Halley's Comet is rare, but it arrives pretty much on schedule, roughly every 75 years. The rare occurrences in sports are not so predictable. You might see a rare feat two or three times in a decade, then never again in your lifetime.
American Pharoah is passing through the sports universe as we speak, but will he light up the sky by winning horse racing's coveted Triple Crown at the Belmont Stakes on Saturday, or will he, like so many others, fall short and leave us waiting for another year, as we have been waiting every year since 1978.