In navigator training back when we actually used a sextant, whenever
our mission called for legs using celestial navigation ATC would give
us a celnav clearance. This allowed us much more airspace than a
"direct" clearance as celestial nav was less precise than other methods
of navigation. On missions which required students to use only ded
reckoning, the pilot would also ask ATC for a celnav clearance in order
to have the freedom to manuever (i.e. wander) off the direct line to
the next turnpoint. Thus, when ATC sends you direct to a point, they
expect you to be pretty darn close to staying on the direct course to
that point. If you decide that your going to use a watch and compass
(dr) or pull out a sextant and you wander significantly off that
straight line and stray into restricted airspace--guess who's going to
get violated?