The Virtual Solar-Terrestrial Observatory (VSTO) is a distributed, scalable education and research environment for searching, integrating, and analyzing observational, experimental and model databases in fields of solar, solar-terrestrial and space physics. Our work on VSTO required us to create a formal, machine understandable representation for concepts, relations and attributes of physical quantities, including concepts to represent scientific instruments and their operating modes, parameters measured by these instruments, and measured from a particular location during a particular time period. The end-user is allowed to search for data based on measured parameters of interest, instruments of interest, geophysical constraints and temporal constraints. To fulfill this need we developed a set of formal encodings of the knowledge in the OWL-DL format. In the current implementation of VSTO (www.vsto.org), however, time coverage information is retrieved as needed from a relational dat...