bpaj Welcome to our 'cabin'. The untrained eye does get confused by two objects moving in 3-D and I am an untrained eye - other than paxing for 45 years.
Last Tuesday, over France at FL380, there was another a/c lower than us and 180 in the opposite direction. Consequently, the closing (ground) speed was about 1,000mph - which made it all happen very fast! The other was probably 1,000ft lower.
This picture illustrates the parallel approach/departure mentioned by
wiggy, above.
Photos: Boeing 737-322 Aircraft Pictures | !!!!!!!!!!!!!!
bpaj, you might try an 'advanced search' of the forum to see other threads about this topic.
TS, when this thread has run it's course, it might one for the FAQ? I have seen this question raised a few times.