Earth shifts: new star maps needed

The slow shift of Earth's axis with respect to the stars isn't apparent to most people. But astronomers and others who want to locate objects accurately on the sky must take it into account. They are well aware that their standard basic star maps and catalogs, which show the sky as it appeared in 1950, are becoming increasingly out of date.

Thus the appearance of new maps and a catalog - referenced to the year 2000 - marks a welcome change in astronomical epoch. Their star positions will need less and less correction in this century.

They also remind us not to take the sky for granted. For example, humanity will one day lose the North Star. Thuban, midway between the Little Dipper's bowl and the bend in the Big Dipper's handle, was pole star to the pyramid builders. Now Polaris serves us. Our descendents a few centuries hence will have no such guide. It will be AD 10,000 before another star (Deneb) comes into position.

The gravitational pull of Moon, Sun, and other planets, acting on Earth's equatorial bulge, is what makes Earth's axis wobble - a motion called precession. Over a roughly 25,800-year period, that axis traces a circle about 47 degrees in diameter on the sky. Astronomers use a coordinate system analogous to latitude and longitude to map star locations. The projection on the sky of Earth's axis and equator are basic references. Since these are constantly moving , star maps based on them represent the sky only at one particular time.

Wil Tirion's Sky Atlas 2000.0 - showing some 43,000 stars down to visual magnitude 8.0 plus 2,500 star clusters, nebulae, and galaxies - should serve anyone as an accurate set of star charts for perhaps 40 to 50 years. It is especially handy since it comes in three different editions - desk (loose sheets with black stars on white background), field (loose sheets with white stars on black sky), and color (a bound edition with important features, such as the Milky Way, highlighted by color coding).

The Sky Catalogue 2000.0 twins well with this atlas. Its positional data for some 45,000 stars to magnitude 8.0 should also be useful for a long time. Other data include such items as a star's distance, color, and proper (intrinsic) motion through space. Some of these data are not so timely. Radial (line-of-sight) velocities have 1953 values, for example. However, this is a minor drawback to an excellent reference work. A second catalog volume is expected for such special objects as double stars, galaxies, and radio sources.

For those who would discern the face of the sky accurately, the new atlas and catalog are a valuable kit of modern tools.

Sky Atlas 2000.0: Twenty-six Star Charts Covering Both Hemispheres, by Wil Tirion. (Cambridge University Press/Sky Publishing. Desk or Field Edition $14.95 , ordered as a pair $24.95; Color Edition $29.95).

Sky Catalogue 2000.0, Vol. 1: Stars to Magnitude 8.0, Alan Hirshfeld and Roger W. Sinnott Eds. (Cambridge University Press/Sky Publishing. 604 pp. $44.95 cloth, $29.95 paper.)

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