View source for Astrophysics Data System

{{short description|Digital library portal operated by the Smithsonian}}
{{Use dmy dates|date=October 2023}}
{{Multiple issues|
{{Tone|date=March 2023}}
{{update|reason=most content is from 2005, ignoring the updates and improvements since then|date=May 2023}}
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{{infobox bibliographic database
| title       = SAO/NASA Astrophysics Data System
| image       = Ads logo full light background.svg
| caption     = 
| producer    = [[Smithsonian Astrophysical Observatory]] for the [[National Aeronautics and Space Administration]]
| country     = United States
| history     = 1992 to present
| languages   = 
| providers   = 
| cost        = Free
| disciplines = [[Astronomy]] and [[physics]]
| depth       = Index & abstract & full-text
| formats     = 
| temporal    = 
| geospatial  = Worldwide
| number      = 
| updates     = 
| p_title     = 
| p_dates     = 
| ISSN        = 
| web         = {{nowrap|{{URL|ui.adsabs.harvard.edu}}}}
| titles      = 
}}

The '''SAO/NASA Astrophysics Data System''' ('''ADS''') is a [[digital library]] portal for researchers on [[astronomy]] and [[physics]], operated for [[NASA]] by the [[Smithsonian Astrophysical Observatory]]. ADS maintains three bibliographic collections containing over 15 million records, including all [[arXiv]] e-prints.<ref>{{Cite web |title=About ADS |url=https://ui.adsabs.harvard.edu/about/ |access-date=2024-07-20 |website=ui.adsabs.harvard.edu}}</ref> Abstracts and full-text of major astronomy and physics publications are indexed and searchable through the portal.

==Historical context==
[[Johann Friedrich Weidler]] published the first comprehensive history of astronomy in 1741 and the first astronomical bibliography in 1755. This was an effort to archive and classify earlier astronomical knowledge and works.

This effort was continued by [[Jérôme Lalande|Jérôme de La Lande]] who published his ''Bibliographie astronomique'' in 1803, a work that covered the period from 480 BCE to the year of publication.

The ''Bibliographie générale de l’astronomie, Volume I and Volume II'', published by J.C. Houzeau and A. Lancaster, followed in 1882 until 1889.<ref>{{cite book |last1=Houzeau |first1=J. C. |title=Bibliographie générale de l'astronomie |date=1887 |publisher=F. Hayez, Imprimeur de L'Académie Royale de Belgique |url=https://books.google.com/books?id=xJhA9noqjT0C&q=Bibliographie+g%C3%A9n%C3%A9rale+de+l%27astronomie+volume+I |language=fr}}</ref><ref>{{cite book |last1=Houzeau |first1=Jean-Charles |title=Bibliographie générale de l'astronomie ou catalogue méthodique des ouvrages, des mémoires et des observations astronomiques publiés depuis l'origine de l'imprimerie jusqu'en 1880: Mémoires et notices insérés dans les Collections académiques et les Revues |date=1882 |url=https://books.google.com/books?id=GzxeAAAAcAAJ |language=fr}}</ref>

As the number of astronomers and astronomical publications grew, bibliographical efforts became institutional tasks, first at the [[Royal Observatory of Belgium|Observatoire Royal de Belgique]], where the ''Bibliography of Astronomy'' was published from 1881 to 1898, and then at the [[Astronomical Calculation Institute (Heidelberg University)|Astronomischer Rechen-Institut]] in Heidelberg, where the yearly ''Astronomischer Jahresbericht'' was published from 1899 to 1968. After 1968, this was replaced by the yearly [[Astronomical Calculation Institute (Heidelberg University)|''Astronomy and Astrophysics Abstracts'']] book series, which continued until the end of the 20th century.

