Might I just interject into the thread that this sub-forum is for 'spotters' and therefore the likelihood of naive/stoopid/ridiculous questions (from the perspective of the professionals) is higher than elsewhere on pprune :-) I recall a situation over East London a few years back where a newspaper showed 2 aircraft 'almost colliding', when in fact to most on pprune it was obvious that one was going into Southend, the other holding before heading into LHR and they were more than the required 1000ft apart. As an aside I was in the jumpseat when another aircraft flew slowly past us to the side. On commenting to that it seemed closer that 1000ft, the captain informed me that the measurement was from the centre of our fuselage to theirs and with the larger aircraft with longer wingspans the distance was shorter and more impressive visually.