1 Introduction

The NASA Astrophysics Data System Abstract Service (hereafter ADS, except in Sect. 2) is now a central facility of bibliographic research in astronomy. In a typical month (March 1999) it is used by more than 20 000 individuals, who make $\sim$580 000 queries, retrieve $\sim$ 10 000 000 bibliographic entries, read $\sim$ 400 000 abstracts and $\sim$ 110 000 articles, consisting of $\sim$ 1 100 000 pages. The ADS is a key element in the emerging digital information resource for astronomy, which has been dubbed Urania (Boyce 1996). The ADS is tightly interconnected with the major journals of astronomy, and the major data centers.

The present paper serves as an introduction to the system, a description of its history, current status, use, capabilities, and goals. Detailed descriptions of the ADS system are in the companion papers: The design and use of the search engine is in; hereafter SEARCH. The architecture, indexing system, and mirror maintenance is in ; hereafter ARCHITECTURE. Finally the methods we use to maintain and update the data base, and to maintain communication with our collaborating data centers and journals (primarily via bibcodes, Schmitz et al. 1995) is in ; hereafter DATA.

In Sect. 2 we discuss the history of the ADS, paying particular note of the persons and events which were most important to its development. Section 3 briefly discusses the current status of the system, the data it contains, and the hardware, software, and organizational methods we use to maintain and distribute these data. Urania, and especially the ADS role in it, is discussed in Sect. 4. The current capabilities and use of the system are shown in Sect. 5; with Sect. 5.1 showing example queries, and Sect. 5.2 showing how ADS use has changed over time. In Sect. 6 we show how current use varies as a function of the age of an article and the journal it was published in; in Sect. 6.1 we develop a multi-component model which accurately describes the whole pattern of article use as a function of age; in Sect. 6.2 we compare the similarities and differences of readership information with citation histories; in Sect. 6.3 we examine several aspects of the readership pattern for the major journals. Finally, in Sect. 7, we estimate the impact of the ADS on astronomy.