The late Jim Harrison wrote a poem some time ago called, “Weak winter sun,” in which he said, “It is stupidly human to rush the season. The boy cleans up his trout equipment. Only two more months to the fishing opener and the dry flies and streamers are impatiently waiting.”

The other morning, in the inflatable dome that rises like a mystery bubble in the heart of Stillwater, the sun itself was invisible. This was early, 7 a.m. or so, and inside the bubble along its outer margins, walkers and runners counted laps silently.

Bob Nasby, Mark Newman and Bill Hilton were not among these Fitbit buffs. Instead, encircled by them, the three men held fly rods in their hands and looped long lines from the middle of the bubble to its edges.

This was exercise, the casting, but also artistry, as the men imagined that instead of being surrounded by runners and walkers, they were afloat on the St. Croix in June, casting to smallmouth bass, or on Lake of the Woods in August, patrolling for muskies, or, more immediately, atop the endless aquamarine flats of the Bahamas, turtle grass undulating.

Otherwise productive members of society, these guys were born to fish, and more than that, born to cast. Put another way: In winter, they do not drill holes in lake ice and jig for unseen bluegills and sunnies.

Instead, invoking memories of long days passed on open water, and summoning also their imaginations, beginning with December’s north winds and continuing even now, they cast and cast again.

“Try this one,” Nasby said.

Nasby makes a living teaching casting, and all of his heroes are casters: Lefty Kreh, Steve Rajeff, Joan Wulff.

There are others, too, some of whom cast single-handed, using traditional equipment, and hauling and double-hauling to gain line speed and distance.

Others are two-handed casters.

Nasby is demonstrating the latter now, or what is known as spey casting, in which rods up to 14 feet long are used to throw flies as far as 90 feet, employing two hands.

“Here is your ‘anchor,’ ” Nasby said, imagining that he is not inside, standing on artificial turf on this morning, but rather knee-deep in the Brule River in northern Wisconsin, fishing for steelhead.

Were Nasby in that river, or another, his leader, his fly and the front tip of his line would have provided the resistance necessary to “load” his spey rod as he initiated a cast by forming what is known as a “D” loop.

Properly fashioned and collected in the air behind the caster, the D loop provides energy to the forthcoming forward cast.

Newman paid close attention while Nasby completed his cast.

Having already landed enough 300 muskies on psychedelic streamers to wallpaper a fly shop, and preparing soon to leave for the Florida Keys to chase permit, tarpon and bonefish, Newman, an emergency room physician, is calculating how spey casting can make him an even more efficient angler.

First developed by Scottish salmon fishers more than 150 years ago, spey casting in recent decades has become the preferred line-throwing method among western steelhead anglers who often immerse themselves in wide, fast-running rivers.

Increasingly, spey casting is catching on for use on the smaller streams of Minnesota and Wisconsin, as well.

Performed correctly in its many variations (Single Spey, Skagit, Snake Roll, Snap T, Snap Z, Spiral, among others), the spey cast allows anglers to quickly cover a lot of water with their flies, using minimal effort.

Handing the rod to Newman, Nasby watched as the latter formed a perfect back cast before smoothly bringing the rod forward and stopping it sharply when it was almost perfectly perpendicular to the bubble floor.

The line shot into the far distance as if exiting a cannon, landing nearly at the feet of the runners and walkers.

“Feel how the rod and line do all of the work?” Nasby said. “If you try to power it ahead with your arms and shoulders, the cast will collapse. Let the rod and line provide the effort. It’s in the technique.”

To the runners and walkers circling the inflated mystery bubble, all of this must have seemed like so much wasted energy. Pretending to fish when there were no fish, nor even water, would appear to be the ultimate folly.

But more was happening here than mere calisthenics in preparation for landing a bass, muskie, walleye or permit.

At 45 degrees north latitude, open water, whether beneath blue skies or gray, in wind or no wind, is difficult to find during mornings of weak winter sun.

Except in the imagination. And memory.

On those mornings, holding a fishing rod, and casting a line, can inspire both.