Preface

This, the 20th edition of Norton's Star Atlas, has been completely redesigned and reset to give it a fresh, modern appearance for a new century. The star charts have been relabelled to make them more legible and attractive. Among the updates and improvements to the reference handbook and its data tables, we have introduced new sections on computer-controlled telescopes and CCD imaging, both of which have changed the face of amateur astronomy in the five years since the 19th edition appeared. We have also enlarged the section on observing deep-sky objects, which are popular targets for amateurs. Other major improvements include completely new maps of the Moon and Mars. Throughout, the needs of the active observer have been kept uppermost in mind.

In its previous editions, Norton's earned the reputation of being the most famous and most widely used star atlas in the world, and its reference handbook has become an indispensable companion for observers of all standards. We believe that this latest edition will carry the tradition of Norton's Star Atlas well into the twenty-first century.

History

Norton's Star Atlas first appeared in 1910. It achieved immediate success, due largely to its uniquely convenient arrangement of charts in slices, or 'gores', each covering approximately one-fifth of the sky, and its inclusion of stars down to sixth magnitude, the naked-eye limit. The Atlas was intended for owners of small telescopes, particularly those who wanted to find the objects of interest that were listed in two famous observing guides by nineteenth-century amateur astronomers: Celestial Objects for CommonTelescopes by the Rev. T. W. Webb, and Cycle of Celestial Objects by W. H. Smyth. Over the years Norton's established an international reputation, becoming a standard reference work for amateur and professional astronomers alike.

The author of the Atlas, Arthur Philip Norton (1876–1955), was an amateur astronomer; his full-time occupation was as a schoolmaster. Had it not been for his Atlas he would have remained almost unknown in the world of astronomy.

Norton was born in Cardiff, Wales, the son of a clergyman. His interest in astronomy started when, as a schoolboy, he was given a telescope that had belonged to his great-grandfather. After receiving his BA degree from Trinity College, Dublin, he taught at various schools in England. For 22 years Norton taught geography and mathematics at the Judd School, Tonbridge, Kent, retiring in 1936. Norton published no books other than the Atlas on which his fame rests (and a simplified version, the Popular Star Atlas, which appeared in 1949), but during his lifetime it went through numerous editions, and he updated the star charts twice.

Back in 1910, when Norton's Star Atlas first appeared, there were no officially recognized boundaries to the constellations — a deficiency that the International Astro-nomical Union rectified in 1930. For the 5th edition of his Atlas, published in 1933, Arthur Norton redrew the charts to incorporate the newly defined IAU constellation boundaries, and he set the magnitude limit of the stars at 6.2, based on the Harvard Revised Photometry catalogue (the magnitude limit of the 1st edition had not been precisely defined). Norton by now had to cope with the fact that the sight in his left eye was badly blurred as a result of a blood clot behind the retina, but it did not affect the quality of his charts.

Celestial cartographers are faced with a problem that does not afflict their terrestrial counterparts: the coordinates of all stars are gradually changing with time, because of an effect called precession. This means that all star charts are bound to become progressively out of date. The epoch (i.e. the reference date for the star positions) of the original Norton's was 1920. For the 9th edition, published in 1943, Norton redrew his charts again, this time for the standard epoch of 1950.0, and further extended the magnitude limit of the stars to 6.35. That version of the charts remained in print long after his death.

Inevitably, with the passage of time, another change of epoch became necessary. For the 18th edition of Norton's, published in 1989, the charts were redrawn to the standard epoch of 2000.0, using technology that Norton could hardly have dreamed of. And for the first time Norton's contained nothing by Arthur Norton himself, although his influence lived on in more than just the title.

The Charts

An early decision in preparing the star charts for the new epoch was to retain their existing arrangement, which had stood the test of time. A subtle difference was that both the polar charts and the equatorial gores now used the same projection, known as Lambert's azimuthal equi-distant projection, which allows large areas of sky to be represented with little distortion. Norton never stated the projections that he used; the gores were apparently plotted on a modified globular projection of his own devising.

In the new charts, the plane of the projection surface touches the celestial sphere at the poles for the polar charts, and at the celestial equator for the gores. To minimize distortion, each gore has been projected from its central meridian at the equator. All the projections were generated by computer for maximum accuracy.

The projection software was written at the cartographic company of John Bartholomew & Son in Edinburgh. The outlines of the Milky Way, the Magellanic Clouds, the galactic equator and the ecliptic were added by our cartographic consultant, Mike Swan. In addition to being a former professional cartographer with the Ordnance Survey, he is a deep-sky observer with the Webb Society. His expertise in both astronomy and cartography was a vital ingredient in the project.

