[time-nuts] gnat sizing

[Peter Vince]

Maybe I was brought up in a more genteel part of West London, but the unit of measurement I was brought up with was a 
Gnat's Whisker.  I don't think that was just my parents cleaning it up, as that expression seemed pretty common.  Sometimes 
abbreviated to just "a gnat's", but it was definitely its whisker that was understood.

Peter Vince  (G8ZZR, London, England)


On Wed Aug 19 22:49 , 'Lux, Jim (337C)' <james.p.lux at jpl.nasa.gov> sent:

>I realize we are straying afar.. but inquiring minds may wish to know.
>Referring to "A handbook of the gnats or mosquitoes: giving the anatomy and life history of the Culicidae" by George 
Michael James Giles, 2nd edition, 1902  (thank you google for digitizing this book from the Stanford library) The forward 
says that the second ed is much better than the first "...the result of a couple of months of constant work with the 
microtome." So I think we can consider this a reliable reference.
>
>Now I readily confess that this book seems devoted to only members of family Culicdae, and it's not clear that when 
referring to gnat anatomy as an unit of measure whether these are the gnats being referred to. The common name gnat 
seems to be applied to many small (often biting) Dipterid Insects, and Wikipedia seems to restrict the gnat terminology to 
other families.
>
>It would appear that the rectum of the gnat is about 1/10th the diameter of the abdomen (there's a drawing of a transverse 
section of the abdomen on page 91). If the page is about 6" wide (judging from the type size, and the image of the checkout 
card in the back page this is reasonable.. it's probably octavo size), then the 100x drawing is 2" across, so that rectum is 
.002 inches across (call it 0.05 mm, or 50 microns) .  This is much larger than the 1E-4 inches (2.5 microns) previously 
cited, but well within the range for human hair diameters (given as 17 to 181 micron in a variety of online sources, but a 
much smaller range of 50-90 micron is cited in "Forensic Examination of Hair", albeit for scalp, J. Robertson, Ed.)
>
>Now, to return to the original question of position accuracy for your timing receiver.  Whether 50 microns will result in a 
significant timing error? 1 nanosecond is 300 mm light time. 300 microns is 1 picosecond, so that 50 micron position error 
is down in the femto seconds..
>
>
>James Lux, P.E.