On Mon, Sep 12, 2005 at 09:16:42AM -0700, keith_w wrote: > Godfrey DiGiorgi wrote: > > >On Sep 12, 2005, at 1:42 AM, mike wilson wrote: > > > >>>A "large parallel cylinder of light" is the same thing as light > >>>coming from an infinitely distant point source. Consider light from > >>>the sun: all rays are parallel at 92 million miles distance, unless > >>>scattered by atmosphere. In the vacuum of space, they are absolutely > >>>parallel. So even though the sun is several hundred times the > >>>diameter of the Earth and is a light source, it is a point light > >>>source. > >> > >> > >>Point of information: 8-) > >> > >>The sun's light rays are not parallel. If they were, then it would > >>appear the same size at whatever distance you saw it. Think of a > >>laser spot from a pointer. Apart from atmospheric diffusion and the > >>flaws of production, the spot should be the same at one foot, one > >>hundred yards or one mile. _Because the sun is so far away_, the > >>light rays are very close to parallel. But they are not. > > > > > >True, point taken. But they are very close to parallel, the deviation > >being extremely small across so short a distance as the diameter of the > >earth. I should have written "... all rays are 'effectively' parallel > >at 92 Milliion miles distance ... ". > > > >Godfrey > > He may indeed have scored a point, but the net result is, it doesn't > really matter one tiny bit. It's calculable, but not observable... > > No-one on earth can measure the difference in angles involved . .
Oh yes they can. In fact it's quite easy to do, even with the naked eye (as long as you use a piece of smoked glass or some other sun filter). There's around half a degree of difference in the angle of light from one side of the sun to the other - that's why the sun appears as a disc instead of a point light source.
