  The apparent source region (or regions) of long-period comets as well as the
definition of the dynamically new comet are still open questions.The aim of
this investigation is to look for the apparent source of selected long period
comets and to refine the definition of dynamically new comets. We show that
incorporation of the non-gravitational forces into the orbit determination
process significantly changes the situation. We determined precise
non-gravitational orbits of all investigated comets and next followed
numerically their past and future motion during one orbital period. Applying
ingenious Sitarski's method of creating swarms of virtual comets compatible
with observations, we were able to derive the uncertainties of original and
future orbital elements, as well as the uncertainties of the previous and next
perihelion distances. We concluded that the past and future evolution of
cometary orbits under the Galactic tide perturbations is the only way to find
which comets are really dynamically new. We also have shown that a significant
percentage of long-period comets can visit the zone of visibility during at
least two or three consecutive perihelion passages.