== History ==
The first suggestion of a digital database of journal paper abstracts was made at a conference on ''Astronomy from Large Data-Bases'' held in [[Garching bei München]] in 1987.<ref>{{cite journal |last1=Squibb |first1=G.F. |last2=Cheung |first2=C.Y. |year=1988 |title=NASA astrophysics data system (ADS) study |journal=European Southern Observatory Conference and Workshop Proceedings |volume=28 |page=489 |bibcode=1988ESOC...28..489S}}</ref><ref>{{cite conference |last1=Adorf |first1=H.-M. |last2=Busch |first2=E.K. |year=1988 |title=Intelligent access to a bibliographical full text data base |volume=28 |page=143 |bibcode=1988ESOC...28..143A |journal=European Southern Observatory Conference and Workshop Proceedings}}</ref><ref>{{cite conference |last=Rey-Watson |first=J.M. |year=1988 |title=Access to astronomical literature through commercial databases |volume=28 |page=453 |bibcode=1988ESOC...28..453R |journal=European Southern Observatory Conference and Workshop Proceedings}}</ref><ref>{{cite conference |last1=Rhodes |first1=C. |last2=Kurtz |first2=M.J. |last3=Rey-Watson |first3=J.M. |year=1988 |title=A library collection of software documentation specific to astronomical data reduction |volume=28 |page=459 |bibcode=1988ESOC...28..459R |journal=European Southern Observatory Conference and Workshop Proceedings}}</ref> 

An initial version of ADS, with a database consisting of 40 papers, was created as a [[proof of concept]] in 1988. The ADS Abstract Service became available for general use via proprietary network software in April 1993, and it was connected to [[SIMBAD]] a few months later. In early 1994 the ADS web-based service was launched, which effectively quadrupled the number of active users in the five weeks following its introduction.<ref name="overview">{{cite journal |last=Kurtz |first=M.J. |author2=Eichhorn G. |author3=Accomazzi A. |author4=Grant C.S. |author5=Murray S.S. |author6=Watson J.M. |year=2000 |title=The NASA Astrophysics Data System: Overview |journal=Astronomy and Astrophysics Supplement Series |volume=143 |issue=1 |pages=41–59 |arxiv=astro-ph/0002104 |bibcode=2000A&AS..143...41K |doi=10.1051/aas:2000170 |s2cid=17583122}}</ref> 

In 2011 the ADS launched ADS Labs Streamlined Search which introduced [https://www.researchgate.net/publication/228582891_Retrieval_from_facet_spaces facets] for query refinement and selection. In 2013, ADS Labs 2.0 started featuring a new search engine, full-text search functionality, scalable facets, and an API was introduced. In 2015, the new ADS, code-named Bumblebee, was released as ADS-beta. The ADS-beta system features a micro-services API and client-side dynamic page loading served on a cloud platform. In May 2018 the beta label was dropped and Bumblebee became the default ADS interface—with some legacy features (ADS Classic) remaining available.<ref>{{Cite conference |last1=Accomazzi |first1=Alberto |last2=Kurtz |first2=Michael J. |last3=Henneken |first3=Edwin |last4=Grant |first4=Carolyn S. |last5=Thompson |first5=Donna M. |last6=Chyla |first6=Roman |last7=McDonald |first7=Steven |last8=Shaulis |first8=Taylor J. |last9=Blanco-Cuaresma |first9=Sergi |last10=Shapurian |first10=Golnaz |last11=Hostetler |first11=Timothy W. |last12=Templeton |first12=Matthew R. |last13=Lockhart |first13=Kelly E. |date=January 2018 |title=ADS Bumblebee comes of age |conference=231st Meeting of the American Astronomical Society |at=362.17 |bibcode=2018AAS...23136217A}}</ref> Development continues to the present day, with an extensible [[API]] available: enabling users to build their own utilities on top of the ADS bibliographic record.

