Originating within astronomy as a technical term in the first half of the 18th century, the term “personal equation” spread into a litany of other fields including medicine, where it was used widely and variously from the late 19th century to the middle of the 20th century. We explore the personal equation in the medical literatures of the United States and Britain through a systematic analysis of over 700 articles in four prominent medical journals in conjunction with additional relevant source materials. After tracing the term’s dispersion from astronomy into medically allied fields, we examine its striking polysemy while using its rich usage as a lens to examine prevailing tensions within contemporary American and British medicine. Stretching from patient and clinician variability to observer variability and error, the personal equation’s various meanings reflect debates about the art and science of medical care that persist into the present day.