  Distant starlight passing through the Earth's atmosphere is refracted by an
angle of just over one degree near the surface. This focuses light onto a focal
line starting at an inner (and chromatic) boundary out to infinity - offering
an opportunity for pronounced lensing. It is shown here that the focal line
commences at ~85% of the Earth-Moon separation, and thus placing an orbiting
detector 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 of
thickness $W H_{\Delta}/R$, yielding an amplification of $8 H_{\Delta}/W$,
where $H_{\Delta}$ is the Earth's refractive scale height, $R$ is its
geopotential radius and $W$ is the detector diameter. These estimates are
verified through numerical ray-tracing experiments from optical to 30 micron
light with standard atmospheric models. The numerical experiments are extended
to include extinction from both a clear atmosphere and one with clouds. It is
found that a detector at one Hill radius is least affected by extinction since
lensed rays travel no deeper than 13.7 km, within the stratosphere and above
most clouds. Including extinction, a 1 metre Hill radius 'terrascope' is
calculated to produce an amplification of ~45,000 for a lensing timescale of
~20 hours. In practice, the amplification is likely halved in order to avoid
daylight scattering i.e. 22,500 ($\Delta$mag=10.9) for $W=$1 metre, or
equivalent to a 150 metre optical/infrared telescope.
