= Long-wavelength source approximation
{title2=$d/\tau\ll1$}
Dropping the position-dependent source delay in the leading far-zone retarded field requires source size $d$ to be small compared with its variation time $\tau$, in units $c=1$. The <retarded quadrupole field> also requires $d/r\ll1$. For a typical nonrelativistic bound source, $\tau\sim d/v$ implies this long-wavelength condition; an arbitrarily rapidly oscillating small displacement is not controlled merely by its instantaneous speed.
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