  The quasar 0917+624 has been one of the best studied intraday variable (IDV)
radio sources. However, debate continues as to whether the underlying cause is
intrinsic or extrinsic. Much previous work has assumed the IDV to be intrinsic
and which implies an extraordinarily compact source for the radio emission; in
contrast, an extrinsic variation due to interstellar scintillation (ISS)
implies a relatively larger source diameter, though at the smaller end of the
range expected for relativistic jet models. Kraus et al. (1999) reported a
marked slowing of the IDV at 6cm wavelength in September 1998, and suggested a
change in the source was responsible. However, here we show that the slowing is
consistent with the annual modulation in scintillation time-scale expected for
ISS, under the assumption that the scattering medium moves with the local
standard of rest (LSR). The ISS time scale is governed by the ISS spatial scale
divided by the Earth's velocity relative to the scattering plasma. It happens
that in the direction of 0917+624 the transverse velocity of the Earth with
respect to the LSR varies widely with a deep minimum in the months of September
to November. Hence the slowing of the IDV in September 1998 strongly suggests
that ISS, rather than intrinsic variation of the source is the dominant cause
of the IDV.