The ADS service is distributed worldwide with twelve [[Web mirror|mirror sites]] in twelve countries and with the database synchronized by weekly updates using [[rsync]], a mirroring utility which allows updates to only the portions of the database which have changed. All updates are triggered centrally, but they initiate scripts at the mirror sites which "pull" updated data from the main ADS servers.<ref name="architecture">{{cite journal |last1=Accomazzi |first1=A. |last2=Eichhorn |first2=G. |last3=Kurtz |first3=M.J. |last4=Grant |first4=C.S. |last5=Murray |first5=S.S. |year=2000 |title=The NASA Astrophysics Data System: Architecture |journal=Astronomy and Astrophysics Supplement Series |volume=143 |issue=1 |pages=85–109 |doi=10.1051/aas:2000172 |bibcode=2000A&AS..143...85A |arxiv=astro-ph/0002105 |s2cid=7182316}}</ref>

==Data in the system==
At first, the journal articles available via ADS were exclusively [[Image scanner|scan]]ned [[bitmap]]s created from the paper journals and the abstracts created using [[optical character recognition]] software. Some of these scanned articles up to around 1995 are available for free by agreement with the journal publishers,<ref name="data">{{cite web |date=23 June 2005 |title=NASA ADS Abstract Service Mirroring Information |url=http://ads.harvard.edu/mirror/ |access-date=2 November 2008 |publisher=Harvard-Smithsonian Center for Astrophysics}}</ref> with some dating from as far back as the early 19th century. Eventually, because of a wider spread of online editions of journal publications, abstracts would start to instead be loaded into ADS directly.

Papers are indexed within the database by their bibliographic record which contains the details of the journal they were published in, and various associated [[metadata]], such as author lists, [[Image resolution|reference]]s and [[citation]]s. Originally this data was stored in [[ASCII]] format but eventually the limitations of this encouraged the database maintainers to migrate all records to an [[XML]] (Extensible Markup Language) format in 2000. Bibliographic records are now stored as an XML element with sub-elements for the various metadata.<ref name="architecture" />

Scanned articles are stored in [[TIFF]] format at both medium and high [[Image resolution|resolution]]. The TIFF files are converted on demand into GIF files, for on-screen viewing, and [[PDF]] or [[PostScript]] files for printing. The generated files are then [[cache (computing)|cached]] to eliminate needlessly frequent regenerations for popular articles. As of 2000, ADS contained 250 [[gigabyte|GB]] of scans, which consisted of 1,128,955 article pages comprising 138,789 articles. By 2005 this had grown to 650 GB and was expected to grow further to about 900 GB by 2007.<ref name="data" /> No further information has been published (2005).

The database initially contained only astronomical references, but has now grown to incorporate three databases, covering [[astronomy]]
references (including planetary sciences and solar physics), [[physics]] references (including instrumentation and geosciences), as well as preprints of scientific papers from [[arXiv]]. The astronomy database is by far the most advanced and its use accounts for about 85% of the total ADS usage. Articles are assigned to the different databases according to the subject rather than the journal they are published in, so that articles from any one journal might appear in all three subject databases. The separation of the databases allows searching in each discipline to be tailored, so that words can automatically be given different [[weight function]]s in different database searches, depending on how common they are in the relevant field.<ref name="architecture"/>

Data in the preprint archive is updated daily from arXiv which is the dominant repository of physics and astronomy preprints. The advent of preprint servers has, like ADS, had a significant impact on the rate of astronomical research, as papers are often made available from preprint servers weeks or months before they are published in the journals. The incorporation of preprints from arXiv into ADS means that the search engine can return the most current research available, with the caveat that preprints may not have been peer-reviewed or [[proofread]] to the required standard for publication in the main journals. The database of ADS links preprints with subsequently published articles wherever possible, so that citation and reference searches will return links to the journal article where the preprint was cited.<ref>{{cite conference |url=http://meetings.aps.org/Meeting/MAR07/Event/61837 |title=myADS-arXiv: A fully customized, open access virtual journal |conference=March Meeting 2007, American Physical Society |volume=52 |issue=1 |at=U20.9 |access-date=30 October 2008}}</ref>

==Software and hardware==
The software runs on a system that was written specifically for the ADS, allowing for extensive customization for astronomical needs that would not have been possible with general purpose [[database]] software. The scripts are designed to be as [[platform independent]] as possible, given the need to facilitate mirroring on different systems around the world, although the growing use of [[Linux]] as the [[operating system]] of choice within astronomy has led to increasing optimization of the scripts for installation on that platform.<ref name="architecture"/>

