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Distant starlight passing through the Earth's atmosphere is refracted by anangle of just over one degree near the surface. This focuses light onto a focalline starting at an inner (and chromatic) boundary out to infinity - offeringan opportunity for pronounced lensing. It is shown here that the focal linecommences at ~85% of the Earth-Moon separation, and thus placing an orbitingdetector between here and one Hill radius could exploit this refractive lens.Analytic estimates are derived for a source directly behind the Earth (i.e.on-axis) showing that starlight is lensed into a thin circular ring ofthickness $W H_{\Delta}/R$, yielding an amplification of $8 H_{\Delta}/W$,where $H_{\Delta}$ is the Earth's refractive scale height, $R$ is itsgeopotential radius and $W$ is the detector diameter. These estimates areverified through numerical ray-tracing experiments from optical to 30 micronlight with standard atmospheric models. The numerical experiments are extendedto include extinction from both a clear atmosphere and one with clouds. It isfound that a detector at one Hill radius is least affected by extinction sincelensed rays travel no deeper than 13.7 km, within the stratosphere and abovemost clouds. Including extinction, a 1 metre Hill radius 'terrascope' iscalculated to produce an amplification of ~45,000 for a lensing timescale of~20 hours. In practice, the amplification is likely halved in order to avoiddaylight scattering i.e. 22,500 ($\Delta$mag=10.9) for $W=$1 metre, orequivalent to a 150 metre optical/infrared telescope.

The "Terrascope": On the Possibility of Using the Earth as an Atmospheric Lens