Transit of Venus - Why it comes in pairs every 100ish years

Venus passes in front of the Sun from the Earth's point of view very rarely. It occurs in eight-year pairs, but then with gaps of over 100 years between those pairs. CORRECTIONS: Ok, so I mistaked a few words. There's an early “2017“ that should have been “2117“ and at least one “day“/“year“ swap. I really was recording that right before the transit and only had time for one take. More importantly, I said that 105.5 years before the next transit meant it would be mid-way through 2117. Actually, 105.5 years from now is the end (December) of 2117. Finally, I made a lot of simplifications, including setting my spreadsheet to use this 2012 transit as the zero-point for the sweet-spots. All improvements welcome!