When Masaccio and Brunelleschi created the apparent diminution of the Holy Trinity fresco barrel vault, they depended on the orthographic and stereographic projection techniques informing medieval astronomical instruments of measure, especially those inscribed on a widely admired siting and surveying device known as the astrolabe. Using these projections, medieval astronomers had structured a perfectly ordered macrocosmic space according to a set of coordinates directly useful to an artist wishing to chart the reflected perfection of a measured and measurable microcosmic space. The diagrammatic protocols governing astronomical graphics were based not only on the belief in the geometrically perfect universe, but also in the idea that the abstract mathematical point could be thought of as a substitute for the seeing human eye. The astrolabic articulation of these two ideas represent a symbolic sanction and a technical underpinning for the fundamental elements of Renaissance perspective as they exist in the Holy Trinity fresco: the relationship between the viewer and the horizon line, the transfer of projected coordinates from one plane to another through a process of rotation, and the mathematically controlled, proportionate relationship of transverse quantities.