Dear Teachers<br/><br/><br/><br/>1. There is a sun in the sky.<br/><br/>2. There is the sun in the sky.<br/><br/>3. When the sun rises or sets, it looks much bigger.<br/><br/>4. When a sun rises or sets, it looks much bigger.<br/><br/>Which are correct? <br/><br/>When should I use <b>the sun or a sun? </b><br/><br/><br/><br/>Thanks a lot.