## Observer’s Challenge – Measuring the size of the Great Red Spot

At the 2019 Winchester week-end, Paul Abel and Pete Lawrence presented a series of observing challenges which members might like to have a go at during the year. Some are relatively easy, some really are quite challenging indeed.

Jupiter’s Great Red Spot (GRS) is a vast storm which sits in Jupiter’s southern equatorial belt (see figure 1). To measure the GRS you’ll need the free software WINJUPOS. You’ll also need at least a 4 inch telescope to see it. You can use a blue filter to help darken the spot and make it easier to see.

Check when the GRS is visible; the aim is to record the times (in UT) that the preceding edge and the following edge appear on Jupiter’s central meridian (CM), an imaginary line connecting the north and south poles.

When the preceding edge (see figure 2) is on the CM, record the time in UT and put the date and time into the fields in the WINJUPOS programme, WINJUPOS will then give you the CM2 value:

You’ll need to wait about 12 minutes or so for the other end of the spot to cross the CM (see figure 3); when this happens record the time and put this into WINJUPOS to get a CM2 value for the other edge:

We can now get the length of the spot in CM2 longitude by subtracting the two values:

• Length (CM2) = 313.8° − 296.2° = 17.6°

Finally, multiply this length by 1187 to convert CM2 longitude into kilometres:

• Length (km) = 17.6 × 1187 = 20891.2 km

The value here is a bit too large (because we’ve used a WINJUPOS graphic rather than a real observation) but the principle is the same. If you do this a number of times, you can work out if the spot is shrinking or staying the same size.

Please do post any observations you make to your BAA Member Page

Paul G. Abel & Pete Lawrence

[Thumbnail image by Geof Lewis]

BAA Members can access the 2019 BAA Handbook here.