The Astronomical and Astrophysical communities - as part of the International Virtual Observatory Alliance - is in the process of creating several "vocabularies" useful for describing a range of Astronomical terms, be it related to scientific concepts, data processing, or some other area of Astronomy. The current status of this work is described in the Vocabularies in the Virtual Observatory document, a proposed recommendation of the IVOA. The aim of the work is to standardise the methodology for the creation and output of multiple sets of vocabularies for identifying the things that Astronomers care about, and is based on the W3C's Resorce Description Framework (RDF) and Simple Knowledge Organization System (SKOS) specifications.
The latest version of this work has produced several candidate vocabularies, from which I have selected the IVOA thesaurus and A&A keywords for use in this server. All I have done is to extract the "fragment" part of each URI from these two vocabularies and use this as a tag (after converting to lower case). This works well for simple terms like "moon", but can lead to somewhat unlikely tags such as "astronomicalinstrumentationmethodsandtechniques". I have also used the contents of the skos:prefLabel and skos:altLabel elements as tags - after removal of non-alphanumeric characters and some minor clean up - which means that agn and activegalacticnucleus both refer to http://www.ivoa.net/Documents/PR/Semantics/Vocabularies-20080729/rdf/IVOAT#activeGalacticNucleus.
There is also a project to create an Astronomical Ontology written using the OWL Web Ontology Language, which uses the IVOA Thesaurus as one of its sources. I have performed a similar conversion for this ontology as I did with the SKOS vocabularies. Many of the tags overlap with the IVOA values, such as "abellcluster", but there are separate terms - e.g. "meanfreepath" - and there are also tags that should probably be removed such as "unitlessnumberof". Not all the entities in this ontology have been converted to tags; those items that are OWL properties (i.e. have a class something like owl:ObjectProperty) have not been included.
The UMBEL ontology can be searched and browsed using the UMBEL Web Services.
It would be interesting to increase the number of "vocabularies" that are present in this server; for instance Astronomical terms present in the DBpedia project, OpenCyc ontology, and UMBEL ontology (although these may not be entirely dis-joint), as well as more Astronomy-specific collections. I have added several items from the UMBEL ontology, such as star, galaxy, and blackhole but a more complete set could be added.