  The analysis of modern cosmological data is becoming an increasingly
important task as the amount of data multiplies. An important goal is to
extract geometric information, i.e. the metric of the cosmos, from
observational data. The observer metric is adapted to the reality of
observations: information received along the past null cone, and matter flowing
along timelike lines. It provides a potentially very good candidate for a
developing general numerical data reduction program. As a basis for this, we
elucidate the spherically symmetric solution, for which there is to date single
presentation that is complete and correct. With future numerical implementation
in mind, we give a clear presentation of the mathematical solution in terms of
4 arbitrary functions, the solution algorithm given observational data on the
past null cone, and we argue that the evolution from one null cone to the next
necessarily involves integrating down each null cone.
