Toboggan sun solves 400-year-old mystery

JOHN THOMPSON

In 1597, the ill-fated Dutch explorer Willem Barents and his crew watched the spectacle of the planet Jupiter sliding across the central mountain range of Novaya Zemlya. It was a strange sight, given that Jupiter should have been well below the horizon.

The phenomenon helped fool Barents’ party into believing their longitude was 74 degrees east of Venice, an estimate off by nearly 30 degrees. Their confusion led to heated discussions between scientists during the centuries to follow.

If only Barents knew about the “toboggan effect,” a term coined by Wayne Davidson, a weather station operator in Resolute Bay, who co-published an article this month in the journal Applied Optics with researcher Siebren van der Werf.

It works like this: layers of warm and cool air floating above the horizon cause light to bend, distorting the shape and location of astral bodies like the sun, or in this case, Jupiter. These pockets of warm and cool air are called temperature inversions, and it turns out these distortions grow when there’s a hillside involved.

That’s because cold winds corralled against mountain slopes tend to push the temperature further down. What might be a mild temperature inversion over a plain or the sea suddenly grows when it hits the hillside.

Temperature inversions can cause the sun to look squashed or square as it hovers over the horizon in the Arctic. Put these warped effects on a tilt, and it begins to look as if the bottom of the sun has slid from beneath itself and turned into a crazy carpet as it coasts down the hill.

The crew member of another doomed polar explorer, Captain Robert F. Scott, could have been the first to write about the toboggan effect. On Aug. 26, 1911, H.G. Ponting writes that Scott invited him to walk across the icebergs and climb a nearby slope to witness the first sunrise of the season.

“Within a few minutes a flame seemed to burst from the serrated lava ridge above Cape Barne; then the upper rim of the sun crept out, not rising by sidling along the slope. A few minutes more, and the blazing orb had cleared the land, and for the first time in more than four months we were bathed in his grateful rays. The world was once more golden.”

But Scott never understood the science behind what he saw, and never thought of naming the phenomena.

“Scott had his chance. He failed,” Davidson said.

Davidson has watched the sun slide down Cape Sherington in Resolute, as a long, flattened disc pressed against the hillside. In its various forms, he says the “toboggan sun” can resemble an anvil or a Chinese lantern.

It takes around four minutes for the sun to slide across half a kilometre. It occurs during the spring, for a single sunset, once a year.

Davidson describes the toboggan sun as a special variant of the Novaya Zemyla effect, a broader, already-recognized phenomena that causes the sun to appear above the horizon when it is, in fact, as much as five degrees below it. Again, temperature inversion is the culprit, which bends light along an invisible duct of air.

One of Barents’ crew members, Gerrit de Veer, first recognized the Novaya Zemyla effect.
De Veer’s observations were later derided by critics as inaccurate, but today, it’s recognized as a common occurrence in the Arctic.