Cosmic Patterns Provide Delight And Insight

A 22-degree halo circled the sun this past Friday afternoon. Halos like this one form in high cirrostratus clouds composed of microscopic hexagonal (6-sided) pencil shaped ice crystals. Sunlight refracted through billions of them spreads into a circle with a radius of 22 degrees. Click to learn more. Photo: Bob King

There’s always something happening in the sky. It’s a big place after all with many dimensions. There are near-space phenomena like ice halos and weather and a much deeper dimension that includes the most distant galaxies in the universe and the entire history of time.

If I were to pick one night to view the three bright planets at dusk, it would be tonight. Look very low in the northwestern sky starting about 40 minutes after sunset. Venus is the brightest of the three. Created with Stellarium

Patterns abound as well. Solar halos form when billions of ice crystals refract or bend sunlight and planets align when they appear along our line of sight. If you still haven’t seen the Jupiter-Venus-Mercury gathering in the evening sky, tonight’s the best night. All three worlds will fit inside a circle just 3 degrees (6 full moons) wide. Get lucky with a clear sky and wide-open view to the northwest and you’re good.

Saturn’s north polar hexagon photographed by the Cassini spacecraft. Each side of the hexagon measures about 8,600 miles long. The feature has persisted for at least 20 years. Credit: NASA/JPL-Caltech/SSI

Speaking of patterns, especially hexagons, I’m continually amazed by the quality of planetary photos taken by amateur astronomers. Philippine astrophotographer Christopher Go recently shared images of Saturn that blew me away. In them you can discern the planet’s north polar hexagon, a mysterious six-sided wave pattern centered on Saturn’s north pole.

You can easily make out the straight sides of Saturn’s north polar hexagon in this May 25, 2013 image taken through a 14-inch telescope. Credit: Christopher Go

It was first discovered during the Voyager flybys of the 1980s. Back then the pole was in sunlight. Until recently it was tilted away from the sun in shadow during the 7.5-year-long winter season in Saturn’s northern hemisphere. Now the hexagon’s back out in sunshine and the Cassini spacecraft has taken stunning photos of this only partially understood feature.

Photo taken by the Cassini probe last February shows Saturn’s polar hexagon emerging from shadow into sunlight during northern hemisphere spring. The Earth is shown for comparison. About 4 Earths would fit inside the feature. Credit: NASA/JPL-Caltech/SSI

The hexagon may be what physicists call a standing wave pattern. You can picture this by imagining two people holding either end of a jump rope and shaking it in sync. If done carefully, the rope will form a regular pattern of oscillating up and down waves that travel in place. Perhaps wavelike movements of air in the polar atmosphere create the standing hexagonal wave.

Animation of Saturn’s north polar hexagon. Credit: NASA/JPL/U. of Arizona

Ana Aguiar and her team at the University of Oxford have an alternative explanation. It turns out there’s a steep contrast in wind speeds on the planet at 78 degrees north latitude – the hexagon’s perimeter – that’s perfect for inducing instabilities in the atmosphere leading to the formation of oddball waves and eddies. They recently re-created a similar 6-sided pattern using fluorescent dyes in a dual-speed water tank in the laboratory. For a fascinating write-up on their experiment, please check out the Planetary Society’s Emily Lakdawalla’s blog on the topic.

Searching for the underlying cause of the patterns in nature has always been one of humanity’s specialties. Our lives have depended on it since the beginning when we tracked animals for food and followed the movement of the sun to forecast the seasons. That same spirit brought Saturn’s hexagon into the lab.

2 Responses

  1. Giorgio Rizzarelli

    Indeed amazing the Go picture – I didn’t imagine one can see the hexagon with an amateur scope

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