An instrument for measuring the strength and direction of magnetic field.

A region between 100 and 200 kilometres 60 and 120 miles thick that is the boundary layer between the magnetosphere and the solar wind.

The region around the Earth, or any other planet, within which its natural magnetic field is constrained by the solar wind.

The part of the magnetosphere of the Earth, or of any other planet, which on the planet’s nightside extends like a comet tail in the direction opposite the Sun. The Earth’s magnetotail extends from around 8 to 10 Earth radii distant on the nightside, to 80, and perhaps as far as 1,000, Earth radii.

The factor by which the angular size of an object is apparently increased when it is imaged by a particular combination of telescope and eyepiece. The magnification of an image made by a telescope is given by the focal length of the telescope divided by the focal length of the eyepiece. A high magnification is not always an advantage. It results in a small field of view in which the effects of bad seeing and any optical shortcomings are emphasized. The contrast between markings on planets and the Moon may also be reduced. Eyepieces should therefore be selected to be appropriate to the type of observations being made.

A measurement of the brightness of a star or other celestial object. On the magnitude scale, the lowest numbers refer to objects of greatest brightness. The magnitude system was initially a qualitative attempt to classify the apparent brightness of stars. The Greek astronomer Hipparchus c. 120 BC ranked stars on a magnitude scale from first for the brightest stars to sixth for those just detectable in a dark sky by the unaided eye. This qualitative description was standardized in the mid-nineteenth century. By this time it was understood that each arbitrary magnitude step corresponded roughly to a similar brightness ratio. In other words, the magnitude scale is a logarithmic scale of brightness. In 1856, N. R. Pogson proposed that a difference in magnitude of 5 should correspond to a brightness ratio of 100:1, a system that is now universally accepted. If two stars differ by one magnitude, their brightnesses differ by a factor equal to the fifth root of 100, i.e. 2.512. This number is known as Pogson’s ratio. The zero point of the scale was set by assigning standard magnitudes to a small group of stars near the north celestial pole, called the North Polar Sequence. The brightness of stars as observed from the Earth, and hence their apparent magnitude, depends on both their intrinsic luminosity and their distance. Absolute magnitude is a measure of intrinsic luminosity on the magnitude scale, defined as the apparent magnitude an object would have at the arbitrary distance of ten parsecs. The magnitude of an object varies with the wavelength range of the radiation observed. Visual magnitude corresponds to the normal sensitivity of the human eye. Photographic magnitude usually refers to the response of a standard photographic emulsion, which is chiefly in the blue and violet part of the spectrum. However, different photographic materials and detectors may have very different colour responses, and quoted magnitudes should include information about the method of measurement. Bolometric magnitudes take account of all radiation, both visible and outside the visible range. Magnitudes measured over a defined wavelength range are often described as colours. The accurate determination of such magnitudes is achieved by photometry.

eclipse.

Medieval Jewish philosopher and physician, also knowns as Rabbi Moses ben Maimon, or Rambam. Maimonides is considered to be the greatest Jewish scholar and philosopher of the Middle Ages. He had a more powerful impact on Jewish tradition than any other human being. In 1180, Maimonides completed the Mishneh Torah, the first code of Jewish law. The Mishneh Torah, a fourteen volume work, includes sections on astronomy, as well as on medicine and metaphysics. Maimonides’ main contribution to astronomy was a complete rejection of astrology. In a letter written to a group of rabbis in southern France in 1194, Maimonides said that he had studied astrology, and that it did not deserve to be called a science. Maimonides argued that man should believe only what can be supported either by rational proof, by the evidence of the senses, or by trustworthy authority. He claimed that the idea that a man’s fate could depend on the constellations is ridiculous; such a theory would rob life of its purpose and make human beings slaves to destiny.

A narrow band running from the upper left to lower right on the Hertzsprung-Russell diagram, on which stellar luminosity is plotted as a function of temperature, with temperature decreasing towards the right. The temperature and luminosity of most stars places them on the main sequence. This reflects the fact that these two fundamental quantities are largely determined by the mass of the star, some variation being introduced by differences in chemical composition. The main sequence is thus a mass sequence. The points for the most massive stars lie at the upper left and those for the least massive at the lower right. In stars on the main sequence, the fusion of hydrogen into helium in the stellar core is the source of energy. Stars in stages of stellar evolution before and after this phase are represented by points elsewhere on the HR diagram. For example, after hydrogen has been exhausted in the core, adjustments taking place internally cause a star to evolve in a way that carries it away from the main sequence towards the upper right of the HR diagram. Most stars spend about 90 per cent of their observable life on the main sequence.

A star with a temperature-luminosity combination that places it on the main sequence of the Hertzsprung-Russell diagram.

Any of the nine planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune or Pluto. The term minor planet is used interchangeably with asteroid.

A reflecting telescope incorporating a deeply curved meniscus lens, which corrects the optical aberrations of the spherical primary mirror to give high-quality images over a wide field of view. It was invented by D. D. Maksutov 1896-1964. The practical arrangement is typically a Cassegrain system. A small secondary mirror is fixed to the back of the correcting lens and the image is formed just behind the primary mirror, which has a small central hole. The difficulty of making large correcting lenses limits the professional application of the design, but the compactness of the tube, the wide field of view and low focal ratio have made Maksutov telescopes popular with amateurs.

A star of spectral type B with a peculiar spectrum in which the lines of manganese and a number of other exotic elements, notably mercury, gallium and yttrium, are exceptionally strong.

The layer in the structure of a planet or satellite lying below the crust and overlying the core. Earth’s mantle contains 65 per cent of the planetary mass.

Literally sea, a term used for extensive dark areas on the Moon. Its use dates from a time when it was believed that the darker features on the Moon were of liquid water; this is now known to be untrue. In recognition of its use over a long period, the term has been retained in official names for these features on the Moon. The largest of the maria is called ocean oceanus rather than sea - Oceanus Procellarum. The lunar maria are actually seas of solidified lava, dating from the era shortly after its formation more than 4,000 million years ago when the Moon was volcanically active. The molten lava flowed into huge basins that had been excavated by the impacts of large meteorites. By this stage in the Moon’s history, the frequency of meteoritic impacts had fallen. There is a noticeably lower density of craters on the lunar maria than on the brighter highland areas terrae.