What does 'Sacred dew' mean in the poen, "To the Evening Star"?

It's not really clear to me what 'Thy sacred dew' means. I think it should metaphorically represent someyhing too as it is with her sacred dew which Venus protects the sheep or the innocent. If it means something, then why is it 'sacred'? 

I've also had a confusion on what 'silver dew' means. I think it is some sort of power to make the flowers sleep. But it may be wrong... 

Just wonder if I could have the meaning of the above 2 phrases.

(Studying literature for GCE)

:-) Thanks a lot!!