  The sum of sunspots number over an odd numbered 11 yr sunspot cycle exceeds
that of its preceding even numbered cycle, and it is well known as Gnevyshev
and Ohl rule (or G--O rule) after the names of the authors who discovered it in
1948. The G--O rule can be used to predict the sum of sunspot numbers of a
forthcoming odd cycle from that of its preceding even cycle. But this is not
always possible because occasionally the G--O rule is violated. So far no
plausible reason is known either for the G--O rule or the violation of this
rule. Here we showed the epochs of the violation of the G--O rule are close to
the epochs of the Sun's retrograde orbital motion about the centre of mass of
the solar system (i.e., the epochs at which the orbital angular momentum of the
Sun is weakly negative). Using this result easy to predict the epochs of
violation of the G--O rule well in advance. We also showed that the solar
equatorial rotation rate determined from sunspot group data during the period
1879--2004 is correlated/anti-correlated to the Sun's orbital torque during
before/after 1945. We have found the existence of a statistically significant
$\sim$ 17 yr periodicity in the solar equatorial rotation rate. The
implications of these findings for understanding the mechanism behind the solar
cycle and the solar-terrestrial relationship are discussed.
