N-SING 赤道 The equator is an imaginary line around themiddle of the earth at an equal distance from the North Pole and the SouthPole.
这里用了an.
The equator is an imaginary line around the middle of the earth at an equal distance from the North Pole and the South Pole.
主语和表语互换句法功能，即让表语作主语，主语作表语：
An imaginary line around the middle of the earth at an equal distance from the North Pole and the South Pole is the equator.
句子成立。
由此可知，这个表语为确认性表语。确认性表语也可以用不定冠词，只不过较少而已。
答疑网有不少关于确认性表语、归属性表语的问答，你应该参读。知道了基本定义后，这本来不该成为问题的。
