Menzel Donald Howard

scientists looked back to the writers of antiquity for their definitive standards in observation and explanation, generally believing that the Greeks had already uncovered the great truths of nature. Preservation rather than progress was the aim, yet much useful observation and invention came from them.

Between the end of the Roman world in the 5th century ad and the 12th century, only fragments of ptolemy, aristotle and other Greek scientific writers were available in the West. Astronomy was learned from encyclopedic digests, such as those by Pliny the Elder (ad 23-79) and Boethius (c. 480-524). Nonetheless, the basic structures of the classical cosmos were familiar to all educated people. The Earth was not flat, but a sphere, set motionless at the centre of a series of 'crystalline' spheres that carried the Moon, Sun, planets and stars, and rotated around us at different speeds; the Moon's sphere in 28 days, Saturn's in 29 years. The stars were all the same distance away, and were gathered into Ptolemy's 48 constellations, including the 12 zodiacal signs.

One of the main reasons for the cultivation of astronomy in medieval Europe was the refinement of the calendar, and in particular, the accurate determination of the date of Easter, the most sacred of Christian festivals, which was calculated from a formula governed by the 'Paschal' or full moon following the vernal equinox. 'The Venerable' bede became Britain's first astronomer when he developed superior techniques for calculating Easter. Throughout the medieval period, the requirements of the calendar, and, to a lesser extent, of observing the daily motions of the stars to determine the times for monastic prayers, kept astronomy and the Church closely wedded. Not until 1582, by which time astronomers had sufficient data on calendrical errors, and established the Earth's rotation period to within seconds of the modern value, could they refine calendrical calculations to produce the gregorian calendar we still use today.

It was only after the mid-12th century that astronomy came to be extensively cultivated in Europe, partly as a result of contacts with islamic astronomy in Spain and Palestine. The Arabs had already translated Ptolemy, Aristotle and other writers into Arabic. These works, in turn, came to be translated into Latin, so that for the first time European scholars had access to complete versions of the leading classical texts. They also acquired Latin translations of the original researches of various Arab astronomers. Europeans, such as the Frenchman Gerbert of Aurillac (c. 940-1003), visited Muslim Spain, and it is Gerbert who is credited with introducing the astrolabe into Europe.

The large number of astronomical books being translated into Latin gave astronomy an assured place in the curricula of Bologna, Paris, Oxford, and the other emerging universities of the 12th century. In the quadrivium, students were instructed in astronomy, geometry, arithmetic and music: the four 'sciences' of mathematical proportion. Johannes de Sacrobosco (d.c.1256) wrote the best-selling De sphaera mundi ('On the Sphere of the World') around 1240, which would be a reference for students of astronomy for the next 400 years. Through the universities especially, astronomical knowledge became widespread in educated society. The poet Geoffrey Chaucer (c.1340-1400) wrote Treatise on the Astrolabe (c.1381), the first technological book in the English language, being a practical manual describing the use of the astrolabe.

One of the most adventurous branches of medieval astronomical thought was cosmology. While celestial mechanics was explained in terms of Aristotle's spheres and Ptolemy's epicycles, several theologian-astronomers had some remarkably modern-sounding ideas about time and space. Archbishop of Canterbury Thomas Bradwardine (c. 1290-1349), Bishops Jean Buridan (c.1295-c.1358), Nicole de Oresme (c.1323-82) and Nicholas of Cusa (1401-64), and others asked such questions as, could time have existed before God

MENSA (gen. mensae, abbr. men)

Small and very faint southern constellation between Hydrus and Volans. Mensa was named Mons Mensae (Table Mountain) by Lacaille in the 18th century, because the southern part of the Large Magellanic Cloud in the northern part of the constellation reminded him of cloud overlying Table Mountain, South Africa. The brightest star is a dim mag. 5.1.

created the Universe? Could there be such a thing as an infinite universe? And was motion relative? Space, time and infinity fascinated medieval scholars. No one was burnt at the stake for asking such questions, for the academic clergy saw them as lying within the legitimate bounds of university discussion. Apart from an ultimately unsuccessful attempt to ban aspects of Aristotelian science in Paris in 1277, the medieval Church had no specific policies on astronomy, and would not have until the 17th century.

From the sheer number of manuscripts and, after 1460, printed astronomical books, astrolabes, dials and artefacts in libraries and museum collections, it is clear that astronomy had a high profile in medieval European culture. It was essential to Church administration, it was a major component of the university curriculum and it even penetrated vernacular literature. It was also suspicious of astrology. Where it differed essentially from the astronomy of the scientific revolution, however, was in its conservative, as opposed to the latter's progressive, approach. Without an already established astronomical culture, the developments of renaissance astronomy could not have taken place.

Megrez The star 8 Ursae Majoris, visual mag. 3.32, distance 81 l.y., spectral type A2 V. Its name comes from the |U| Arabic maghriz, meaning 'root' (of the tail), referring to its position in Ursa Major.

MEM Abbreviation of maximum entropy method meniscus lens Thin lens usually having one convex and one concave surface and resembling the shape of the meniscus at the surface of a liquid such as water. Common examples are contact lenses. In astronomy, meniscus lenses are used to improve image quality in reflecting telescopes. Examples are the corrector plate in schmidt-cassegrain, Maksutov-Cassegrain and Maksutov-Newtonian telescopes. In all of these, a large meniscus lens with little optical power is mounted at the entrance to the optical tube; it is often referred to as the corrector plate. As the light enters the telescope its path is altered slightly by the meniscus lens so that it hits the main mirror at the optimum angle for forming sharp images right across the whole field of view. In the Schmidt-Cassegrain telescope the meniscus lens appears to be a flat plate, although it has a mild aspheric shape to one surface. In the maksutov telescope the meniscus lens is steeply curved.

Menkalinan The star p Aurigae, visual mag. 1.90, distance 82 l.y., spectral type A1 IV. It is an eclipsing binary of period 3.96 days, undergoing two minima of 0.1 mag. in each orbital cycle. Its name comes from the Arabic mankib dhi al-'inan, meaning 'shoulder of the charioteer'.

Menkar The star a Ceti, visual mag. 2.54, distance 220 l.y., spectral type M2 III. Binoculars show an apparent companion of mag. 5.6, but this is an unrelated background star. Its name comes from the Arabic mankhar, meaning 'nostril' or 'nose'.

Mensa See feature article

Menzel, Donald Howard (1901-76) American solar astronomer, astrophysicist and astronomy administrator who directed Harvard College Observatory (1952-66)

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