# Explanatory Supplement to the Astronomical Almanac.

P. Kenneth Seidelmann, editor (University Science Books, 1992). 752 pages. ISBN: 0-935702-68-7. $65.00.BEGINNING WITH the editions for 1960, the American Ephemeris and Nautical Almanac issued by the U.S. Naval Observatory and the Astronomical Ephemeris from the Royal Greenwich Observatory were identical, except for the introductory pages. With the edition for 1981, the two almanacs became completely unified, under the new title The Astronomical Almanac.

A separate volume, the Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, published in 1961, contained the complete explanations of the ephemerides and their computation. It was reprinted several times, with amendments, the last being in 1977. Fifteen years later, the long-awaited, entirely revised and rewritten edition finally appeared.

This authoritative source contains a wealth of information on spherical astronomy, time and calendars, celestial reference systems, eclipses, orbital and physical ephemerides of the Sun, Moon, planets and satellites, and much more. The updated volume incorporates the new astronomical constants adopted by the International Astronomical Union, the new standard epoch and equinox J2000.0, and the new standard ephemerides.

The Supplement's preface succinctly defines the work's purpose: "to provide users of The Astronomical Almanac with more complete explanations of the significance, sources, methods of computation, and use of the data given in the almanac than can be included annually in the almanac itself."

The volume is not intended as a basic textbook on spherical or dynamical astronomy. Nor is it for beginners. The reader is supposed to be familiar with the principal concepts of spherical astronomy and to understand the significance of the quantities discussed in the book.

Vector and matrix notations, absent from the earlier Supplement but now used in many places, might bewilder some readers. For example, the classical formula giving an object's altitude as a function of latitude, declination, and hour angle must now be deduced from a matrix expression. Chapter 11, however, provides some explanation on this computational technique, together with other formulas and hints that should prove to be a great help in astronomical calculations.

A comparison with the former Supplement shows the progress of concepts and techniques made during the last two or three decades. To mention only a few: relativistic effects, Barycentric Dynamical Time, measurement of Atomic Time, clock synchronization by very-long-baseline interferometry, planetary terms in nutation (used only for arc-millisecond accuracy!), the FK5 system, laser ranging, and rotation parameters for satellites of the planets.

Some classical subjects, such as the sections on the Hebrew, Islamic, and Indian calendars, have been expanded. On the other hand, while the former Supplement had 15 pages about transits of Mercury and Venus, the new work contains only 26 lines on this subject.

Missing from the new version are detailed, step-by-step numerical examples. Merely 10 are given, and they generally contain only a few lines and concern "easy" subjects (calculating the day of the week and determining the date of Easter in the year 2010). Examples about more complicated subjects, such as solar eclipses or the position of Titan, are sorely lacking. The previous Supplement had 20 detailed numerical examples for solar eclipses alone!

The chapters were written by a variety of authors. This may explain the inconsistency of the symbols used for some quantities throughout the book. For example: the altitude of a celestial body is identified as a on page 209 but h on page 489, while the Sun's mean anomaly is alternately g (page 42), M (page 44), and G (page 484).

All in all, this Supplement is highly recommended to all persons seriously involved in astronomical calculation. They surely will use it intensively.

By profession, Meeus is a meteorologist at Brussels Airport, though he has an international reputation for his computational wizardry.

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Author: | Meeus, Jean |
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Publication: | Sky & Telescope |

Article Type: | Book Review |

Date: | Mar 1, 1993 |

Words: | 620 |

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