UFO investigator Russell Kellett of Filey was excited by this video footage shot by his friend Jody Holden, 23.
So he has sent it to Prof Roger Green of Warwick University for close analysis.
"Prof Green is open-minded about UFOs," said Russell, a member of the British UFO Hunters group.
"He can tell us what the object is not. He can eliminate things like aircraft, space debris and so on.
"Jody spotted this object in the sky above Abbot's Road, Selby, about 4pm on Mother's Day. It seemed about 200ft up and was perhaps a mile away.
"He said it had a purple/black colour and he saw it change shape from a castle into tank and then a shamrock and finally into a saucer over two minutes. He finally lost sight of it behind houses."
Russell says that Selby, along with Filey, is now one of Britain's top UFO "hot spots."
He also has video footage of balls of light seen over the sea at Filey. One of his group, a former RAF pilot, is convinced they are not terrestrial craft.
Russell says he has also seen flying triangles shooting out of the sea.
He now has a huge video collection of UFO sightings which he is thinking of auctioning on Ebay.
Anyone who wishes to make an early bid can contact him on 01723 514700.
howard.williamson@ypn.co.uk