The difference between an actual observed quantity and its predicted or expected value. Residuals may result from observational error, basic limitations on the accuracy with which the observations can be made, or inaccuracy in the prediction with which the observation is compared.

The size of the smallest detail that can be distinguished with an imaging instrument such as a telescope or spectrograph.

The ability of an optical system to distinguish detail. Theoretically, the resolution is limited by the size of the aperture, through the effects of diffraction. Because of diffraction, the image of a point source is actually a disc surrounded by a number of rings. The diameter of the disc, known as the Airy disc, is given in radians by the formula 1.1λ/Δ resolving power. In practice, however, the resolving power of a large ground-based optical telescope is limited by the quality of seeing rather than the aperture.

A situation in which one orbiting body is subject to a regular periodic gravitational disturbance by another. Resonances occur between orbital positions that are linked by periods in whole-number ratios e.g. 1:1, 2:1, 3:2. They are responsible for phenomena such as the Kirkwood gaps in the asteroid belt and divisions in the planetary rings, such as exist in those around Saturn. See also: commensurability.

Kirkwood gaps.

The difference between the times of Moonrise on successive nights.

Fine lines or wires at the focus of an optical instrument to aid in the measurement of angular distances in the image.

A small southern constellation introduced by Nicolas L. de Lacaille in the mid-eighteenth century. Its two brightest stars are of third magnitude.

Motion of an object on the celestial sphere in the east-west direction, or, for orbital motion or axial rotation in the solar system, motion that is clockwise as observed from north of the ecliptic. See also: direct.

The outermost layers of a star in which the absorption lines are presumed to be impressed on the continuous spectrum of the hotter gas beneath. The idea of a reversing layer is, however, an oversimplification since both emission and absorption take place through all the layers of a stellar atmosphere.

Abbreviation for Royal Greenwich Observatory.

The second-largest satellite of Saturn, discovered by Giovanni Cassini in 1672. Voyager 1 images show that Rhea’s light-coloured, icy surface is saturated with craters. At the low temperatures prevailing this far from the Sun, the ice is like rigid rock. There is little evidence that the surface has changed since the era when the impact craters were formed.

A large nebulous region near the star Rho Ophiuchi ρ Oph. It is a mixture of reflection nebulae, emission nebulae, dark absorption nebulae and molecular clouds, and is relatively close, at a distance of about 700 light years. Infrared observations reveal the presence of a cluster of at least 40 stars within the dark cloud. This is a region of very active star formation and contains many T Tauri stars and Herbig-Haro objects.

The brightest star in the constellation Orion. Its Bayer letter is Beta, despite the fact that, at magnitude 0.1, it is slightly brighter than Betelgeuse, which is designated Alpha. Rigel is a supergiant B star, with a seventh magnitude companion. The name, derived from Arabic, means leg of the giant.

One of the coordinates used to define position on the celestial sphere in the equatorial coordinate system. It is the equivalent of longitude on the Earth but is measured in hours, minutes and seconds of time eastwards from the zero point, which is taken as the intersection of the celestial equator and the ecliptic, known as the First Point of Aries. One hour of right ascension is equivalent to 15 degrees of arc; it is the angle through which the celestial sphere appears to turn in one hour of sidereal time, as the Earth rotates. See also: declination.