[BobSpence (OP)]

Here I go again, being 'picky'.[]

This instrument (the 4 black and silver vanes on a spindle in an evacuated glass vessel has been subject of some debate over the years,

Apparently, if the direct 'impact' of photons was what caused it to spin, the vanes would spin away from the shiny side, because there the photons are bouncing off and so experiencing a reversal of velocity which changes their momentum by up to twice their original amount, wheras on the dark side they get absorbed, and so simply impart all their momentum to the vane. This can be observed in highly evacuated containers and very low friction bearings, because the force is quite small.

It used to be thought that the usual motion in the opposite direction was due to the dark surface getting warmer and heating up the nearby gas molecules which hid the surface harder, but apparently its more subtle than that. Its apparently due to the thermal gradient around the edges of the vanes, and a bit more complicated to explain.

See this site:
newbielink:http://science.howstuffworks.com/question239.htm [nonactive]

Regards,
Bob S.