Astrometric Angles

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824------]OOB OOKS ------NATURE VOL. 31927 FEBRUARY 1986 a uniform B field", quoting the B field basis of brief but lucid accounts of electro inside a long solenoid as an example. They magnetic waves in empty space and con assume that aBlat is constant, claiming ductors, and there are also good discus that, according to Maxwell's equations, sions of the Poynting vector and radiation the "time-varying B field gives rise to an pressure. Overall, this book only gives a E field", and adding "we have a static E brief and simplified development of the field yet one that is not generated by physical principles of Maxwell's equations charges". As an alternative "physical" and their applications, but students may interpretation, it is a straightforward find it a useful companion to the more boundary value problem to show that, in detailed accounts of the subject. side the solenoid, in cylindrical coordin Students should do a large number of ates the vector potential is As = wzlrl2; problems to develop their expertise in then, using electromagnetism. To this end, CM. Ler B = V x A and E = (- V - aAlat) ner's Problems and Solutions in Electro for quasi-stationary conditions we obtain magnetic Theory contains 425 solved B, = WZI and Es = - (1/2)WZr(dIldt). problems covering all levels from first It is straightforward to show that year undergraduate to graduate; some V x E = -aBlat for these fields. problems are in cgs units, the others in SI Hence both B and the induced electric units. The solutions are given in full, all field E can be determined independently the steps are explained clearly, and the from the vector potential and the equation book will be a very helpful complement V x E = -aBlat is just a relation be to the standard textbooks on electromag Andromeda Galaxy, photographed through the tween these fields. By integrating the rela netism. O.9m telescope of the Lick Observatory. The pic tion V x E = -aBlat at a fixed time The central theme of Electromagnetic ture is taken from the paperback edition of The I we can equate fE.dl to the rate of change Wave Theory by J.R. Wait is the applica New Astronomy, by Nigel Henbest and Michael I of magnetic flux, without relating E and B tion of the impedance concept to wave Marten,. to be published next week by Cambridge I Umverslty Press. Pnce IS £9.95, $14.95. i back to their common source, and without phenomena. This approach has been par saying that the V x E produces the ticularly fruitful in ionospheric physics, pleasing to note that precession is treated -aBlat or vice versa. antenna theory and geophysics, areas here in terms of three rotations, rather When discussing the displacement cur where Wait himself has made outstanding than by the algebraically simpler method rent, Thomas and Meadows again adopt contributions. This is an advanced book used by Smart. A very welcome new addi the old ether viewpoint, writing "a non which gives a comprehensive account of tion to a book at undergraduate level is an zero aElat gives rise to a spatially vary the methods concerned for the benefit account of the astrometry of radioastro ing B"; yet, for example, the aElat term of the more general reader at graduate nomy; this is a field where astrometric never appears as one of the sources of A, level. 0 results of the highest precision are being and hence of B = V x A, in the expres obtained and it is likely to playa leading W. G. V. Rosser is a Reader in the Department oj sion for the retarded vector potential. Physics, University of Exeter, Exeter EX44QJ, role in future work on fundamental refer Maxwell's equations are then used as the UK. ence systems. There are a few niggles. For example, more use might have been made of vector been made in recent years, and more are methods. On the other hand, while a text Astrometric angles likely in the next decade with the advent of of this sort is under continual tension be Derek McNally the astrometric satellite HIPPARCOS tween the need for clarity and the need for and the Space Telescope. While a rigorous conciseness, the lack of an extended relativistic treatment has been given by account of the calendar, seasons, rising, Spherical Astronomy. By Robin M. Murray in Vectorial Astrometry (1983), setting and twilights is regrettable. It is Green. Cambridge University Press: 1985. there has remained a need for a book surprising, too, to see a discussion of de Pp.520. Hbk£40, $69.50; pbk £J5, $27.95. intermediate in level between Smart! pendences under plate measurement - McNally and Murray. That need is well dependences are really only justified as a AT A time when astrometry is assuming an met by the present text. quick laboratory technique giving equato increasingly prominent role in astrono The book has the advantage that it gives rial or rectangular equatorial co-ordinates mical research, it is essential to have avail a simpler introduction than Murray to the directly. able a good, up-to-date undergraduate relativistic basis for astrometric measure Altogether, though, Dr Green has pro textbook on the subject. R.M. Green was ment. It also retains a derivation of the duced a good standard work, pitched at responsible for the last (1977) revision of older, first-order formulae for day-to-day about second-year undergraduate level, Smart's classical Spherical Astronomy, use. It is important, as it always was, that which sets out the modern basis and termi but while there is a kinship between that an astrometric textbook lays the founda nology of astrometry. One hopes it will book and this new one published under the tion for a thorough development of the lead to a much better understanding, same title, the content has been changed subject, while recognizing that only a few appreciation and use of astrometry by fu entirely. of its readers will ever want to use the full ture generations of astronomers. Many of A feature of textbooks such as Smart's detail. today's astronomers remain unaware of and my own Positional Astronomy of 1974 Dr Green develops his arguments vec the contributions, actual and potential, of was the exclusion of mathematical techni tori ally , which is a concise and useful way the subject - they now have little excuse ques which would not be familiar to begin of working and provides a direct link to for not ensuring that their successors are ning students. Both Smart and myself also Murray's monograph. He explains the astrometricallyaware. 0 concentrated on providing explanation of recent changes in the form of dynamical simplified practices which would allow time used in the Astronomical Almanac Derek McNally is a Senior Lecturer in the De partment of Physics and Astronomy, University rapid determination of parameters of and also gives a short summary of the College London, and Assistant Director of the limited accuracy. However, considerable mechanics of precession as well as the University of London Observatory, Mill Hill advances in astrometric precision have more usual description of its effects. It is Park, London NW72QS, UK.