Why is Lupin afraid of the floating silver orb?

[usergoogol at yahoo.com]

No: HPFGUIDX 26884

In Prisoner of Azkaban, the thing which Lupin is apparently afraid of is a floating silver orb.

> The legless spider had vanished. For a second, everyone looked
> wildly around to see where it was. Then they saw a silvery-white
> orb hanging in the air in front of Lupin, who said, "Riddikulus!"
> almost lazily.

and then...

> "I wonder why Professor Lupin's frightened of crystal balls?" said
> Lavender thoughtfully.

It really makes little sense, and I don't think it's explained at all during the concluding chapters, but I'll find out when I continue reading the books. What do YOU think, sirs?