Yes, "Що на небі?" is correct too. But I'd say there's a different feeling about them.
"Що є на небі?" is actually closer to "What is there in the sky [in general]?" Answer: There are clouds, planes, etc.
"Що на небі?" sounds more like "What is in the sky [that we were talking about or in some context]?" Answer: The superman is in the sky.
Just different patterns like "There is [stuff]" and "[Stuff] is ...".
Funnily enough, "What is there in the sky" is not accepted in this exercise. Please consider applying to be a mod for the course :) there are probably thousands of unprocessed reports!
Whoa, well, I am not a native English speaker, so I wouldn't always know whether my English translation is correct. But actually I would love to contribute more. I applied before the course was open but was not accepted. I actually don't know how to apply now :) Just write to someone? There seems to be no such procedure on the website...
It's OK, the current course team also doesn't have native-level English :) about joining, usually you'd apply in the incubator, but I think this might be a special case, in that the team is almost inactive, so there's no guarantee they'll even read the application. If you are up to it, I suggest writing to Duolingo directly and bringing it up on various forums, so the course could be revived.
Well, yeah, I applied in the incubator before it was hatched and got an email saying "no we have enough people" (ok also in my application i wrote a lame pun/joke, so maybe that :D) I guess I could just contact one of existing moderators and ask. Maybe they really actually don't more help or help from me in particular. Thanks for suggestion and your comment! I'm happy that I can be useful :)