"According to Norton's Star Atlas, there have been twenty fairly bright novae between 1899 and 1936. No less than five of them have been in one small area of the sky, in the constellation Aquila. There were two in a single year (1936), and the 1918 Nova Aquila was one of the brightest ever recorded.

What's going on in this constellation? Why did 25 percent of the novae in a forty-year period appear in only 0.25 percent of the sky? Is the front line moving in our direction?"
--Arthur C. Clarke*

I had a copy of Norton's when I was a kid, and I noticed the same unusual grouping Clarke did. I thought it might make a great plot for an SF novel, an interstellar war where the belligerents were blowing up each other's suns, the ultimate scorched-earth tactic.

As I grew older, I realized there was an alternative explanation. These novae are centered around the celestial equator, near a Right Ascension of 19h. This puts them in the southern Milky Way, at approximately 33d from the galactic center.
It only makes sense that as you look into areas of increasing star density, you're going to see more novae.

So why doesn't Norton's show increasing novae further south, towards the galactic nucleus? The answer is a selection effect, during the years when the data for Norton's was being collected, most of the world's astronomers and observatories conducting photographic patrols and mapping were in the Northern Hemisphere, particularly Europe. They couldn't see clearly much further south than the equator. Astronomical research photography started in earnest in the late 19th century, and Norton's stopped adding new data before mid-century, so the spatial clustering of novae and their temporal proximity is an artifact of the way the data was collected and presented, not of any phenomenon itself.

This is a classic example of what astronomers call a "selection effect", where the data collection process unavoidably skews the observations.

*Reprinted from "The Scientist Speculates" by I. J. Good, General Editor, Copyright 1962. Capricorn Books, New York,
as reproduced in Cosmic Search: Issue 1 (Volume 1 Number 1; January 1979).
http://www.bigear.org/CSMO/HTML/CS01/cs01p22.htm