  Historically, long-slit spectroscopic observations were carried out using the
parallactic angle for the slit orientation if slit loss was an important
consideration (either to maximize the signal-to-noise or to do
spectrophotometry). This requires periodic realignment of the slit position
angle as the parallactic angle changes. This is not possible for multi-slit
observations where one slit position angle must be chosen for the entire
exposure. Common wisdom suggests using the parallactic angle at the meridian
(HA=0). In this paper, I examine what the best strategy is for long, multi-slit
exposures. I find that in extreme cases (very long exposure time) the best
choice is to orient the slit \emph{perpendicular} to the parallactic angle at
the meridian. There are two effects to consider: the increasing dispersion with
increasing airmass and the changing angle between the parallactic angle and the
slit. In the case of \emph{traditional} slit orientation, the two effects
amplify each other, thus rendering a significant fraction of the observation
useless. Using the perpendicular orientation, the two processes work against
each other, thus most of the observation remains useful. I will use, as an
example, our 8 hour Lockman Hole observations using the Keck telescope, but
generic methods are given to evaluate a particular observation. I also make the
tools available to the community.
