The mystery of Fenway

As Adrian Gonzalez can attest, Fenway Park is not an easy place for lefties to hit home runs. Jim Rogash/Getty Images

Tomorrow, Fenway Park will be 100 years old. Getting to turn 100 is tough enough as a human -- currently, only 1 in 4,400 Americans have completed their first century. As a baseball park, it's even more difficult, with Fenway being the first major league park to host major league games for 100 years. Only 11 parks in history have managed to make it to 50, Wrigley Field at 98 being the only other remaining one. The 15th-ranked ballpark in age, Bank One Ballpark, is a mere 14 years old.

When you have a century to look at a ballpark, one of the more interesting things of note is just how much a park can change, even when the park's measurements change little. Despite its reputation of being a home run park, Fenway hasn't been a home run park for two decades now, despite no significant change in distance to any of the walls since the early 1940s.

Fenway spent most of its early years as a very tough place to hit home runs. The first time the park had a home run factor above 100 (league average) was 1941, thanks to a significant shortening of the park's dimensions. Some of these changes were significant, such as left-center being as far back as 488 feet in the 1920s, now 390 feet.

Starting in 1941 with Fenway's 107 five-year HR factor, the next time that number crept below league-average was after the 1986 season; and except for a very brief blip after 1991 and 1992, Fenway has been a more difficult than average place to hit home runs. Why? No one really knows, but we can test some possible theories.