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Like human skin... a litte sunlight is ok, in moderation.
UV in sunlight is an ionising form of radiation, meaning it has the power to break molecular bonds. This is the mechanism by which most of the damage is done. I imagine the first thing to happen would be fading to the knits and exposed lining. And it looks like the longer term effects to the leather has already been covered in detail. If non-ionising radiation is found to be an issue, I could be in a lot of trouble...
So my thoughts: don't let 'em laze in the sun like some tourist on a resort beach; get them out & active in all weather, riding on your back, sharing life's adventures.
fyi: i have an mint 1947 star g-1 that has knits [perfect condition] that have faded to a reddish color much like an aero a-2. the muton collar also is faded. interestingly, the goatskin hides are still dark, creaky and as i say, in pristine condition. when i first got the jac, i thought that the knits were replaced, as i have never seen reddish knits on a g-1. close inspection proved that the knits are indeed original. all of this led me to poring over as many wwll pix of g-1s as i could find, to see if other originals had reddish knits, and yup, this does seem to the case in a few jackets. dye lot? specific contract? dunno, but it looks sharp.
So my thoughts: don't let 'em laze in the sun like some tourist on a resort beach; get them out & active in all weather, riding on your back, sharing life's adventures.