When gathering article metadata from the MediaWiki API for constructing a page lead object, MCS only takes the first set of geo coordinates (if any) and reports them without regard to which terrestrial body they're referring to. In fact, the MediaWiki API may provide more than one set of coordinates, the first one may not (at least in principle) be the primary set of coordinates for the page, and the coordinates may not refer to the planet Earth.
Further, clients may assume that the coordinates reported for an article are valid Earth coordinates; I know that the Android app does this, for instance. However, for articles like [[ https://en.wikipedia.org/api/rest_v1/page/mobile-sections-lead/Mare_Tranquillitatis | Mare Tranquillitatis ]] this is an incorrect assumption.
Rather than naively taking the first set of coordinates reported and returning them without the remaining context, I propose simply returning the full set of coordinates reported by the MediaWiki API and allowing clients to make decisions about which to use.
For now, we can do this in a purely additive, backward-compatible way, but I think we should deprecate the current 'geo' property.