Abbreviation for New General Catalogue.

Abbreviation for Next Generation Space Telescope.

An alternative name for airglow.

The former name of the extinct martian volcano now known as Olympus Mons.

Abbreviation for noctilucent clouds.

Abbreviation for National New Technology Telescope.

Abbreviation for National Optical Astronomy Observatories.

A high-atmosphere phenomenon appearing as luminous bluish clouds in the summer twilight sky. They occur at heights of about 80 kilometres 50 miles and show a range of structures. Noctilucent clouds are very thin and scatter only a very small proportion of the sunlight incident on them, so they cannot be seen from the ground during daytime or in bright twilight. Since they are a summer phenomenon, they cannot be observed from the highest latitudes, where the sky never gets dark enough. However, they are also a high-latitude phenomenon, so the range of latitudes from which they can in practice be seen, between 50° and 65°, is quite narrow. The clouds form in the presence of nuclei on to which water can condense and freeze into ice. It is not known just what these nuclei are, but they may be ions created by solar ultraviolet radiation or micrometeoritic particles. The main prerequisite is a low enough temperature, which is calculated to be 120 K -150°C at a height of 80-90 kilometres 50-56 miles. This is created by the pole-to-pole air flow at these heights and is not affected by the level of solar radiation. There is observational evidence to suggest an overall increase in the occurrence of NLCs over recent decades, and this has been linked to an increase in water vapour in the upper atmosphere as a consequence of increasing amounts of methane. The number of sightings also varies inversely with the solar activity cycle.

A system of short, narrow canyons on the surface of Mars, east of the Tharsis region. The canyons are apparently grabens that formed as a result of the uplift of the Tharsis bulge.

A simple instrument for telling the time at night by observation of the position of the two stars in the constellation Ursa Major known as the Pointers. The continuation of the line joining these stars passes very close to the north celestial pole, and can effectively be used as a giant clock-hand in the sky as the Earth’s rotation causes them to sweep out a circle daily. The nocturnal consists of two concentric discs and a sighting arm, fixed at the centre by an eyelet through which the Pole Star can be sighted. The lower disc is graduated with the days of the year and the upper one with the 24 hours of the day. In use, the sighting arm is aligned with the Pointers. The dials are needed to convert the direct measurement, which is effectively the sidereal time, to mean solar time.

Either of the points on the celestial sphere where the plane of an orbit intersects a reference plane. The position of a node is one of the orbital elements used to specify the orientation of an orbit.

In a radio receiver, the characteristic hiss produced by the random motion of electrons in the components of the receiver. Cosmic radio waves are also produced by random motions of electrons, so the signal detected by a radio telescope is indistinguishable in nature from the noise generated locally. For this reason, very-low-noise receivers are required for radio astronomy.

In astronomy, the electromagnetic radiation produced by an electron travelling at a speed close to that of light when it is forced to change its velocity. The commonest form is synchrotron radiation, caused when electrons spiral round a magnetic field. The detection of non-thermal radiation from an object is a clear indication that high-energy processes are at work. More generally, the term is used for any electromagnetic radiation not produced by thermal processes.

A 2.56-metre 101-inch reflecting telescope at the Observatorio del Roque de los Muchachos in the Canary Islands, operated jointly by Denmark, Finland, Norway and Sweden since 1989.

A small and insignificant southern constellation introduced by Nicolas L. de Lacaille in the mid-eighteenth century. It contains no star brighter than fourth magnitude.