How far away is that bird? White-winged Scoter at Musselburgh

 There's an American White-winged Scoter out there somewhere amongst the Velvet Scoter but just bobbing up above the waves for a split second at a time. Reports said 300m off shore but it looked a lot more to me. So I decided to work out how to apply the little finger trick to my telescope. The island of Inchkeith subtended half a little finger to my eye, which is about half a degree but through the telescope it took up about half the field of field.

 Conclusion is that the field of view at 40x is one degree or 0.0175 radians. Well, let's say 0.02 radians as this is all very guesstimated. I tried to photograph a duck in the distance but all I managed was a gull left of centre below. Measurements suggest it subtends about 0.0004 radians. Wingspan is about 1.20m. By the arc length rule, distance is therefore 1.20/0.0004 = 3000m. This might be inaccurate but it is clearly more than the 300m quoted. Inchkeith was 10km away as measured on a map. I can only dream of seeing the ducks that close!