This app was mentioned in 5 comments, with an average of 5.40 upvotes
Hey Speaking of the rotation of the earth, a few years ago I wrote a free app for reddit that will calculate how long of an exposure you can use before the earths rotation will cause streaking.
https://itunes.apple.com/us/app/dark-skies/id843351673?mt=8
There is an android version but admittedly it is not as nice as the iOS version (I'm a better iOS developer than Android)
https://play.google.com/store/apps/details?id=com.strykeout.darkskies&hl=en
Enjoy ~
Nice. My next purchase is actually gonna be the 35mm Rokinon. Also, I double check my exposure times with this app https://play.google.com/store/apps/details?id=com.strykeout.darkskies, just to be sure the star trails are kept to a minimum. It says your 35mm can do 10secs with a crop sensor camera. Probably want to shoot a f2.4 or above to keep the stars a little sharper. I'm sure a full frame will allow a little bit longer exposure time as well. Hope you get some clear skies!
There's an App for that. https://play.google.com/store/apps/details?id=com.strykeout.darkskies&hl=en