[not enough space above!] (...the difference corresponding to the angular distance masked off from our visual field by three fingers held out at the end of an outstretched arm.

I agree this is tricky to translate as the original is only clear to someone who already understands the astronomical use of angular distances such as parsecs. I hope my version is slightly less opaque(!)

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Note added at 2003-02-18 22:13:44 (GMT)
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Two of you have, quite rightly, asked where the \"masked off\" came from, since there is no such phrase in the original. The basic problem is not vocabulary but that the original is - as you suggest - not very enlighteneing as an explanation to a non-expert since it assumes the understanding of the very concept (angular distance) it is supposed to explain. I thought that my addition made it more obvious that the distance concerned is that covered up (masked off - c.f. masking tape, mask drawings etc.) by the now famous three fingers. I also put in the visual field bit. Without these additions, I think that it may not be clear to a non-expert what the hell the three fingers are supposed to be doing stretched out into space. Given that, as far as I know, three fingers is not an SI unit, I assumed that the context is fairly informal and approximate, rather than an extract from an academic science journal and so open to greater freedom of translation. Anyway, I often find myself improving the original in translation (except legal docs, of course, even though they\'re usually so badly written). There\'s so much bad writing out there...