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Wed, 22 May 2013 - Saturn's hexagon

The anatomy of Saturn. Click for larger view
Saturn from Australia. Click to enlarge
Saturn photographed from St Albans by Martin Lewis. Click to enlarge

For many observers around the world Saturn has put on an excellent display during 2013; however from the UK the ringed planet has been rather low in the sky and this has presented some difficulties with observation and imaging. On a good night the planet has still had the ‘wow’ factor when viewed at the eyepiece but on many occasions the need to view it through a considerable depth of our own atmosphere, combined with the poor that weather the UK has ‘enjoyed’, has meant the view from the UK has often been spoiled by unsteady seeing conditions.  None-the-less some fascinating features have been on view. 

Saturn is moving into northern hemisphere summer: this means the North Polar Region (NPR) is tilted towards the Sun and we are able to get views of this region that have not been visible from Earth since Saturn’s last summer, some 29 years ago. When the Voyager 1 probe passed the planet in November 1980 it photographed a strange cloud pattern surrounding the NPR at around 78 degrees north latitude. This took the form of a hexagonal band of cloud with each side of the hexagon around 14,000 Km long. At the time this feature had not been glimpsed from Earth and it rapidly became invisible as the tilt of Saturn moved the NPR into the permanent dark of the Saturnian winter. When the Cassini probe arrived at the planet in 2006 this region was still un-lit however the infra-red cameras were able to pick out the hexagonal pattern due to variations in heat within the polar cloud formations. Since 2009 this region has moved slowly into daylight and been visible from the probe in normal light; now it is, just about, visible from Earth.

The accompanying diagram shows the region of the hexagon surrounding the NPR as well as highlighting the major rings and divisions in the rings that have been on view this year; the top photograph show an example of what has been captured by amateur observers. Not surprisingly the clearest images have come from observers further south than the UK, who can view Saturn higher in the sky where our atmosphere has less chance to distort the view. Milika and Nicholas are observing from Sedan in South Australia and have given permission for their image to be used here. They report that seeing conditions were good but not excellent however their image shows very well all the major features on view including the bizarre and still mysterious hexagon feature. This cloud pattern is rotating with a period of some 10 hours and 40 minutes, matching the deep rotation rate of the interior of this gas-giant, so the number of visible points of the hexagon is varying through the observing period and makes this a fascinating target to observe.

 The lower image was taken by Martin Lewis from St.Albans in the UK with a 222mm Dobsonian telescope and a DMK21 camera and it shows that UK observers can still capture all the major features on view when the seeing is steady, despite the low elevation of the planet. The Cassini probe has recently discovered a major hurricane-type storm settled over the North Pole of Saturn and as this region tilts slowly towards us this storm may even become visible from Earth.  

In 2014 the North-Pole of Saturn will be tilted around 5 degrees further towards us and, despite a low elevation in the UK sky this will give further opportunity for observation of this strange weather feature.


Added by: Alan Clitherow