From time to time, a baseball player puts up numbers so outlandish they just don’t register with anyone.

You could characterize Barry Bonds’s 2001-04 run this way, putting aside the issue of how exactly it was accomplished. Last year, Bonds walked 232 times, breaking the National League record for the fourth time; Babe Ruth still holds the American League record he set in 1923, when he walked 170 times, which also happens to be the highest total in a season by anyone other than Bonds. To put Bonds’s accomplishment in context, the difference between first and fourth place on the all-time walk list is equal to the difference between fourth place and 118th place. For all the hype and press coverage surrounding Bonds since 2001, it will probably be quite some time before we realize just what we witnessed over the last few years.

Another such performance is taking place this season in Houston, where Roger Clemens is leading the league in ERA with a mark of 1.51. He is allowing a .182 opponents’ batting average and a 0.95 WHIP, both the best in his long and storied career.

This is bizarre and inexplicable. Had Clemens retired before this season – or, for that matter, before last season, when he went 18-4 and quite nearly pitched the Astros into the World Series – he would have been rightly lauded as probably the greatest pitcher in the history of baseball. Still, the seven-time Cy Young winner will turn 43 in a month, and he is just not supposed to have a 1.51 ERA.

Clemens’s magnificent 2005 campaign has been relatively unremarked upon, for several reasons. For one, Houston is a horrible team, and that’s affected Clemens’s record. The Rocket has given up three or fewer runs in all but one of his 15 starts this year, but with the worst offense in baseball behind him, he has gone just 6-3. At one point, he worked three consecutive games in which he pitched seven innings, gave up no runs, and earned no decision.

A second reason for the oversight is that Houston, despite its enormous size and importance to the national economy, lies on the margins off the national press’s map. The most important factor, though, is that it’s just hard to be surprised by anything Clemens does. Supposedly washed up at 34, he won two Cy Youngs with Toronto; at 38, seemingly having settled into a role as a reliable workhorse but not a top-tier ace, he went 20-3 for the Yankees; at 41, after he’d retired, he won yet another Cy Young. Why shouldn’t he have his best season at 42?

It should surprise no one then, that Clemens is on pace to have the best season in history for a 42-year-old pitcher, and in fact by any starter 40 or older. One could argue for the 41-year-old Cy Young’s 1908 campaign, when he pitched 299 innings with a 1.26 ERA, but that was obviously a different game than the one played today. In the interests of apples-to-apples comparisons, consult the chart below, which measures Clemens’s season against the best had by starters 40 and older since baseball began to integrate in 1947.

Aside from reminding us that Dennis Martinez has a stronger Hall of Fame case than anyone seems to think, the comparison to other excellent 40-somethings shows that Clemens is pretty much on another planet this year. Comparing him just to other older starters doesn’t really do him any justice; there have, after all, been only 67 seasons in history where a player 40 or older pitched enough innings to qualify for the ERA title. Clemens is after bigger game this year: At this point, he is on track for one of the greatest seasons in history.

Clemens’s current ERA, if it holds up, will be the best, relative to his league, in baseball history. It should also be noted that he is accomplishing this feat while playing in a good hitter’s park in front of a defense that features, among other horrors, a 39-year-old outfielder – Craig Biggio – at second base and a first baseman coming off knee surgery – Lance Berkman – playing left field.

Can Clemens keep it up? Almost certainly not, if only because his performance is unprecedented in the long history of the game. Even so, it doesn’t much matter whether he ends up winning an eighth Cy Young Award. What does matter is that one of the very best ever to play the game is showing that something we would have thought was impossible isn’t, provided a player is willing to put in the work. The great Satchel Paige was still a very good pitcher at 46 years old, and he didn’t have the benefit of modern medical technology. Can Clemens do what only two men have ever done, and win his 400th game? Probably not, but I wouldn’t put it past him.