well constructing linear points is a valid part of a compass.
simply construct a circle, and mark the point you left off on, or if you can't do that,
pick a point on the circle, draw a circle keeping the compass the same radius.
then from one of the points of intersection, construct another circle. the two newest circles meet in the middle.
and then most new and the first circle meet, you can then construct another point using a similar process
for a pentagon, you can use the golden ratio, (1+sqrt(5))/2. which is easy enough to construct.
with a heptagon, its impossible. see http://en.wikipedia.org/wiki/Heptagon