  The general relativistic gravitomagnetic clock effect involves a coupling
between the orbital motion of a test particle and the rotation of the central
mass and results in a difference in the proper periods of two counter-revolving
satellites. It is shown that at O(c^-2) this effect has a simple analogue in
the electromagnetic case. Moreover, in view of a possible measurement of the
clock effect in the gravitational field of the Earth, we investigate the
influence of some classical perturbing forces of the terrestrial space
environment on the orbital motion of test bodies along opposite trajectories.
