The ninth planet of the solar system, discovered as a fifteenth magnitude object on 18 February 1930 from the Lowell Observatory by Clyde Tombaugh. Searches for a planet beyond Neptune had started in 1905, stimulated by apparent discrepancies between the calculated and observed orbits of Uranus and Neptune. However, it is now known that the mass of Pluto is less than one-fifth that of the Moon, insufficient to have any gravitational effect on Uranus and Neptune. Pluto’s orbit is more highly inclined to the ecliptic and more eccentric than that of any other planet. Its distance from the Sun ranges between 30 and 50 AU. Perihelion occurred in 1989 and, between 1979 and 1999, Pluto’s orbit brings it nearer the Sun than Neptune. The discovery of Pluto’s satellite, Charon, in 1978, made it possible to obtain improved VALUES (for the planet’s diameter and mass. The diameter is 2,300 ± 40 kilometres. Pluto’s overall density is approximately twice that of water and it is thought likely to consist of a thick layer of water ice overlying a core of partially hydrated rock. Charon and Pluto are locked in synchronous rotation with a period of 6.39 days. Pluto’s rotation axis is inclined at 122° to the plane of the ecliptic so that, like Uranus, it rotates in a retrograde sense, lying on its side. A rare series of mutual occultations and transits took place between 1985 and 1990. Such events, as viewed from Earth, take place only twice in the planet’s 248-year orbital period. They made it possible to distinguish the spectral signatures of Pluto and Charon and to construct the first approximate albedo maps of Pluto’s surface. These confirmed previous suspicions of a highly non-uniform and variable surface based on the change of brightness over the rotational period, and in the longer term. In contrast with Charon, which is grey, Pluto’s surface is reddish in colour. Methane ice was detected on Pluto in 1976 by infrared spectroscopy. The occultation of a star by Pluto in 1988 revealed the presence of an extended tenuous atmosphere. Nitrogen and carbon monoxide ices were discovered on the surface in 1992. The surface temperature is about 40 K. In 1996 observations with the Hubble Space Telescope resolved broad light and dark features on Pluto’s surface for the first time.

magnitude.

The stars Alpha α and Beta β in the constellation Ursa Major, so called because the line joining them points almost directly to the Pole Star.

A small class of short-period, variable binary stars characterized by X-ray emission. Their light is strongly polarized and the polarization varies over the orbital period, which is between one and four hours. These close systems appear to consist of a normal star and a strongly magnetic white dwarf with its spin locked in synchrony with the orbital period. Matter is transferred from the normal star to the white dwarf but, because of the strong magnetic field, an accretion disc cannot form. Instead, the material is channelled along the magnetic field lines and is deposited at the poles. Polars are also known as AM Herculis stars after the star considered to be the prototype for the class. Intermediate polars are similar, but they have longer orbital periods of several hours. They emit pulsed radiation at the spin rate of the white dwarf, which is not locked to the orbital period in these systems and is typically less than one hour. Their white dwarfs are thought to have weaker magnetic fields, making it possible for an outer accretion disk to form, though material close to the white dwarf is channelled on to the magnetic poles. The pulsed emission is a searchlight effect seen as the accreting pole of the white dwarf sweeps across the line of sight. Intermediate polars are also known as DQ Herculis stars after their prototype.

One of the two rotation axes about which a telescope on an equatorial mounting can turn. The polar axis must be accurately oriented parallel to the Earth’s rotation axis, i.e. at an angle to the horizontal equal to the latitude of the place where it is located, and in the north-south plane. Rotation about the polar axis results in a change in the right ascension of the direction in which the telescope is pointing, but not in declination.

A roughly circular area of limited extent around a pole of rotation of a planet. In the case of Earth and Mars, the term is applied to the areas covered by ice or frost in the two polar regions.

The angular distance along a great circle on the celestial sphere between an object and either the north or south celestial pole.

A slow and very slight movement of the Earth’s geographic poles relative to the surface of the Earth not relative to the stars. It does not affect the celestial coordinates of a star, but does affect the reduction of positional measurements, for example those made with a transit circle. The origin is geophysical: principally that lack of exact coincidence between the Earth’s axes of symmetry and rotation. The magnitude of the effect is typically 0.3 arc seconds, and there are periodicities of 433 days and one year. Much smaller variations also take place over short timescales, ranging between two weeks and three months, due to surface air pressure changes. See also: Chandler wobble.

The measurement of the state of polarization of a beam of electromagnetic radiation. The instrument used is called a polarimeter, and it must include optical elements capable of altering the state of polarization of the beam being tested. Such elements are described as optically active.

The brightest star in the constellation Ursa Minor, lying within one degree of the north celestial pole. It is a Cepheid variable and its magnitude changes between about 1.95 and 2.05 over a period of four days.

The non-random distribution of electric field direction among the photons in a beam of electromagnetic radiation. In linear polarization, the vectors representing electric field are parallel. In circular polarization, the direction of polarization changes continuously in such a way that the electric field vector rotates with the frequency of the radiation. Elliptical polarization is similar to circular polarization except that the magnitude of the electric field vector also changes continuously, but at twice the frequency of the radiation. The properties of a beam of polarized light can be described by a set of four numbers known as the Stokes parameters.

An optical component that transmits only light that is linearly polarized in a particular direction. See also: polarization.

Popular name for the star Polaris.

The brightest star in the constellation Gemini. It was nevertheless given the Bayer letter Beta rather than Alpha. It seems unlikely that Pollux has brightened since Bayer’s time 1572-1625. Pollux is an orange-coloured, giant K star. Castor and Pollux were the twin sons of Leda in classical mythology.

English name for the constellation Puppis.