I normally stack lots of short exposures. Due to the limitations of my telescope, I need to spend a lot of time re-centering whatever I’m taking pictures of. This is OK for star clusters or galaxies or nebulae. I can spend hours collecting as many frames as I like. These objects barely change over a period of hundreds of years. So I did the same with Jupiter.

This picture was taken on 21 Dec at about 2 am. It’s a stack of about ten frames. It’s OK, but I thought I could improve it by stacking more exposures. But it seemed that the more frames I tried to stack, the worse the result became. What’s going wrong?

It took me ages to figure out the reason: Jupiter has changed! It may seem obvious, but the planet Jupiter rotates. In fact it rotates fast. Despite being large enough to swallow 1,000 earths, it’s day only lasts 10 hours.

This adds another complication when you’re trying to photograph it. For a planet that’s rotating fast it means I don’t have much time. Here’s the same picture on the right. This time contrasted with another, taken 20 mins earlier.

The planet does a full rotation of 360 degrees in 10 hours.
36 degrees in an hour.
12 degrees in 20 mins. 12 degrees is a huge amount.

Even in these low resolution, blurry images you can see that cloud features have moved significantly in 20 mins. Here’s the same comparison again. This time there’s a yellow rectangle highlighting a small collection of three greyish clouds against a brown background. The same three clouds are visible on the right hand image too but have clearly moved.

If only the planet would just stay still while I photograph it!