  We observed the 2019 January total lunar eclipse with the Hubble Space
Telescope's STIS spectrograph to obtain the first near-UV (1700-3200 $\r{A}$)
observation of Earth as a transiting exoplanet. The observatories and
instruments that will be able to perform transmission spectroscopy of
exo-Earths are beginning to be planned, and characterizing the transmission
spectrum of Earth is vital to ensuring that key spectral features (e.g., ozone,
or O$_3$) are appropriately captured in mission concept studies. O$_3$ is
photochemically produced from O$_2$, a product of the dominant metabolism on
Earth today, and it will be sought in future observations as critical evidence
for life on exoplanets. Ground-based observations of lunar eclipses have
provided the Earth's transmission spectrum at optical and near-IR wavelengths,
but the strongest O$_3$ signatures are in the near-UV. We describe the
observations and methods used to extract a transmission spectrum from Hubble
lunar eclipse spectra, and identify spectral features of O$_3$ and Rayleigh
scattering in the 3000-5500 \r{A} region in Earth's transmission spectrum by
comparing to Earth models that include refraction effects in the terrestrial
atmosphere during a lunar eclipse. Our near-UV spectra are featureless, a
consequence of missing the narrow time span during the eclipse when near-UV
sunlight is not completely attenuated through Earth's atmosphere due to
extremely strong O$_3$ absorption and when sunlight is transmitted to the lunar
surface at altitudes where it passes through the O$_3$ layer rather than above
it.
