Why have sightings of unidentified flying objects around the nation more than tripled since 2001? Why is July the busiest month for U.F.O. sightings? Why did they spike in Texas in 2008, or in New Mexico in September 2015?
And how in the world, or out of it, has Manhattan racked up New York State's second-highest tally of U.F.O. sightings in this century?
These questions and many others emerge from the first comprehensive statistical summary of so-called close encounters: 121,036 eyewitness accounts, organized county by county in each state and the District of Columbia, from 2001 to 2015.
The unlikely compendium, "U.F.O. Sightings Desk Reference," is the work of a couple in Syracuse, who crunched unruly data on U.F.O. reports collected by two volunteer organizations: the Mutual U.F.O. Network, or Mufon, and the National U.F.O. Reporting Center, or Nuforc.
It is the reference "U.F.O. researchers dreamed of having," Gordon G. Spear, emeritus professor of physics and astronomy at Sonoma State University in California, writes in the foreword.
The book contains no narrative or anecdotal accounts, just 371 pages of charts and graphs that slice and dice the geography and timing of the incidents and the various shapes that witnesses reported: flying circles, spheres, triangles, discs, ovals, cigars.
Many of the sightings turn out to be explainable, the authors say, but a small percentage defy resolution.
The authors are Cheryl Costa, 65, a former military technician and aerospace analyst, and her wife, Linda Miller Costa, 62, a librarian at Le Moyne College and a former librarian at the National Academy of Sciences, NASA and the Environmental Protection Agency.
Working on PCs amid sewing tables in the upstairs parlor — the warmest room in their hundred-year-old house — the two spent weekends for the last 16 months extrapolating figures from sightings reports and laying out the graphics.
Cheryl Costa was writing New York Skies, a U.F.O. blog for The Syracuse New Times, when the Costas decided to expand their tallies of U.F.O. sightings nationwide. "We wanted to do our bit for disclosure," she said. "It's something the government should have been doing."
The Costas realize some might find this a strange way to spend weekends. But both say they have spotted U.F.O.s themselves and want to detoxify the subject.
"We're doing scientific research," Cheryl Costa said. "What's crazy is not being willing to look at research."
She came to the collaboration roundabout, having served as a cable lineman in the Air Force in Vietnam, and afterward in the Navy's submarine service, as a man before undergoing gender-reassignment surgery in the 1980s. Ordained as a Buddhist nun, she was running a theater group in Maryland when she met Linda. They wed in 2011.
U.F.O. trackers welcomed their publication.
"With this compendium, Cheryl and Linda Costa have reminded the public and the media the extraterrestrial phenomenon continues unabated," said Stephen Bassett, founder and executive director of the Paradigm Research Group, which lobbies for disclosure of official U.F.O. records.
Rebutting a common perception that U.F.O. sightings are on the wane, the Costas' book shows that sightings have risen in waves, to 11,868 nationwide in 2015 from 3,479 in 2001. Only a small fraction of sightings are actually reported to Mufon or Nuforc.
Their labor of love is about the numbers, just the numbers, and the Costas refrain from speculating on what exactly is happening. "We really don't know," Linda Costa said. "But all these people are seeing these things."
The government officially quit the U.F.O. business in 1968, with the finding in the Condon report from the University of Colorado that there was nothing significant to investigate, although some 30 percent of the incidents were unexplained.
Mufon's 500 volunteer investigators, however, continue to check out many of the sightings reported to the group. Roger Marsh, a Mufon spokesman, said that of the 270 cases his group investigated in Manhattan from 2002 through 2016, 44 eluded explanation and remained "unknown."
One of the most intriguing occurred on the afternoon of Sept. 17, 2011, when a man on the roof terrace of the New Museum on the Bowery photographed a fast-moving diamond-shaped object with windows and flashing blue and red lights against the TriBeCa skyline.
According to Mufon, it resembled an unknown flying object photographed in Round Rock, Tex., two weeks earlier.
The Costas listed 426 sightings in New York County from 2001 to 2015, second in the state's tallies only to Suffolk County, on the tip of Long Island, with 554. How so many sightings in the nation's densest core and around its toniest beach resorts have escaped wider notoriety is just part of the mystery.
For the U.F.O. enthusiast, the pages of graphs and charts are a treasure trove of hard-to-find detail.
The District of Columbia, with 9,856 people per square mile, had the fewest sightings: 154. (A political snub from deep space?) Wyoming, with 5.8 people per square mile, had more than twice as many: 337.
Fireballs made up nearly 8 percent of the sightings in Indiana (230) and fewer than 5 percent in Colorado (157).
California, the most populous state, led the nation in U.F.O. reports (15,836, more than the next two states, Florida and Texas, combined). Los Angeles County alone had more sightings than 40 states, followed by Maricopa County, Ariz., which includes Phoenix.
Population fails to explain the figures conclusively, the Costas said. Washington State, with 6.7 million people according to the 2010 census, ranks No. 4 in sightings, ahead of Pennsylvania, with 12.7 million people, and New York State, with 19 million.
Rather, the Costas theorize, the figures may reflect good West Coast weather, which draws more people outside where they may spot U.F.O.s. Nationwide sightings peak in July, they found, and drop off between December and February.
Still, in Mississippi, U.F.O. reports spike in January and November; in New Mexico, in September.
The arduous breakdown by the nation's more than 3,000 counties was notable for revealing clusters of sightings in remote regions, places where U.F.O.s are almost never mentioned. But every county in the United States appears to have seen at least one U.F.O.
In the end, the Costas noted, the spikes may have a lot to do with media coverage.