The main ADS server is located at the [[Harvard-Smithsonian Center for Astrophysics|Center for Astrophysics {{!}} Harvard & Smithsonian]] in [[Cambridge, Massachusetts]], and is a dual 64-bit X86 [[Intel]] server with two quad-core 3.0 [[GHz]] [[Central processing unit|CPU]]s and 32 GB of [[random access memory|RAM]], running the [[CentOS]] 5.4 [[Linux]] distribution.<ref name="data" />  As of 2022, there are mirrors located in China, Chile, France, Germany, Japan, Russia, the United Kingdom, and Ukraine.<ref>{{cite web |title=SAO/NASA ADS at SAO: Mirror Sites |url=http://doc.adsabs.harvard.edu/mirrors.html |archiveurl=https://web.archive.org/web/20220128092136/https://doc.adsabs.harvard.edu/mirrors.html |archive-date=28 January 2022 |access-date=26 July 2024 |publisher=Harvard-Smithsonian Center for Astrophysics}}</ref>

==Indexing==
ADS currently (2005) receives abstracts or tables of contents from almost two hundred journal sources. The service may receive data referring to the same article from multiple sources, and creates one bibliographic reference based on the most accurate data from each source. The common use of [[TeX]] and [[LaTeX]] by almost all scientific journals greatly facilitates the incorporation of bibliographic data into the system in a standardized format, and importing [[HTML]]-coded web-based articles is also simple. ADS utilizes [[Python (programming language)|Python]] and [[Perl]] scripts for importing, processing and standardizing bibliographic data.<ref name="architecture" />

The apparently mundane task of converting author names into a standard ''[[Surname]], Initial'' format is actually one of the more difficult to automate, due to the wide variety of naming conventions around the world and the possibility that a given name such as Davis could be a [[first name]], [[middle name]] or surname. The accurate conversion of names requires a detailed knowledge of the names of authors active in astronomy, and ADS maintains an extensive database of author names, which is also used in searching the database (see below).

For electronic articles, a list of the references given at the end of the article is easily extracted. For scanned articles, reference extraction relies on OCR. The reference database can then be "inverted" to list the citations for each paper in the database. Citation lists have been used in the past to identify popular articles missing from the database; mostly these were from before 1975 and have now been added to the system.

==Coverage==
The database now contains over fifteen million articles. In the cases of the major journals of astronomy (''[[Astrophysical Journal]]'', ''[[Astronomical Journal]]'', ''[[Astronomy and Astrophysics]]'', ''[[Publications of the Astronomical Society of the Pacific]]'' and the ''[[Monthly Notices of the Royal Astronomical Society]]''), coverage is complete, with all issues indexed from number 1 to the present. These journals account for about two-thirds of the papers in the database, with the rest consisting of papers published in over 100 other journals from around the world, as well as in conference proceedings.<ref name="data" />

While the database contains the complete contents of all the major journals and many minor ones as well, its coverage of references and citations is much less complete. References in and citations of articles in the major journals are fairly complete, but references such as "private communication", "in press" or "in preparation" cannot be matched, and author errors in reference listings also introduce potential errors. Astronomical papers may cite and be cited by articles in journals which fall outside the scope of ADS, such as [[chemistry]], [[mathematics]] or [[biology]] journals.<ref>{{cite web |url=http://adsabs.harvard.edu/abs_doc/journals1.html |title=ADS Bibliographic Codes: Journal Abbreviations |publisher=Harvard-Smithsonian Center for Astrophysics |access-date=30 October 2008 |archiveurl=https://web.archive.org/web/20080430032916/http://adsabs.harvard.edu/abs_doc/journals1.html |archive-date=30 April 2008}}</ref>

