Comet Encke.

A form of equatorial mounting for a telescope.

Descriptive popular name for a planetary nebula with the catalogue designation MyCn18. Public attention was drawn to its spectacular form by a Hubble Space Telescope image published in 1996.

A process by which jets streaming from radio galaxies draw energy from the surrounding interstellar and intergalactic material. As material is sucked in, turbulent eddies are set up. These in turn create shocks which can heat the gas to extremely high temperatures. Pockets of the gas collect and cool; ultimately star formation takes place within them.

A gaseous region surrounding one or more stars or any other astronomical object.

One of the Hirayama families of asteroids. The members lie at a distance of 3.02 AU from the Sun and are intermediate in type between carbonaceous and silicaceous.

The Moon’s age i.e. stage in its cycle of phases at the beginning of a calendar year.

The difference between a solar year and a lunar year of 12 lunar months. This period, of about 11 days, is known as the annual epact.

The difference between a calendar month and a lunar month, known as the monthly epact.

A table giving the celestial coordinates, magnitude and other data for astronomical bodies such as the Moon, Sun, planets and comets. The term is also used for a book containing a compendium of such tables and other astronomical data.

The time used prior to 1984 in calculations involving gravitational theories of the solar system. In 1984 it was replaced by dynamical time.

Motion in a circle, the centre of which is in turn moving around a larger circle called the deferent. Epicyclic motion was a fundamental aspect of the geocentric model of the solar system propounded by Ptolemy in the second century AD. To improve the accuracy of predictions based on the model, Ptolemy had the centre of the epicycle revolve with uniform angular motion, not about the centre of the deferent, but about a point called the equant, which he displaced to one side. By also placing the Earth to the other side of the centre of the deferent and choosing the radii of the epicyle and deferent appropriately, the Ptolemaic theory could be used to predict planetary positions to an accuracy of one degree.

A small satellite of Saturn, discovered in 1980. It is co-orbital with Janus, the orbit lying just beyond the limit of the main ring system. The two satellites may be fragments of a larger object that was shattered by an impact. It is irregular, measuring 140 × 100 kilometres. See also: co-orbital satellites

A precise moment in time for which given celestial coordinates or orbital elements are strictly correct. It is necessary to define the epoch for such measurements because of the effects of precession on celestial coordinate systems and gravitational perturbations on the orbits of bodies such as comets and planets. It has been the practice to change the standard epoch used for star charts and catalogues every 50 years. Those prepared since the 1970s have typically been for the epoch 2000.0; the previous standard epoch was 1950.0.

A device incorporated by Ptolemy in his geocentric model of the solar system to take account of the observed irregular motion of the planets. He considered that the motion of the planets was uniform, not around the Earth itself, but around a point in space called the equant. See also: epicycle.