The Great Red Spot

Jupiter in the early hours of the 11th February 2016 with the Great Red Spot (GRS) on the limb. Taken from our backyard using my QHY5-11 planetary camera and my 127mm Meade apo-refracting telescope with a x2.5 Barlow lens

The Great Red Spot is a meteorological feature in the Jovian atmosphere, an anticylcone of stupendous proportions that has been raging since 1878. In all probability the GRS has been in existence for much longer, with some evidence that it was first observed by Giovanni Domenico Cassini in 1665 or by Sir Robert Hooke in 1664.  As after the 17th century, there was a long break in documented observation, it is uncertain as to whether the earlier sightings were either real or related to some other shorter termed phenomenon.  I'm a big 'Hooke' fan so I like to think the 'great man' did see the 'Great Red Spot'!

The Great Red Spot can be seen on my above image sitting  in a hollow within the South Equitorial Belt on the eastern limb of Jupiter's disc. This year the GRS is much redder than  in recent previous years but it is clearly shrinking in size. In the late 19th century the Great Red Spot measured more than 40,000km across and took more than an hour to cross Jupiter's central meridian. It is currently only 16,000km wide and transits the planet's central meridian in approximately 20 minutes.

A number of white ovals can be seen in the South Temperate Belt. Like the GRS these are storm systems within the Jovian clouds.

Jupiter has a fast differential rotation. If you look closely at my image you can see that the disc is not completely circular. It is instead 'oblate' that is, its diameter measured along its equator is larger than that measured through its poles. Jupiter is a gas giant, so as it rotates-faster at its equator, centrifugal force acts to increase its diameter. The rotation is 'differential' in that the central portion (only the Equatorial Zone) moves faster than the rest (including the North and South Equatorial belts and Polar Regions).  The central zone, known as rotation System 1 has a rotational period (once around) of 9 hours 50 minutes whilst the rest of the planet known as rotation System 2 has a rotational period of 9 hours 56 minutes.  A differential of 6 minutes doesn't seem a lot but it is enough to be noticeable, with meteorological features appearing to move relative to each other from night to night.

I never tire of looking at and imaging Jupiter because it is such a dynamic world of gargantuan proportions. I
 particularly enjoy spotting the GRS. Question is for how much longer will it exist? At the current rate of shrinkage the GRS will be circular by 2040. Whatever happens, I will cease to exist before it does!  But I would like to think my grandchildren and great grandchildren might be able to point a telescope at the King of Planets and see the Great Red Spot appear around  from behind Jupiter's limb just as Sir Robert Hooke probably did  352 years ago!

Time lapse monochrome image (taken through a blue filter every 10 hours) of the GRS on the Jovian disc from the Voyager 1 spacecraft over 28 days in 1979: NASA-JPL (The fast moving  flashes and black dots in this image are caused by Jupiter's moons and their shadows)

Credits: NASA/JPL, Wikipedia and Astronomy Now