  We present a direct observation of the geometrical jitter in single photon
detection by a straight superconducting nanowire. Differential measurement
technique was applied to the 180-{\mu}m long nanowire similar to those commonly
used in the technology of superconducting nanowire single photon detectors
(SNSPD). A non-gaussian geometrical jitter appears as a wide almost uniform
probability distribution (histogram) of the delay time (latency) of the
nanowire response to detected photon. White electrical noise of the readout
electronics causes broadened, Gaussian shaped edges of the histogram.
Subtracting noise contribution, we found for the geometrical jitter a standard
deviation of 8.5 ps and the full width at half maximum (FWHM) of the
distribution of 29 ps. FWHM corresponds to the propagation speed of the
electrical signal along the nanowire of $6.2\times10^{6}$ m/s or 0.02 of the
speed of light. Alternatively the propagation speed was estimated from the
central frequency of the measured first order self-resonance of the nanowire.
Both values agree well with each other and with previously reported values. As
the intensity of the incident photon flux increases, the wide probability
distribution collapses into a much narrower Gaussian distribution with a
standard deviation dominated by the noise of electronics. We associate the
collapse of the histogram with the transition from the discrete, single photon
detection to the uniform bolometric regime
