  We study the Earth's electric field monitored at PYR (Greece) monitoring
site, for a period of more than six years (May 23rd, 2003 to September 7th,
2009). It is compared, in particular its oscillating component of T = 365 days,
with the Perihelion - Aphelion dates of the Earth's orbit around the Sun, with
the same component of the Earth's magnetic field, with the corresponding same
period tidal oscillation and with the occurred large EQs of the same period of
time. The obtained results suggest that the oscillating electric field
component is generated by large scale piezoelectricity triggered by the Earth's
shape - lithospheric oscillating deformation. The driving mechanism (yearly
tidal variation) precedes the Aphelion - Perihelion dates for a month complying
with the corresponding tidal friction behavior of the Earth's shape
deformation. The Earth's oscillating electric field peaks coincide with the
Perihelion - Aphelion dates while the triggered large EQs are clustered very
close to the Perihelion - Aphelion dates. Moreover it is shown that the
observed Earth's oscillating electric field is not related to or induced by the
corresponding Earth's magnetic field. In conclusion, the Earth's oscillating
electric field character could be used as medium to long term electric seismic
precursor of large EQs.
  Keywords: tidal waves, Earth deformation, preseismic electric signals,
magnetic field, Perihelion, Aphelion, earthquake prediction.
