Supermoon estimation is prediction of the timing and distance of a perigee full moon. Some existing relevant tools can be opaque (lacking implementation details), functionally limited (giving a subset of the required information) or give mutually inconsistent results. A new supermoon estimator has been implemented to address drawbacks in, and to help benchmark, such existing tools. Specifically, the case of the 21st century supermoons is handled. Implementation in R is discussed, including numerical precision caveats, first-class function leveraging, general optimisations (e.g. Horner's method) and time scale transformations.