CGEL by Huddleston and Pullum says on page 171:
8.3 Non-aspectual uses of the progressive
(a) The progressive futurate
 i a. The sun sets at five tomorrow. b. #The sun is setting at five tomorrow.
The progressive is restricted to cases where human agency or intention is involved – hence the anomaly of examples like [ib].
By the last line, I think, they mean that the progressive futurate is restricted to cases where human agency or intention is involved. Right?
Also, I was wondering why human agency or intention would need to be involved to express future time with the progressive. Could someone articulate the reason for this?
Finally, I'd like to know if you could ever use these other variations:
c. The sun is going to set at five tomorrow.
d. The sun will set at five tomorrow.
e. The sun will be setting at five tomorrow.