In astronomy, a planetary transit is when a planet closer to the Sun passes in front of the Sun's disc as seen from a more distant planet. From the Earth, transits of Mercury and Venus are visible; recently, observers were treated to these spectacles in back-to-back years: 2003 and 2004.

In the run-up to the 2004 transit of Venus, I became obsessed with one of those silly questions which, once they sink their claws into one's mind, won't go away without being answered yes or no. Is there ever a simultaneous transit of Mercury and Venus visible from the Earth?

The naïve answer is, “Of course not (you idiot)! Transits can occur only when inferior conjunction with the inner planet coincides with its crossing the ecliptic. Transits of Venus always occur in June and December, transits of Mercury in May and November, and thus a simultaneous transit can never happen.”

But this doesn't take into account the evolution of planetary orbits over time. Analytical planetary theories such as VSOP87 are useless beyond the period for which they are fit (say, −4000 to +8000 years Gregorian). To go beyond that, you need to do full-up numerical integration of the motion of the Sun and planets.

Well, that's what computers are for, isn't it? So, I found a high-precision numerical integration code for the Solar System written by Steve Moshier, built a back-end to search for transits, and set it looking for this extraordinarily rare event. Since I didn't want to burn months of computing time with nothing to show for it, I decided to prepare a canon of all Solar System transits (excluding marginal graze events) visible in the time interval I chose, namely a quarter million years centred on the start of the so-called “Common Era”. Here it is.