==Search engine==
[[Image:A complex ADS search.png|thumb|right|An example of a complex search combining object, title and abstract queries with a date filter]]
Since its inception, the ADS has developed a highly complex [[search engine]] to query the abstract and [[Object database|object databases]]. The search engine is tailor-made for searching astronomical abstracts, and the engine and its [[user interface]] assume that the user is well-versed in astronomy and able to interpret search results which are designed to return more than just the most relevant papers. The database can be queried for author names, [[astronomical object]] names, title words, and words in the abstract text, and results can be filtered according to a number of criteria. It works by first gathering synonyms and simplifying search terms as described above, and then generating an "inverted file", which is a list of all the documents matching each search term. The user-selected logic and filters are then applied to this inverted list to generate the final search results.<ref name="search">{{cite journal |last1=Eichhorn |first1=G. |last2=Kurtz |first2=M.J. |last3=Accomazzi |first3=A. |last4=Grant |first4=C.S. |last5=Murray |first5=S.S. |year=2000 |title=The NASA Astrophysics Data System: The search engine and its user interface |journal=Astronomy and Astrophysics Supplement Series |volume=143 |issue=1 |pages=61–83 |doi=10.1051/aas:2000171 |bibcode=2000A&AS..143...61E |arxiv=astro-ph/0002102 |s2cid=2787647}}</ref>

===Author name queries===
The system indexes author names by surname and initials, and accounts for the possible variations in spelling of names using a list of variations. This is common in the case of names including accents such as [[Umlaut (diacritic)|umlaut]]s and transliterations from [[Arabic alphabet|Arabic]] or [[Cyrillic script]]. An example of an entry in the author synonym list is:

:''AFANASJEV, V''
:''AFANAS’EV, V''
:''AFANAS’IEV, V''
:''AFANASEV, V''
:''AFANASYEV, V''
:''AFANS’IEV, V''
:''AFANSEV, V''

===Object name searches===
The capability to search for papers on specific astronomical objects is one of ADS's most powerful tools. The system uses data from the [[SIMBAD]], the [[NASA/IPAC Extragalactic Database]], the [[International Astronomical Union]] Circulars and the [[Lunar and Planetary Institute]] to identify papers referring to a given object, and can also search by object position, listing papers which concern objects within a 10 [[arcminute]] radius of a given [[Right Ascension]] and [[Declination]]. These databases combine the many catalogue designations an object might have, so that a search for the [[Pleiades]] will also find papers which list the famous [[open cluster]] in [[Taurus (constellation)|Taurus]] under any of its other catalog designations or popular names, such as M45, the Seven Sisters or Melotte 22.<ref>{{cite web |url=http://doc.adsabs.harvard.edu/abs_doc/help_pages/search.html#SIMBAD/NED/LPI/IAUC_Object_Names/Position |title=SAO/NASA ADS HELP: Abstract Query Form |at=2.2.2.2 - SIMBAD/NED/LPI/IAUC Object Names/Position |publisher=Harvard-Smithsonian Center for Astrophysics |access-date=30 October 2008 |archiveurl=https://web.archive.org/web/20080509091500/http://doc.adsabs.harvard.edu/abs_doc/help_pages/search.html#SIMBAD/NED/LPI/IAUC_Object_Names/Position |archive-date=9 May 2008}}</ref>

===Title and abstract searches===
The search engine first filters search terms in several ways. An M followed by a space or [[hyphen]] has the space or hyphen removed, so that searching for [[Messier catalogue]] objects is simplified and a user input of M45, M 45 or M-45 all result in the same query being executed; similarly, [[New General Catalogue|NGC]] designations and common search terms such as [[Comet Shoemaker-Levy 9|Shoemaker Levy]] and [[T Tauri star|T Tauri]] are stripped of spaces. Unimportant words such as AT, OR and TO are stripped out, although in some cases [[case sensitivity]] is maintained, so that while '''a'''nd is ignored, '''A'''nd is converted to "[[Andromeda (constellation)|Andromeda]]", and '''H'''er is converted to "[[Hercules (constellation)|Hercules]]", but '''h'''er is ignored.<ref>{{cite web |url=http://doc.adsabs.harvard.edu/abs_doc/help_pages/search.html#Stop_Words |title=SAO/NASA ADS HELP: Abstract Query Form |at=2.2.1.2 - Stop Words |publisher=Harvard-Smithsonian Center for Astrophysics |access-date=30 October 2008 |archiveurl=https://web.archive.org/web/20080509091500/http://doc.adsabs.harvard.edu/abs_doc/help_pages/search.html#Stop_Words |archive-date=9 May 2008}}</ref>

