  Equation of motion for a comet in the Oort cloud is numerically solved.
Orbital evolution of the comet under the action of the gravity of the Sun and
the Galaxy is presented for various initial conditions.
  Oscillations of the Sun with respect to the galactic equatorial plane are
taken into account. Real values of physical quantities concerning the
gravitational action of the galactic neighbourhood of the Sun are important.
The results are compared with currently used more simple models of the galactic
tide. It turns out that physically improved models yield results which
significantly differ from the results obtained on the basis of the conventional
models. E.g., the number of returns of the comets into the inner part of the
Solar System are about two times greater than it is in the conventional models.
  It seems that a comet from the Oort cloud can be a source of the dinosaurs
extinction at about 65 Myr ago. A close encounter of a star or an interstellar
cloud disturbed a comet of the Oort cloud in the way that its semi-major axis
increased/decreased above the value 5 $\times$ 10$^{4}$ AU and the comet hit
the Earth.
