This morning the sky was partially cloudy but with patches of clear sky. Whilst standing out by my car at about 6:50 am I could see Jupiter very clearly in a clear patch of sky to the South. Just to the right of Jupiter there seemed to be another small dim object at the limit of what I was able to see. I didn't have time to go back in and get my binoculars or telescope but in my jacket pocket I had a set of car keys and on the key ring was my Tesco club card. I held my club card up to the sky narrow side on (about 0.5mm) and carefully moved it over Jupiter. With the glare from Jupiter diminished there next to Jupiter on the right was a faint spot of light. I checked the Shallow Sky website when I got back home later on about 3:30 pm and sure enough Ganymede's position was a good match for what I saw.
Checking on line I found that I am not the first person to see Ganymede as when it is at greatest elongation from Jupiter , as it was this morning, it is quite easy to see. See Denis Dutton's blog. http://www.denisdutton.com/jupiter_moons.htm
In fact the Chinese astronomer Gan De may well have seen Ganymede in 365 BCE. However, I bet he did not have a Tesco club card!!