===Synonym replacement===
Once search terms have been preprocessed, the database is queried with the revised search term, as well as synonyms for it. As well as simple [[synonym]] replacement such as searching for both [[plural]] and [[Grammatical number|singular]] forms, ADS also searches for a large number of specifically astronomical synonyms. For example, [[spectrograph]] and [[spectroscope]] have basically the same meaning, and in an astronomical context [[metallicity]] and [[Abundance of the chemical elements|abundance]] are also synonymous. ADS's synonym list was created manually, by grouping the list of words in the database according to similar meanings.<ref name="architecture"/>

As well as [[English language]] synonyms, ADS also searches for English translations of foreign search terms and vice versa, so that a search for the [[French language|French]] word ''soleil'' retrieves references to [[Sun]], and papers in languages other than English can be returned by English search terms.

Synonym replacement can be disabled if required, so that a rare term which is a synonym of a much more common term (such as '[[dateline]]' rather than '[[calendar date|date]]') can be searched for specifically.

===Selection logic===
The search engine allows selection [[logic]] both within fields and between fields. Search terms in each field can be combined with OR, AND, simple logic or [[Boolean logic]], and the user can specify which fields must be matched in the search results. This allows complex searches to be built; for example, the user could search for papers concerning [[NGC 6543]] OR [[NGC 7009]], with the paper titles containing (radius OR velocity) AND NOT (abundance OR temperature).

===Result filtering===
Search results can be filtered according to a number of criteria, including specifying a range of years such as "1945 to 1975", "2000 to the present day" or "before 1900", and what type of journal the article appears in [–] non-peer-reviewed articles such as [[academic conference|conference]] proceedings. These can be excluded or specifically searched for, or specific journals can be included in or excluded from the search.

==Search results==
[[Image:ADS search results.png|thumb|right|Search results page from ADS – A, F, G, C, R etc. are links to associated data for each abstract such as full-text article, citations, also-read papers and so on.]]
Although it was conceived as a means of accessing abstracts and papers, ADS provides a substantial amount of ancillary information along with search results. For each abstract returned, links are provided to other papers in the database which are referenced, and which cite the paper, and a link is provided to a preprint, where one exists. The system also generates a link to "also-read" articles – that is, those which have been most commonly accessed by those reading the article. In this way, an ADS user can determine which papers are of most interest to astronomers who are interested in the subject of a given paper.<ref name="search" />

Also returned are links to the [[SIMBAD]] and/or [[NASA Extragalactic Database]] object name databases, via which a user can quickly find out basic observational data about the objects analyzed in a paper, and find further papers on those objects.

==Impact on astronomy==
ADS is almost universally used as a research tool among astronomers, and there are several studies that have estimated quantitatively how much more efficient ADS has made astronomy; one estimated that ADS increased the efficiency of astronomical research by 333 full-time equivalent research years per year,<ref name="overview" /> and another found that in 2002 its effect was equivalent to 736 full-time researchers, or all the astronomical research done in France.<ref name="impact">{{cite journal |last=Kurtz |first=M.J. |author2=Eichhorn G. |author3=Accomazzi A. |author4=Grant C.S. |author5=Demleitner M. |author6=Murray S.S. |year=2005 |title=Worldwide Use and Impact of the NASA Astrophysics Data System Digital Library |journal=Journal of the American Society for Information Science and Technology |volume=56 |issue=1 |pages=36–45 |arxiv=0909.4786 |bibcode=2005JASIS..56...36K |doi=10.1002/asi.20095 |s2cid=15181632}} ([http://cfa-www.harvard.edu/~kurtz/jasis-abstract.html Preprint])</ref> ADS has allowed literature searches that would previously have taken days or weeks to carry out to be completed in seconds, and it is estimated that ADS has increased the readership and use of the astronomical literature by a factor of about three since its inception.<ref name="impact" />

