Image Credit: Paul Whitfield, Pete Lawrence, Patrick Moore x 2, Paul Abel
PDFs for this ‘how to’ are available here.
Observing Jupiter is one of the most rewarding activities you can do with a telescope.
It’s the largest planet in the Solar System and it also has the largest disc (never less then 30 arcseconds in diameter) seen through a scope.
While a 3-inch refractor with 130x magnification will show the main equatorial belts and the Great Red Spot, with a 6-inch scope more belts become visible and irregularities along their edges start to show.
Under good seeing conditions, allow 25x to 30x magnification for each inch of aperture.
Larger scopes will reveal a wealth of detail, and possibly the stages of the SEB’s return.
In the months following previous fadings, a bright spot began spouting blobs of material, which were stretched into filaments and ovals to form a new belt.
Jupiter has an obviously flattened disc in the eyepiece – wider around the equator than the poles.
This is caused by the same thing that’s behind the pattern of zones and belts: its rapid rotation.
But Jupiter doesn’t rotate like a solid body.
Visible features in its equatorial region move more rapidly than in the higher latitudes, due to a fast moving jet-stream in this central region.
To account for this double-speed rotation, two different systems of longitude are used when recording the easterly or westerly position of features.
System I of longitude covers anything lying about 10° either side of the equator, and it completes one rotation in 9h 50m 30s.
The rest of the planet is covered by System II of longitude, which completes one rotation in 9h 55m 40s.
As recent events show, Jupiter’s features change rapidly, so your observations can have real value.
The British Astronomical Association’s Jupiter Section collates observations to work out the processes occuring in the planet’s layers.
The longitude of spots and storms is one such observation you can submit.
Longitude is measured as a feature travels across the ‘central meridian’, an imaginary line running down the centre of Jupiter’s disc.
You need to note the time a feature crosses the line (the ‘transit time’) to the nearest minute.
You can then convert the transit times for each spot into System I or II longitudes using software such as the free WinJUPOS.
Once you’ve recorded enough transits for a feature, plot them in a chart of longitude against date.
This reveals how much a spot is moving and keeps track of the jet streams in the planet’s atmosphere.
What you’ll need
Eyepieces – A selection of good planetary eyepieces, giving magnifications of 180x or more, to get the maximum power that the seeing conditions will allow.
Logbook and pencils – A hardback book in which to collate your observations, plus pencils from B to 6B for sketching.
Sketching blanks – These should show Jupiter’s flattened disc correctly. By convention, they have an equatorial diameter of 64mm and a pole-to-pole diameter of 60mm. You’ll find some pre-prepared to print out on the coverdisc this month.
A spotless record?
Sketching the disc is a good way to visualise your recordings.
Spend 10 minutes looking at the planet and then draw the equatorial belts and the poles on a blank of Jupiter’s flattened disc.
You can then fill in other bands and details. Because of Jupiter’s rapid rotation, don’t spend more than 15 minutes on a sketch.
Features on the planet also change colour and brightness over time.
Keeping track of these changes provides an insight into the workings of Jupiter’s chaotic atmosphere.
The intensity of a feature is estimated by giving it a number from 0 (the brightest) to 10 (a black area).
As these are estimates, they have to be done systematically and repeatedly to be of any value, so that any changes can be tracked through the observations you’ve built up.
With the disappearance of one of Jupiter’s main belts, there’s rarely been a more interesting time to take a closer look at the gas giant.
With this guide, maybe you’ll be the first to spot the belt’s return.
Spend 10 minutes looking at Jupiter in the eyepiece.
This is a good time to find the optimal magnification, which suits the seeing conditions favourably.
Magnifications of 180x or more in average seeing will show the main belts and bright zones.
Using the transit form and weather features pdf files available here, make a sketch of Jupiter.
Afterwards, examine the belts and zones for any of the weather features listed.
On the sketch, mark any feature for which you want to make a transit timing with a number.
When your chosen feature is on the central meridian, record the time in UT.
This is the transit time.
Next, make an intensity estimate to record changes in brightness by noting down the feature’s brightness between 0 (the brightest) and 10 (a black sky).
After observing, convert your transit timings into the System I or II longitudes.
Use the table of Jupiter ephemerides in the BAA handbook, the free software WinJUPOS, or an online calculator (www.arksky.org/JupCMCalc.html).
Note the longitudes on your form.
Repeat the process as often as possible.
The aim is to achieve repeated observations of the same features so that your records can be used for scientific analysis.
Remember to transfer your observations (drawings, transit times etc) to a logbook after each session.
For each date, give the equipment, magnification and seeing.
For each feature, give an intensity average and the number of times you observed it.
Send your observations to the BAA.
This ‘How to’ originally appeared in the July 2010 issue of BBC Sky at Night Magazine.