With all the data converted into machine-readable form, the charts were generated on film at Bartholomews. These films then went to Mike Swan for hand-labelling and final checking. Films combining the star charts and labelling were output at Bartholomews, from which printing plates were produced. For the 20th edition of Norton's, the charts were redrawn and relabelled by the book's designer, Charles Nix and his associate Gary Robbins.

Data

For information on positions and magnitudes of stars we adopted the Yale Bright Star Catalogue (BS) and its Supplement. The BS contains the same stars as the Harvard Revised Photometry catalogue that Arthur Norton used for his charts, but with considerably improved magnitude measurements. We chose a magnitude limit of 6.49 (i.e. encompassing all stars of 6th magnitude and brighter), against the 6.35 used for the 1950.0 maps.

Dorrit Hoffleit of Yale University Observatory, senior author of the BS, and Wayne Warren of the National Space Science Data Center in Greenbelt, Maryland, supplied magnetic tapes of the 5th edition of the Bright Star Catalogue. They also supplied tapes of the 1983 Supplement to the BS, from which we extracted stars brighter than mag. 6.50 that were not included in the main BS, having been missed by the original Harvard Revised Photometry.

Data required for the maps were extracted from the BS tapes by the Royal Observatory, Edinburgh, and were supplied to Bartholomew with the constellation boundaries added. Data for the galactic charts were also supplied by the ROE.

Even in a computerized operation such as this, considerable manual intervention was still necessary. Since the BS does not include deep-sky objects, lists of star clusters, nebulae and galaxies were drawn up by Mike Swan for addition to the stellar database. He also spent many hours identifying variable stars, and stars that are both variable and double, for depiction by special symbols on the charts. Ordinary double stars were identified directly from the BS tapes.

The charts for the current edition of Norton's Star Atlas show over 8800 stars. The star symbols are graduated in whole-magnitude steps for ease of identification. The few stars of magnitude 0 and 21 are given the same size symbol as stars of magnitude 11. The percentage of stars in each magnitude range is as follows:

Double and Multiple Stars

Stars that are listed in the BS as double or multiple are identified on the charts by a special symbol (a line bisecting the star dot) if their separation is at least 0.1arcsec. The exceptions to this system are stars whose components are wide enough to be plotted separately; these do not carry the double-star symbol unless they have other, closer companions. Spectroscopic binaries and other exceptionally close doubles (e.g. those found by occultation studies or speckle interferometry) are not denoted by the double-star symbol on the maps.

In the list of interesting objects that accompanies each chart in Norton's Star Atlas, the double stars are restricted to those with a combined magnitude brighter than 6.5. For the 20th edition, position, separation and magnitude data for these stars were checked against the latest edition of the Washington Double Star Catalogue. All the double stars in these lists are labelled on the charts.

Variable Stars

Those variable stars with a range of at least 0.1 mag. and a maximum magnitude brighter than 6.5, as listed in the BS magnitude of the star. Those variables, including novae, whose minimum brightness takes them below our chart limit of mag. 6.49 are denoted by an open circle only. More than 500 variable stars are identified on the charts, including over 40 that are not in the BS or its Supplement but for which we found evidence of maxima above mag. 6.5 (certain classes of variable, particularly those of long period, have ranges of variation that are not precisely bounded). A combined symbol is used to identify nearly 150 stars that are both variable and double.

The lists of variable stars that accompany the charts are believed to include all variables that have an amplitude of at least 0.4 mag. and a maximum brighter than approximately mag. 6.5. All the variables contained in these lists are labelled on the charts.

Deep-sky Objects

Separate symbols are used to distinguish each class of deep-sky object: open star clusters, globular star clusters, diffuse nebulae, planetary nebulae and galaxies. Additionally, the charts depict the true shape and extent of nebulae and galaxies that are larger than about 0()().5 in apparent diameter. In all, over 600 deep-sky objects are shown. The most interesting of them are briefly described in the notes accompanying each chart.

Reference Handbook

Over the years, the Reference Handbook section has become as valuable a part of Norton's as the Atlas itself. In the 1st edition the text amounted to only 18 pages, mostly written by James Gall Inglis (1865–1939). By the 5th edition in 1933 the text had grown to 51 pages, and by the 17th edition in 1978 it covered 116 pages. For the 18th edition the text was rewritten almost entirely, while attempting to retain the essential character of Norton's. The emphasis remained on reference information and practical observing advice that is often difficult to obtain elsewhere, a philosophy that has been maintained in subsequent editions.

As in previous editions we have decided against giving a bibliography, but mention must be made of Burnham's Celestial Handbook (in three volumes) by Robert Burnham Jr. (Dover Publications). Although now dated, this is an invaluable companion to Norton's and remains a classic guide for observers. A more compact handbook, with individual constellation charts and notes on objects of interest, is Guide to Stars and Planets by Ian Ridpath and Wil Tirion (HarperCollins U.K., Princeton University Press U.S.).