The mystery of Orbs

What is an Orb? Relatives of mine took a lot of pictures they suddenly showed up in, around christmas time. Their theory: spirits, coming together when there was celebration and family unions. My husband got an Orb beside him in two pictures, transparent but big. Could it be a long-dead relative coming to say hello?

Our relatives lives by a fjord. The place has an old folk tale attached: Two farmers were driving in a wagon by a horse. Near the "Roesbergura" the horse stopped, and would not go a step further. The men grew angry, and whipped the poor horse, but it stood still even then.
One of the men got an idea this could be something otherwordly, and looked through the steel ring in the horse´s headgear. Then he saw a great wedding crowd crossing the road in front of them. Bride and bridegroom, and the fiddler, and the families. They were small of stature, up to our knees in height, and all clad in blue. Their heads were big in comparison to their bodies.
When the wedding thong had passed, the horse went on by itself.

I have never seen Orbs before. Looks like shining old coins. Dust particles? Why are they only in some pics, not in all? This is an Unsolved Mystery!