I'm hoping that a great grandpa is just visiting....From Wikipedia:
The term orb is the popular name given to typically circular anomalies appearing in photographs. In photography and video, orbs appear to be balls, diamonds, or smears of light with an apparent size in the image ranging from a golfball to a basketball. Orbs sometimes appear to be in motion, leaving a trail behind them.
There are two main trains of thought regarding the cause of orbs in photographs. The first school sees it as a fairly clear-cut case of flash reflection off of dust, particles, insects, or moisture droplets in the air in front of the camera, i.e. they are naturalistic. The opposing school maintains that orbs are paranormal in nature, i.e. non-naturalistic and ghost like.
Some people claim that paranormal orbs are more likely to appear in certain locales, or are attracted to human actitivities, especially those involving children. The images on Internet fora devoted to orbs are taken in graveyards, backyards, attics, and kitchens, living rooms, as well as bars, convention centers, and city streets – in short, anywhere people may be taking photos. Locations in which orbs are frequently observed are often said to be associated with other 'supernatural activity' and haunting taking place.