Flawed
There's really not much accuracy involved in this game and it's not much fun either. The reload time on the cannon is too long... later in the game, choppers come at such fast pace that it is impossible to even have the chance to hit them because you're still reloading--that's not difficulty, that's poor mechanics. To spice things up you should make the cannon change direction via the mouse. This would slightly alleviate the problem of having no chance whatsoever to hit the choppers once they pass you. Speaking of which, I don't see how missing a chopper relates to losing health. Allowing aiming with the mouse would open options like, having the choppers shoot homing missiles which you could also shoot, and if they hit you THEN you lose health. Overall not too much of a "game"...
Also, just some crits on the graphics: if you're take the effort to include night and day, why not go further and include an orbiting sun and moon? The sun does not turn into the moon at night :)