What gallery of Yellowstone is complete without at least one image of Old Faithful? Contrary to popular belief, it does not go off every hour but rather about every 90 minutes. There is a variance of +/- 10 minutes with a certainty of 90%. The periodicity of the geyser can change without notice (though that is rare for this geyser at this point in its life.) or as a result of various Earth quaqes 100's of miles away.