mean solar time.

A system of time measurement based on the rotation of the Earth, which is assumed to be constant. The Earth’s rotation rate is not in fact precisely constant when checked against atomic clocks. Mean solar time has therefore been superseded by International Atomic Time TAI. Leap seconds are occasionally introduced to allow TAI to keep pace with the Earth’s rotation. Since the Earth’s rotation axis is inclined to the plane of the ecliptic and its orbit around the Sun is elliptical rather than circular, the Sun’s apparent motion through the sky is not uniform over the course of a year. Apparent solar time, as measured directly by a sundial, differs from mean solar time by an amount that varies through the year and is known as the equation of time. To define mean solar time, the abstract concept of the mean Sun was introduced. This hypothetical object follows a circular orbit around the celestial equator at a constant speed, completing one circuit in a tropical year.

A concept used to define mean solar time.

The branch of applied mathematics that deals with the motion and equilibrium of bodies. It is subdivided into dynamics motion under the influence of forces, statics equilibrium conditions and kinematics motion without reference to mass or force. Celestial mechanics is the application of mechanics in astronomical situations.

A unit of distance equal to one million parsecs.

Asteroid 18, diameter 162 km, discovered by J. R. Hind in 1852.

Literally, table. The term is used to describe a flat-topped, elevated feature on the surface of a planet.

A faint southern constellation introduced in the mid-eighteenth century by Nicolas L. de Lacaille with the longer name Mons Mensae, the Table Mountain. It contains no stars brighter than fifth magnitude, but part of the Large Magellanic Cloud lies within its boundaries.

One of the two stars of the Plough in Ursa Major with Dubhe called the Pointers. The stars in the Plough have been designated by position rather than in brightness order. It is therefore actually the fifth-brightest star in the constellation, with a magnitude of 2.4. Merak is an A star and its name, of Arabic derivation, means the loin.

The nearest major planet to the Sun and the smallest of the terrestrial planets. Telescopic observation of Mercury from the Earth is very difficult, partly because of its small size and partly because it can never be more than 28° from the Sun on the celestial sphere since its orbit lies well inside the Earth’s. For the same reason, Mercury like Venus, the other inferior planet exhibits a cycle of phases, similar to those of the Moon. Hardly any surface detail can be discerned and very little was known about the planet until the flybys of Mariner 10 in 1974 and 1975. The space probe was put in an orbit around the Sun such that it encountered Mercury three times before it ran out of attitude-control gas. The images returned have allowed about 35 per cent of the surface of Mercury to be mapped. Ancient, heavily cratered terrain accounts for 70 per cent of the area surveyed. The most significant single feature is the Caloris Basin, a huge impact crater with a diameter of 1,300 kilometres - a quarter the diameter of the planet. The basin has been filled by a relatively smooth plain, and terrain of the same type covers parts of the ejecta blanket. The impact took place 3,800 million years ago and produced a temporary revival of the volcanic activity that had mostly ceased 100 million years earlier, creating the smoother areas inside and around the basin. At the point on Mercury diametrically opposite the impact site, there is curious chaotic terrain that must have been created by the shock wave. Characteristic features found on Mercury are lobate scarps rupes, which take the form of cliffs between a few hundred and 3,000 metres high, believed to have formed when the planetary crust shrank as it cooled. In places they cut across craters. The planet’s rotation period is such that a day on Mercury lasts two years. This leads to immense temperature contrasts: at perihelion, the subsolar point reaches 430°C; the night-time temperature plunges to -170°C. The high daytime temperatures and the small mass of the planet make it impossible for an atmosphere to be retained. The small amounts of helium detected may be the product of radioactive decay of surface rocks or have been captured from the solar wind. The average density of Mercury is only slightly less than that of the Earth. Taking account of its smaller size and lower interior pressure leads to the conclusion that Mercury has a substantial iron core accounting for 70 per cent of its mass and 75 per cent of its total diameter. There is also a magnetic field of about 1 per cent the strength of the Earth’s field, providing further evidence for the metallic core.

A series of American spacecraft capable of carrying one astronaut. The first American experiments in manned spaceflight used Mercury capsules, and the first suborbital flight, to an altitude of 187 kilometres 116 miles, was in 1961. The first orbital flight was made by John Glenn in February 1962.

manganese star.

The great circle on the celestial sphere passing through the poles and the zenith.

A line of longitude on the Earth, or on another astronomical body. On the Earth, the meridian through Greenwich marks the zero of longitude and is sometimes referred to as the prime meridian.

transit circle.