Tony Moss <[EMAIL PROTECTED]> writes: > This brought to mind an informal experiment I carried out this morning > with an enlarged version of my Meridian Alidade recently completed for > a US client. The instrument consists of pairs of vertical cylindrical > metal posts spaced 0.4mm apart. (see my webpage for a picture of the > standard model) The posts stand astride a fine centreline on a brass > protractor plate with one pair at each end of the line. > > This one-off custom version has extra high posts for use by a client > at low latitudes. In use, a sunray passing through the gap is aligned > with the central line and the other pair of posts is used as a sight > to lay out a meridian line after certain calculations are performed. > Because of the height-extended posts and low winter sun the sunray, > which diverges slightly for the reasons Bill has mentioned, passed > right across the protractor plate and between the second pair of > posts. Result?...a non-diverging line of light that crossed the whole > table in a very distinct line. > > There's a useful principle for a heliochronometer here I think.
Careful! It is true that you can produce an (almost) non-diverging line of light this way. (By the time the beam has traveled ten times the separation between the post pairs, it will have broadened by about ten times the gap between the posts, in this case to 4 mm. Simple geometrical optics.) But this line is not unique. You will get such a line if the instrument is aligned toward any part of the sun's disk. Do your experiment again (Or was Friday the one sunny day allowed in Britain this year?), but this time wiggle the instrument a little once you have a beam. You will find that you can make a beam anywhere within a few tens of a degree. (To be precise, 0.5 deg at sunrise and sunset, closer to 0.3 deg near noon.) --Art
