An annual meteor shower, the radiant of which lies within the sickle in the constellation Leo. The peak occurs on 17 November and the normal limits are two days either side. Though a small number of meteors are detected each year, spectacular displays are occasionally seen. The Leonids in 1966 gave observers in the USA the richest shower ever recorded, with rates as high as 40 meteors a second. The shower is associated with Comet 55P/Tempel-Tuttle, first recorded in 1865, which has a period of 33 years. The meteoric material is concentrated near the comet and is not evenly spread around the orbit. Good displays are possible only every 33 years, though they are not necessarily seen even then, if the comet passes too far from the Earth’s orbit.

The period from one-millionth of a second to one second after the primordial Big Bang. During this interval, the behaviour of the universe was dominated by the lighter elementary particles, such as electrons and neutrinos, together with a tiny proportion one-billionth of protons and neutrons.

One of the original 48 constellations listed by Ptolemy c. AD 140. It lies immediately south of Orion, possibly representing a hare pursued by the Hunter. It is small but distinctive and contains seven stars brighter than fourth magnitude.

English name for the constellation Canis Minor.

English name for the constellation Hydrus.

A comet discovered by C. Messier in 1770, but named after A. Lexell 1740-84 who investigated its orbit. He showed that a close approach to Jupiter in 1767 had caused a large change in its orbit that brought it close enough to the Earth to be visible. The comet passed within 1.2 million kilometres of the Earth, still the closest recorded approach of a comet. However, another close approach to Jupiter in 1779 further perturbed the orbit, so drastically that it was never seen again.

Abbreviation for local hour angle.

One of the twelve zodiacal constellations listed by Ptolemy c. AD 140, though its stars were previously regarded as part of the Scorpion, which is next to it. Libra is one of the least conspicuous constellations in the zodiac, with just five stars brighter than fourth magnitude.

An observatory belonging to the University of California. The observatory site is on Mount Hamilton in the Californian Diablo Range at a height of 1,300 metres 4,200 feet. This is now only an observing station, and it is administered from the University of California’s Santa Cruz campus. The funds for the observatory were provided to the University by a millionaire businessman, James Lick 1796-1876. The building and a 92-centimetre 36-inch refracting telescope were completed in 1888, twelve years after Lick’s death. He is buried at the base of the telescope. The main research telescope is now the Shane 3-metre 120-inch reflector, in operation since 1959. The 92-centimetre 36-inch Crossley reflector, built by Andrew Common, was presented to the observatory in 1895 by its English owner, Edward Crossley. The most recent instrument is the Nickel 1-metre 40-inch reflector, a modern automated instrument completed in 1980. There is also a 50-centimetre 20-inch astrograph, with twin tubes designed to take photographs simultaneously in the blue and yellow regions of the spectrum.

A colloquial expression for a flux collector.

A graph on which the light output from a variable star or other varying astronomical object is plotted against time.

A reflection of the burst of light from a supernova or a nova by neighbouring interstellar clouds, resulting in a ring of light surrounding the supernova or nova. The ring of light is seen to expand over time.

The scattering of light from man-made sources into the night sky, which increases the background brightness of the sky above its natural level and interferes with astronomical observations. Light pollution is worst close to major centres of civilization. Legislation has been enacted in part of the USA to protect important observatory sites from the damaging effects of unnecessary artificial lighting in nearby cities. However, the problem is a growing one and a matter of worldwide concern for both amateur and professional astronomers.

The distance travelled through a vacuum by light or any other form of electromagnetic radiation in one year. A light year is equivalent to 9.4607 × 1012 kilometres, 63,240 astronomical units or 0.306 60 parsecs.