BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply to thread

Message: <blockquote data-quote="Binastro" data-source="post: 3345586" data-attributes="member: 111403">Thank you Henry,I have always been surprised by the eye's ability to see wires against the sky.I don't know if any 'theory' would predict this beforehand.That is why I consider it essential that one defines what one is looking at when talking about resolution.Just idly talking about resolution out of context does not convey much.If not using the eye and just measuring optics resolution one is on safer ground.Incidentally, sunspot size is penumbral size, so that the sunspot is really grey in the outer parts, not black.Average good eyesight can, with experience, detect 38 arcsecond spots with a safe filter.However, on rare Venus transits, where the planet is 60 arcseconds diameter, it is much easier to see than a similar penumbral size sunspot. Venus appears totally black.Aircraft have hard edges and often are seen as black spots. If you look in front of contrails you can see aircraft at surprising distances.I saw a Mercury transit with exactly 3x safely filtered opera glass, so 12 arcseconds became 36 arcseconds.I am not sure if any human eye could detect it without optical aid with just a safe filter.16 arcseconds, maybe 15, but I am not sure about 12.Mercury is either 12 or 10 arcseconds transiting the Sun.</blockquote>