We see a rainbow when the sunlight hit the right droplets that are just in the right place to hit our eyes.While,other droplets send their sunlight somewhere else.But we actually don't see a full circle because the earth gets in the way.Moreover,the closer the sun is to the horizon more the arc we see.
The Earth has its own magnetic field , it acts like there is huge bar magnet buried inside,but this bar magnet is not on the earth rotational axis it is shifted by some angle.The south of the bar magnet is on the magnetic north and the south polarity is on the magnetic north(image).As opposite polarities attract the magnetic needle (north) is attracted to the south polarity in the magnetic north.That’s why the needle always points north.If you want to know “why earth has magnetic field?”go to letter E.