2017 April 23

Observer’s Challenge: Jupiter’s Great Red Spot

Paul G. Abel

Although the giant planet Jupiter has now passed opposition, it still continues to dominate our evening skies and we will be able to follow it for a few more months before it becomes lost in the sun’s glare as the time of solar conjunction occurs.

Jupiter’s vast size means that even small telescopes will reveal something of interest on the planet’s ever changing clouds. A view through a 3inch (75mm) telescope will show Jupiter as a somewhat flattened yellowish disk, crossed by two dark equatorial belts. Larger telescopes will reveal yet more belts which are separated by intermediate lighter zones. These belts are warmer regions deep inside the Jovian atmosphere, in contrast the bright zones are cold ammonia crystals high up in the atmosphere.

Since Jupiter is a gas giant, it has no solid surface and as a result the many storms which rage in the planet’s dynamic atmosphere can continue for many years – there being no land for them to hit and dissipate their energy. For over a hundred years now, the Jupiter section has been studying these storms as they provide important information about the extreme weather which occurs on Jupiter.

One such storm is the Great Red Spot – a vast hurricane which is about three times the size of the earth, and has been observed for over 200 years. Early drawings show that the spot used to be much larger and darker, but over the many years it has steadily been decreasing. 

Although the spot fades, at the time of writing it is quite prominent and has a distinctly dark orange hue. I have seen the spot in a 4inch (100mm) telescope, and it is a splendid sight in a larger telescopes – the drawing above is a recent one which shows the Great Red Spot and the turbulence which follows it in the Southern Equatorial Belt.

Since we have the planet for a few more months, now’s a good time to get your telescope trained on Jupiter to see if you can see this ancient storm. The table below gives some of the best times when you can catch the spot – at these times it’s preceding edge will be on Jupiter’s central meridian (an imaginary line connecting Jupiter’s north and south poles). 

If you’re having difficulty picking up the spot, try using a light blue filter – this will help darken the Great Red Spot and will help make it a little more obvious. Finally, don’t forget to record your observations – an image or a drawing is ideal, but a simple written account in a good old fashioned log book will also do. Make sure you record the date, time (UT), and telescope and magnification. I have been observing the Great Red Spot for over 20 years and never get tired of looking at it!

Date	Time (UT)	Time (BST)
2017 April 25	2121	2221
2017 April 27	2259	2359
2017 April 30	0037	0137
2017 May 02	2206	2306
2017 May 04	2344	0044 (05 May)
2017 May 07	2114	2214
2017 May 09	2252	2352
2017 May 12	2022	2122
2017 May 14	2200	2300
2017 May 16	2338	0038 (17 May)
2017 May 19	2108	2208
2017 May 21	2246	2346
2017 May 26	2154	2254
2017 May 28	2333	0033 (29 May)
2017 May 31	2103	2203

Paul G. Abel

To search for observations of Jupiter follow this link to the BAA Member Pages.
The BAA Jupiter Section website can be found here.