Many humans are bad at visualizing what measurements really mean unless you give them a comparison. Tell someone a space is 360 feet long and they'll probably just blink; say it's the length of a football field and you might get a nod of comprehension. That's why many writers use size comparisons rather than precise measurements in non-technical works. (It also helps convince people your work wasn't written by a robot.) But the comparisons that writers use reflect the culture and time period they're in—tell an ancient Roman something is the size of a credit card or a car, and you're not going to get very far.

As spotted by Digg, programmer and data visualization whiz Colin Morris recently performed an experiment that demonstrates how these kinds of object comparisons change over time. Morris mined the vast Ngram dataset of English-language Google Books for occurrences of the phrase "the size of ___" between 1800 and 2008, then ranked the top results by popularity overall and in specific centuries. Some of the results made perfect sense (England has phased out the shilling; basketball didn't exist for most of the 1800s), while others were more surprising (why did we stop referring to cats as a popular size comparison in the 21st century?).

Overall, Morris found that items from the natural world have fallen into decline as reference points, while sports analogies have exploded onto the scene. (Morris wonders whether this has to do with the rise of leisure time, and/or the mass media that exposes far more spectators to sports than ever before.) Some of the specific results also have intriguing stories to tell: We no longer talk about the size of pigeon's eggs largely thanks to the extinction of the passenger pigeon, which was once the most numerous bird in the United States. The numbers of city pigeons just don't compare—when was the last time you saw one of their eggs?

There is one clear winner across the centuries, however: peas. These tiny legumes were the most popular reference point in the 1800s and they remain so today. The same is true of runner-up the walnut. Let it not be said we have nothing in common with our ancestors.

Here are the top five items in each century that Morris investigated:

1800s

1. pea
2. walnut
3. pinhead
4. egg
5. hen's egg

1900s

1. pea
2. walnut
3. pinhead
4. egg
5. orange

2000-2008

1. pea
2. walnut
3. quarter
4. football field
5. egg

For the full list, head over to Colin Morris's site.

[h/t Digg]