While sailing of the Dorset coast on a NE course towards Portland Bill, I saw a light in the sky that I can't explaine

The object was a dot of light, similar to a star or a satelight, but could not have been either.  Stars do not move arround as this did and, satelights hold a steady course. Ionly noticed this point of light becauseits behaviour was very unusual.  It was able to move at great speed from a standing start and then stop abruptly.  No speeding up or slowing down. It did this aeveral times, in random directions over both land and sea . There was one other witness, a woman I am no longer in contact with.  I think she was a primary school teacher from New Zealand.  She hadn't been on a yacht at sea befor she was helping me on my watch, I was telling her about stars and satelights to calm her nerves, which was how we came to notice it.  I am not sure of the duration of time we could see it because a fleet of trawlers arrived and I had to consentrate  on sailing by them.  When I looked afterwards it was gone The boats name was ((name of boat deleted)) she was registered in London, I wrote the sighting down in the log book.  No one Ihave asked has any suggestions what it could have been.  I have not seen another UFO before or since.