The time taken to complete one circuit of a closed orbit.

A series of US astronomical satellites launched between 1966 and 1972. OAO-1, launched on 8 April 1966, developed faults the day after and was a write-off. OAO-2 was launched on 7 December 1968. It carried instruments for photometry and sky survey work in the ultraviolet, and continued to be operational until February 1973. The next launch, in 1970, was a total failure. The final satellite in the series, OAO-3, was renamed Copernicus after launch on 21 August 1972. Its main purpose was ultraviolet astronomy, for which it carried a 0.8-metre 31-inch telescope complete with a spectrograph. It also carried a British X-ray astronomy instrument with which important observations were made.

A series of US satellites, launched between 1962 and 1975, for various scientific studies of the Sun, particularly at ultraviolet and X-ray wavelengths.

A huge impact feature on the Moon’s extreme western limb as viewed from Earth, visible only at times of favourable libration. Photographs taken from lunar orbit by spacecraft show a structure of at least three concentric rings. Unlike many other impact basins on the Moon, it is not extensively filled by dark mare material.

A brilliant constellation straddling the celestial equator, widely considered to be the most magnificent and interesting in the sky. Its pattern is interpreted as the hunter brandishing a raised club and a shield. Three bright stars mark his belt, and several fainter ones a sword hanging from it. Orion contains five stars of the first magnitude or brighter and a further ten brighter than fourth magnitude. The most spectacular diffuse nebula in the sky, the Orion Nebula, is faintly visible to the unaided eye in the sword.

The spiral arm of the Galaxy in which the Sun is located.

Orion Nebula.

A bright emission nebula surrounding the multiple star Theta1 Orionis θ1 Ori in the sword of Orion. The luminous nebula is just part of a complex region of interstellar matter at a distance of 1,300 light years occupying much of the constellation of Orion. The Orion cloud is the largest such dark cloud known in the Galaxy. Millimetre-wave observations of the emission from the molecules CO carbon monoxide, HCHO formaldehyde, and many others, reveal the presence behind the visible part of a large molecular cloud, known as the Orion Molecular Cloud OMC-1. This is an important region of star formation, and the group of four young hot stars that make up θ1 Orionis, also known as the Trapezium, are believed to be less than 100,000 years old. The Becklin-Neugebauer object and the Kleinmann-Low Nebula, both detected through their infrared emission, are sites of current star formation. The Trapezium stars are creating an expanding spherical cavity near the edge of the dark cloud. Their ultraviolet radiation is ionizing the gas and blowing away the dust. Relatively recently, in astronomical terms, the bubble broke through on our side of the dark cloud, revealing the stars and ionized hydrogen within. The sharper edges of the nebula are produced by remnants of dust. M43 NGC 1982 is a northern section of the nebula separated from the larger part M42; NGC 1976 by a lane of dust. In photographs, the dominant colour of the luminosity is red from the hydrogen alpha light. Observed visually, the nebula appears greenish because of the eye’s low sensitivity to red light. The green emission is due to oxygen. The nebula occupies an area of sky about one degree across and is faintly visible to the naked eye. It has the highest surface brightness of all nebulae.

Belt of Orion.

An annual meteor shower, the multiple radiant of which lies on the border of Orion and Gemini, near the star Gamma Geminorum γ Gem. The peak of the shower occurs around 22 October, and the normal limits are 16-27 October. The shower is produced by meteoroids that have come from Halley’s Comet.

A working model of the solar system showing the planets, possibly with some of their moons, in their orbits around the Sun. The term orrery was first applied to such a model in 1713 when one was made for the Fourth Earl of Cork and Orrery. The gear systems in such models are usually made so that the orbital periods of the planets are in the correct ratios. However, it is impossible to construct a demonstration model of this kind in which the planet sizes and the distances between them are both to scale because of the great range of distance scales.

A good general-purpose telescopic eyepiece, giving a high-quality image and good eye relief. There are various forms, but the eyepiece typically consists of a convex or planoconvex eyelens with an achromatic triplet as the field lens.

The 1.2-m 48-inch Schmidt camera at Palomar Observatory. It has been in operation since 1948.

A model of the universe in which it goes through cycles of expansion, collapse to a so-called Big Crunch, Big Bang, expansion, and so on, in an infinite sequence.

A measure of the intrinsic strength of a spectral line. It depends on the probability that the transition between atomic energy levels responsible for the line will take place in a particular time. It is independent of the physical conditions under which the transition occurs.