these small pieces have been going round the sun ever since. according to above context , could someone help me understand: 1) what is the difference between round and around? also 2)why ever is used in the above sentence?
"Round" and "around" are approximately equivalent, although round, or more properly 'round is more poetic.
Because "since" by itself leaves the reader/listener hanging. You could get a way with just
round the sun since.
but what's expected is either
round the sun ever since.
round the sun since then/that time.
"Since" indicates the beginning of a period. The astute reader/listener could figure out what that period is from previous context. But generally "ever" or "then" would be added to indicate explicitly what that period is.
Going round the sun
Going around the sun
Just about the same meaning, in this context: orbiting. Somehow around feels slightly more formal, but we often can use either word when describing motion.
As a UK English speaker I feel comfortable using either of these phrases
I went round the corner to the bookshop
I went around the corner to the bookshop
I believe that in the US around is more commonly used.
The work round does have other meanings such as "a round of golf" and also "King Arthur had a round table" here around does not work.
There are also specialised uses of around
I get around
meaning that I travel quite a lot, here round would not work; it's a rather idiomatic expression. However just to sow additional confusion the Beach Boys do sing:
Round, round get around, I get around ...
this I think shows how closely related the two words are.
As for ever since, it's a somewhat idiomatic expression, widely used.
The general idea is of unceasing action started at some point in the past. The planets began to orbit the sun and once started have never stopped. We use since to refer to some particular time in the past, in your example there will have been some earlier explanation for how the "small pieces" were formed and began to orbit. Considering the time since that event, the pieces have been continuously orbiting they have been orbiting