The observatory of the University of Toronto, Canada, located 25 kilometres 15 miles north of the university campus. It was presented to the university in 1935 by Mrs Dunlap as a memorial to her husband. The main instrument is a 1.88-metre 74-inch reflector, the largest in Canada. Domes on top of the administration building house 0.5-metre and 0.6-metre reflectors. The observatory is used for training students and fostering public interest in astronomy, as well as research.

Asteroid 511, diameter 324 km, discovered by R. S. Dugan in 1903. It is one of the largest asteroids.

An empirical formula devised by William Rutter Dawes 1799-1868 for the minimum angular separation of a close pair of star images detectable as double by a telescope of a particular aperture - its resolving power. The rule assumes that observing conditions are good and that the two stars are not of very different brightness. It gives a separation in arc seconds of 11.6/D, where D is the diameter of the telescope aperture in centimetres. If D is in inches, the formula is 4.6/D.

In astronomy, a unit of time defined as 86,400 seconds, where the second is in turn defined in terms of the frequency utilized in a caesium atomic clock. This definition of the day is closely linked with the Earth’s rotation period, though that rotation is not completely uniform. See also: solar day, sidereal day.

An adjustment to the normal civil time, introduced in some countries as a matter of convenience, for part or all of the year. One of the chief reasons for the adjustment is to arrange for the habitual working day to occur as far as possible during the hours of daylight, hence its name.

An exceptionally bright comet discovered independently by Klinkenberg from Haarlem on 9 December and De Chéseaux from Lausanne on 13 December 1743. It reached magnitude -7 and developed a fan of multiple tails: eleven separate tails were noted.

dwarf galaxy.

The shortened form of the title, De revolutionibus orbium coelestium, the book by Nicholas Copernicus 1473-1543 in which he set out his heliocentric theory of the solar system. It was published in the year of Copernicus’s death. Copernicus’s work marked a turning-point in astronomical thought, which led ultimately to the rejection of Earth-centred cosmology, though it was a century before the idea of a Sun-centred solar system gained general acceptance. The theory assumes that the planetary orbits are circular, when they are in fact elliptical, so it provided little if any improvement on existing methods for calculating the positions of the planets. Copernicus could offer no direct proof of the heliocentric theory and the book was subsequently condemned by the Church authorities, who long regarded the ideas it contains as heretical.

A model of the expanding universe proposed in 1917 in which there is no matter or radiation. This unrealistic hypothesis is historically important because it established the idea that the universe might be expanding rather than static. See also: Einstein-de Sitter universe.

Abbreviation for declination.

Low-frequency radio waves, with wavelengths of tens or hundreds of metres. Waves of this type are received, for example, from the planet Jupiter, in bursts that are associated with the interaction between the planet and its satellite Io.

A number that indicates the rate at which the expansion of the universe is being slowed down by the mutual gravitational attraction of the matter within it. For a value greater than 0.5 the expansion of the universe will ultimately be halted, followed by contraction and collapse; a value less than or equal to 0.5 indicates that the universe will continue to expand for ever. See also: critical density.

Radio waves in the wavelength range 10 to 300 centimetres. Radio waves of this type are produced, for example, in the radiation belts surrounding Jupiter, where electrically charged particles are trapped in the planet’s magnetic field.

One of the coordinates used to define position on the celestial sphere in the equatorial coordinate system. Declination is the equivalent of latitude on the Earth. It is the angular distance, measured in degrees, north or south of the celestial equator. Northerly declinations are positive and southerly ones negative. See also: right ascension.

One of the parameters used to describe the direction of the geomagnetic field.