  We present a systematic study of elliptic flow as a function of centrality,
pseudorapidity, transverse momentum and energy for Cu+Cu and Au+Au collisions
from the PHOBOS experiment. New data on elliptic flow in Cu+Cu collisions at
22.4 GeV are shown. Elliptic flow scaled by participant eccentricity is found
to be similar for both systems when collisions with the same number of
participants or the same average area density are compared. This similarity is
observed over a wide range in pseudorapidity and transverse momentum,
indicating that participant eccentricity is the relevant quantity for
generating the azimuthal asymmetry leading to the observed elliptic flow.
