a The standard abbreviations are the first three letters;where this is not the case, the abbreviation is given. b Ancient but now defunct constellation, sometimes called Argo Navis, now divided into Carina, Puppis, Pyxis, and Vela.

exhausted, the sequence began again with RR, and proceeded through the sequences, RS, RT,..., RZ, SS,..., SZ, ...ZZ, AA,..., AZ,...,..., QZ. At this point, the naming scheme switches to V335, V336,..., and so on. See §5.8 for a discussion of the various types of variable stars.

Some asterisms are "nebulae" (clouds) because of their diffuse appearance. A nebula may be a real dust or gas cloud (in space!), a star cluster, or a distant galaxy. Gas and dust clouds, usually illuminated by bright stars embedded in them, are also represented among the asterisms. Examples include the Orion Nebula (M42) and the h Carinae nebula. "Star clusters" are families of stars that were born near the same location in space, travel on parallel orbits around the

Galaxy, and generally have similar chemical compositions. There are two types of star clusters: open (also called "galactic") and globular clusters. Open clusters, typically, are located in or near the Milky Way, are irregular in shape, and are composed of hundreds of stars. Examples are the Pleiades and the Hyades clusters in Taurus and the "Beehive" cluster (also called Praesepe or M44) in Cancer. Globular clusters are more widely distributed around the sky, appear spherical in shape, and are composed of hundreds of thousands of stars. Examples are M13 in Hercules, and 47 Tucanae. Finally, there are the galaxies beyond the Milky Way that can be perceived by the naked eye and thus could be considered asterisms, such as "M31" in Andromeda and the Large and Small Magellanic Clouds. The "M" designations in some of our examples are entries in the Messier Catalogue, a collection of nonstellar objects compiled by Charles Messier (1730-1817), a noted comet discoverer of his time. The purpose of the compilation was to avoid false identifications of new comets with diffuse-looking objects in the sky, with which they could be confused in small telescopes.

Figures B.1 and B.2 in Appendix B place the modern constellations and asterisms on the sky in a coordinate framework, provided for general reference. Figure B.1 are bisected by the celestial equator into northern and southern halves. The chart is a Mercator projection4 of a variant of the equatorial system, one way of viewing the celestial sphere independently of the observer. Figure B.2 provides views of the regions around the north and south celestial poles.

Star charts, regardless of the superimposed constellation and asterism associations, are most useful when they permit identification of precise positions in the sky. Stott (1991/1995, p. 9) informs us that the first (Western) star atlas with sets of (modern) stellar coordinates was that of Paolo Galluci (from 1588). In this case, the coordinates were with respect to the path of the Sun, the ecliptic (see §2.3.3 for a discussion of this system of coordinates). Chinese atlases and charts used measurements somewhat akin to hour angles measured from the beginnings of xius (lunar mansions), and polar distance angles much earlier than this (Needham/Ronan 1981 = Needham 1981a, p. 116). Even in the Almagest, Ptolemy gives a position of a star in a kind of ecliptic coordinate; referring to the beginning of the first point of a zodiacal sign, he also gives an ecliptic latitude. Moreover, Ptolemy describes a device (see §3.3) with which some coordinates can be measured, and the existence of some kind of spherical coordinates is implied by relatively accurate placements of stars on the external surface of a sphere, such as the Farnese globe (§2.1.1). Yet when Galileo noticed a faint object while studying the satellites of Jupiter, he was unable to track and follow the object because his telescope mounting lacked coordinates to record and rediscover it once Jupiter's relatively large motion had moved away from the field. The faint object was not knowingly discovered until after calculations by John Couch Adams (1819-1892) and Urbain Jean Joseph Leverrier (1811-1877) in the 19th century. The object was the planet Neptune. If Galileo had obtained access to some of the classic instruments of antiquity, he could have replaced a sighting tube with his telescope and been able to record positions relative to the nearby stars.

In the following sections, we will show how coordinate systems enable us to find objects on the celestial sphere, in catalogues, and in the sky.

4 This is a projection of spherical coordinates onto a cylinder in such a way that lines of latitude and longitude remain perpendicular. It has the property that longitude lines farther from the equator enclose larger areas. The projection is credited to the Flemish cartographer, Gerhardus Mercator (1512-1594).

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