Hallucinatory Terrain

The official description of the effects and uses of this spell is found in
the _Concordance of Arcane Space_, the book included in the original
Spelljammer Boxed Set (the one with the Pirate and the Illithid on the
cover). On page 81:

"Hallucinatory Terrain: In addition to creating illusionary asteroids and
similar bodies, this spell can be used to disguise ships in space, either
making them appear as part of the background stars or as an asteroid or small
dust cloud, provided the ship is small enough to fit within the area of
effect of the spell."

The description of Hallucinatory Terrain on page 158-159 of the PHB indicates
that the area of effect of the spell is a cube of up to 30' per side per
level. Thus, a 12th level wizard could produce an illusion of 360' x 360' x
360', covering most ships and even some large asteroids quite handily. The
spell lasts for 1 hour per level, and it's level of veracity is described as
being such that "The hallucinatory terrain lasts until a dispel magic spell
is cast upon the area or until the duration expires. Individual creatures may
see through the illusion, but the illusion persists, affecting others who
observe the scene". So whether the illusion persists person to person is
dependent upon how they are affected by the actions of those caught within
the illusion. Note that any ability to see invisible, naturally or magically,
would not be useful unless it also allows the person to see through
illusions; the hallucinatory terrain that makes the ship appear as part of
the background of stars, for example, is not invisibility per se, rather it
is a way to make the ship *appear* to be invisible, while in the proper
context. If the ship disguised as a background of stars was occluded between
the viewer and a large body, say an asteroid, the viewer would see a
background of stars appear between himself and the asteroid, obviously
negating the value of the disguise... the same can be said with an illusion
of an asteroid around the ship, as asteroids do not move at tactical speeds,
so unless the disguised ship were simply drifting, it would be pretty
obvious...

Mystaros