Jon Sobel looked approvingly over little Greenacre Park.

Smaller than a tennis court, its 25-foot waterfall and carefully sculpted gardens are hidden away on 51st Street in the middle of midtown. Tucked between buildings, guarded by a gate and pagoda-like overhangs, you might not know it exists unless you were looking for the patch of green, as Sobel was. He walked its length in a visit earlier this week.

Another park down, thousands more to go.

Sobel, 54, is on a mission to visit every park in New York City. It started in 2010, when he began feeling “a little burned out with city life.” He had been working as an IT director for a small company, sitting in a computer center day after day. Install new hardware. Upgrade software. Run security for the network. Stare at a screen.

He found himself pulled, almost involuntarily, to get away from “the craziness of life in the city,” he says. To do so without leaving the five boroughs, he turned to NYC’s parks.

Open space, hiding in plain sight

According to the city’s parks department, there are more than 1,700 parks in NYC. That includes parks and playgrounds, but not squares, plazas or greenstreets. The number also doesn’t count the city’s privately operated parks, or its state parks, or national recreation areas, for example. Add in the fact that parks are always opening in New York, Sobel knows it’s an almost impossible task.

For his peripatetic purpose, he counts parks that offer “some aspect of passive recreation.” Not solely playgrounds or athletic fields. Where someone can go for a walk without a sidewalk, even if the way is brief and not entirely surrounded by green. By his tally, that includes some 1,300 official parks locations plus assorted other greenery. He has visited about 250. Since leaving the IT job, he has worked with looser hours as a copywriter and theater/music reviewer and editor, leaving him some time to document his travels at parkodyssey.blogspot.com.

His experience has given him a somewhat encyclopedic perspective. He knows, for example, that Greenacre Park’s 2,500 gallons-per-minute waterfall, while enchanting, is only one of the city’s waterfalls, such as the one in Swindler Cove in Inwood — a refurbished river park supported in part by actress Bette Midler.

Central Park and its reservoir are nice, Sobel says, but for a less crowded and more interesting walk, make the trip to Highland Park on the border of Brooklyn and Queens, where an old reservoir sits still and quiet far below the path. Or Staten Island’s Silver Lake Park, for an interior waterfront experience.

Sobel has visited parks that are small or not even entirely green, such as one on City Island suggested by a friend. It was just around a fence marked with a Department of Corrections “No Trespassing” sign — at the edge of the Long Island Sound, a plaza with benches and tables. “Never in a million years would you know it’s there,” Sobel says.

Not every park is a park

Spots like these aren’t technically parkland but were created by developers in exchange for zoning compromises.

Sobel laments that the parks department doesn’t have the budget to do the kind of upkeep in every park that conservancies manage in the city’s large, wealthier preserves. And parts of Brooklyn or even midtown are lacking in any real parks at all, he says.

Therefore, Sobel appreciates the little privately operated parks, too, for making a dent in the city’s concrete, as long as they’re actually treated like parks. Not left as empty spaces with lonely flower pots, or overshadowed as construction continues, as supporters of the Greenacre Park worry will happen in a rezoning.

When a more extensive park is too far, these little spaces can be just enough necessary nature for Sobel, a Long Island native who has fond memories of hiking upstate or in New Hampshire with his father as a child. “Your whole frame of mind changes,” he says.

After taking some pictures for his blog and relaxing in the shady courtyard, Sobel left Greenacre behind. Next up: perhaps St. Mary’s in the Bronx or the old Fresh Kills landfill on Staten Island. Summer weekends make for good park-visiting season, and Sobel thought back in 2010 he’d be able to see every park before fall.

That didn’t end up happening, but his quest continues.