Of course we can always say "the sun is up", but is it acceptable to say "the sun is risen" just as we use verb "to be" before other past participles like "she is gone"? Or should we say "the sun has risen"? Does it make any difference?
Well, I'd have to say it depends on what you want to say with your word usage. If you mean it to sound almost poetic? 'The sun is risen' fits right in. So if say this is for a poem, or used in a context where it is meant to sound almost romantic, it fits just fine.
"Consider what your words say, not just what the words mean."
"The sun has risen" sounds more natural than "..sun is risen"
In this mode not all verbs will agree with "has".
"the man has fallen", good, but "the man is fallen" behaves like an adjective, the man is what? A fallen man.
In this form, the verb "fallen" is added to a noun kinda of lije a gerund.
"the company has risen to the challenge.." this form "has risen to..." is common but
"the company is risen to the challenge" sounds off and I think it's plainly wrong.