In monetary terms, this increase in efficiency represents a considerable amount. There are about 12,000 active astronomical researchers worldwide, so ADS is the equivalent of about 5% of the working population of astronomers. The global astronomical research budget is estimated at between 4,000 and US$5,000 million,<ref name="woltjer">{{cite conference |last=Woltjer |first=L. |year=1998 |title=Economic Consequences of the Deterioration of the Astronomical Environment |conference=23rd General Assembly of the International Astronomical Union |book-title=Preserving The Astronomical Windows. Proceedings of Joint Discussion number 5 of the 23rd General Assembly of the International Astronomical Union held in Kyoto, Japan 22–23 August 1997 |volume=139 |pages=243 |bibcode=1998ASPC..139..243W}}</ref> so the value of ADS to astronomy would be about 200–250 million USD annually. Its operating budget is a small fraction of this amount.<ref name="impact" />

The great importance of ADS to astronomers has been recognized by the [[United Nations]], the [[United Nations General Assembly|General Assembly]] of which has commended ADS on its work and success, particularly noting its importance to astronomers in the developing world, in reports of the [[United Nations Committee on the Peaceful Uses of Outer Space]]. A 2002 report by a visiting committee to the Center for Astrophysics, meanwhile, said that the service had "revolutionized the use of the astronomical literature", and was "probably the most valuable single contribution to astronomy research that the CfA has made in its lifetime".<ref name="awards">{{cite web |url=https://ui.adsabs.harvard.edu/about/awards.html |title=ADS Awards and Recognition |publisher=Harvard-Smithsonian Center for Astrophysics |access-date=25 March 2022}}</ref>

==Sociological studies using ADS==
Because it is used almost universally by astronomers, ADS can reveal much about how astronomical research is distributed around the world. Most users access the system from institutes of higher education, whose [[IP address]] can easily be used to determine the user's geographical location. Studies reveal that the highest per-capita users of ADS are France and Netherlands-based astronomers, and while more developed countries (measured by [[GDP per capita]]) use the system more than less developed countries; the relationship between GDP per capita and ADS use is not linear. The range of ADS usage per capita far exceeds the range of GDP per capita, and basic research carried out in a country, as measured by ADS usage, has been found to be proportional to the square of the country's GDP divided by its population.<ref name="impact" /> Statistics also imply that there are about three times as many astronomers in countries of European culture as in countries of [[Culture of Asia|Asian culture]]s, perhaps suggesting cultural differences in the importance attached to astronomical research.<ref name="impact" /> The amount of basic research carried out in a country is found to be proportional to the number of astronomers in that country multiplied by its GDP per capita, with considerable scatter.

ADS has also been used to show that the fraction of single-author astronomy papers has decreased substantially since 1975 and that astronomical papers with more than 50 authors have become more common since 1990.<ref name="trends">{{cite journal |author-link1=Eric Schulman |last1=Schulman |first1=E. |last2=French |first2=J.C. |last3=Powell |first3=A.L. |last4=Eichhorn |first4=G. |last5=Kurtz |first5=M.J. |last6=Murray |first6=S.S. |year=1997 |title=Trends in Astronomical Publication Between 1975 and 1996 |journal=Publications of the Astronomical Society of the Pacific |volume=109 |pages=1278–1284 |doi=10.1086/134008 |bibcode=1997PASP..109.1278S |s2cid=122859920 |doi-access=free}}</ref>

==See also==
* [[List of academic databases and search engines]]
* [[Bibcode]]
* [[INSPIRE-HEP]]
* [[NASA]]'s [[Planetary Data System]] (PDS)
* [[PubMed]]
* [[Michael J. Kurtz]]

==References==
{{Reflist|colwidth=30em}}

==External links==
* {{official website|https://ui.adsabs.harvard.edu/}}

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