A large lunar crater to the north-west of Mare Nectaris, overlapping another large crater, Cyrillus. Theophilus is 100 kilometres 60 miles in diameter and its terraced walls rise 5 km above the floor. A complex central peak rises to 2.2 km.

General Relativity, Special Relativity.

Electromagnetic radiation arising from the thermal state i.e. temperature of the emitter, as opposed to non-thermal radiation, which is emitted by energetic electrons that are not necessarily in thermodynamic equilibrium. See also: black body radiation.

The state of a physical system in which there is no net exchange of thermal energy between members, and temperature remains constant.

In a total or annular eclipse of the Sun, the point when the edges of the Moon’s disc and Sun’s photosphere are in contact at the end of totality or the annular phase. In a lunar eclipse, third contact occurs when the Moon starts to leave the full shadow umbra of the Earth. The term may also be used to describe the similar stage in the progress of a transit or occultation.

The phase of the Moon when half the visible disc of the waning Moon is illuminated. Third quarter occurs when the celestial longitude of the Moon is 270° greater than the Sun’s.

Asteroid 88, diameter 232 km, discovered in 1866 by C. H. F. Peters.

A planetary feature like a small dome-shaped mountain or hill.

A third magnitude star in the constellation Draco. Despite its designation as Alpha, it is only the seventh-brightest star in Draco. About 5,000 years ago Thuban was the nearest bright star to the north celestial pole. The location of the north pole among the stars changes slowly over time because of precession. The name, derived from Arabic, means dragon.

Asteroid 279, diameter 130 km, discovered by J. Palisa in 1888. At a distance from the Sun of 4.26 AU, it is regarded as marking the outer edge of the main asteroid belt.

The movement of fluids, or stresses induced in solid objects, by a cyclical change in the net gravitational forces acting upon them. On the Earth, ocean tides are produced and modified by the daily, monthly and annual variations in the net gravitational force on the Earth exerted by the combined pull of the Sun and Moon. These variations arise from the Earth’s rotation, the Moon’s orbital motion around the Earth and the Earth’s orbital motion around the Sun.

The means by which the intervals between sequential events are measured. See also: civil time, International Atomic Time, Mean Solar Time, sidereal time, spacetime, Universal Time.

A geographical region throughout which civil time is reckoned to be the same. Time zones are based on longitude bands 15° wide, corresponding to a one-hour difference in local time. There are, however, considerable deviations from regular lines of longitude in the boundaries of time zones in order to take account of the distribution of land over the Earth’s surface and the locations of centres of habitation. The difference between most adjacent time zones is one hour, but there are some instances of half-hour differences aimed at minimizing deviations from local time.

The largest satellite of Saturn and the second-largest in the solar system after Ganymede. It was discovered in 1655 by Christiaan Huygens. Titan is surrounded by a thick atmosphere which consists mainly of molecular nitrogen but also contains methane. The surface pressure is 1.6 times greater than atmospheric pressure at the surface of the Earth. The action of sunlight on the methane and other atmospheric constituents such as carbon monoxide results in chemical changes, producing hydrocarbons and other molecules. These molecules condense in the cold atmosphere, forming a layer of opaque orange-coloured haze 200 kilometres 125 miles above the surface. It is believed that the conditions are such that liquid methane may exist on the surface, where the temperature is 95 K -178°C. There could even be methane rain falling from clouds in the lower atmosphere.

The largest satellite of Uranus, discovered by William Herschel in 1787. The flyby of Voyager 2 in 1986 showed Titania to be peppered with numerous craters, though there are regions where the crater density is lower. This suggests that activity resulting in changes to the surface has taken place in the past. In addition, the surface is scarred by a large number of valleys and fractures, some of which cut large craters in half.