically active. Its large-scale features have all been determined by the creation, destruction, relative movement and interaction of a dozen or so crustal plates - comprising the lithosphere - which slide over the less rigid asthenosphere below. Collisions between plates produce folded mountains and zones of seismic activity are concentrated along the plate boundaries. Seismic waves generated during earthquakes reveal the internal structure of the Earth by the way they propagate. At the centre, there is a molten metallic core of iron and nickel, possibly with a solid core at the very centre. The central temperature is around 4,000°C. A silicate mantle overlies the core. The outermost crust is about 10 kilometres 6 miles thick under the oceans and 30 kilometres 20 miles thick where there are continents. In planetary terms, the surface of the Earth is very young. The basaltic rocks forming the ocean floors are among the youngest. The Precambrian shields - which occupy about 10 per cent of the surface - are the oldest, and the nearest approximation to the cratered terrain that forms a large part of other planetary surfaces. Weathering has removed all but a few traces of whatever impact craters there were. The molten metallic core gives rise to the Earth’s magnetic field and magnetosphere. A layer of electrically charged particles between heights of about 50 and 600 kilometres 30 and 400 miles form the ionosphere. The funnelling of charged particles by the magnetic field to regions between latitudes of 60° and 75° creates the phenomenon of the aurorae. Satellite measurements have shown that the Earth is also an intense source of radio waves at kilometre wavelengths, though these are generated high up and are not detectable at ground level.

A technique in radio astronomy that uses the rotation of the Earth to enable small radio interferometers to achieve the resolution of a dish antenna many kilometres in diameter. This form of aperture synthesis was developed at Cambridge University.

A comet or asteroid whose orbit brings it relatively close to the Earth.

A faint illumination of what would otherwise be the dark part of the Moon when its phase is a thin crescent. The effect is caused by sunlight reflected by the Earth towards the Moon. A popular expression describes the appearance as the new Moon in the old Moon’s arms.

English name for the constellation Pictor.

Displaced relative to some specified centre.

anomaly

One of the parameters used to describe the shape of curves belonging to the family known as conic sections: circles, ellipses, parabolas and hyperbolas. The orbit of a body moving under the influence of a central gravitational force, such as a planet travelling round the Sun, is necessarily one of the conic section curves. Thus eccentricity is one of the important elements used to describe an orbit. Circles and ellipses are closed curves. A circle is defined as having e = 0. The eccentricity of an ellipse is a measure of how much it deviates from being a circle. If c is the distance from the centre of an ellipse to one of its focal points, and a the semimajor axis of the ellipse, the eccentricity is given by the ratio c/a. The eccentricity of an ellipse must be less than unity. Parabolas and hyperbolas are open curves. The observed orbits of non-periodic comets are typically parabolic. A parabola has e = 1 and a hyperbola e > 1. See also: orbital elements.

A spectrograph in which dispersion of the incident light into a spectrum is achieved by the use of an echelle grating. The particular characteristic of an echelle grating is that the profile of the parallel grooves is step-like or zigzag, and their spacing is relatively wide. The light to be dispersed is directed on to the grating at right angles to the faces of the grooves. The direction of the light is thus at a large angle with the normal to the grating as a whole. This produces a set of many overlapping spectra with a high degree of resolution. A second, low-dispersion grating, or a prism, arranged perpendicular to the echelle is used to separate out the overlapping spectra.