  Ground-based interferometers are not perfectly all-sky instruments, and it is
important to account for their behavior when considering the distribution of
detected events. In particular, the LIGO detectors are most sensitive to
sources above North America and the Indian Ocean and, as the Earth rotates, the
sensitive regions are swept across the sky. However, because the detectors do
not acquire data uniformly over time, there is a net bias on detectable
sources' right ascensions. Both LIGO detectors preferentially collect data
during their local night; it is more than twice as likely to be local midnight
than noon when both detectors are operating. We discuss these selection effects
and how they impact LIGO's observations and electromagnetic follow-up. Beyond
galactic foregrounds associated with seasonal variations, we find that
equatorial observatories can access over $80\%$ of the localization
probability, while mid-latitudes will access closer to $70\%$. Facilities
located near the two LIGO sites can observe sources closer to their zenith than
their analogs in the South, but the average observation will still be no closer
than $44^\circ$ from zenith. We also find that observatories in Africa or the
South Atlantic will wait systematically longer before they can begin observing
compared to the rest of the world, although there is a preference for
longitudes near the LIGOs. These effects, along with knowledge of the LIGO
antenna pattern, can inform electromagnetic follow-up activities and
optimization, including the possibility of directing observations even before
gravitational-wave events occur.
