Completeness of the Gaia-verse I: when and where were Gaia's eyes on the
  sky during DR2?
  The Gaia space mission is crafting revolutionary astrometric, photometric and
spectroscopic catalogues that will allow us to map our Galaxy, but only if we
know the completeness of this Gaia-verse of catalogues: what stars does it
contain and what stars is it missing? We argue that the completeness is driven
by Gaia's spinning-and-precessing scanning law and will apply this principle to
the Gaia-verse over this series. We take a first step by identifying the
periods in time that did not contribute any measurements to Gaia DR2; these
gaps create ribbons of incompleteness across the sky that will bias any study
that ignores them, although some of these gaps may be filled in future data
releases. Our first approach was to use the variable star photometry to
identify the 94 gaps longer than 1% of a day. Our second approach was to
predict the number of observations of every point on the sky, which in
comparison to the reported number of detections revealed additional gaps in the
astrometry and spectroscopy. Making these predictions required us to make the
most precise, publicly-available determination of the Gaia scanning law. Using
this scanning law, we further identified that most stars fainter than $G=22$ in
DR2 have spurious magnitudes due to a miscalibration resulting from a
thunderstorm over Madrid. Our list of gaps and precision scanning law will
allow astronomers to know when Gaia's eye was truly on their binary star,
exoplanet or microlensing event during the time period of the second data
release.
