You're thinking of the Prime Meridian -- 0 degrees, and the International Date Line -- 180 degrees. Both are lines of longitude.
A line running north and south/ up and down is called a vertical line.
A geodesic line connecting the north and south poles.
Yes, every line of longitude runs from the North Pole to the South Pole.
the equator
The difference is 180 degrees of longitude. They're two different [imaginary] lines on the earth's surface, directly opposite each other. The Prime Meridian connects the north and south poles and runs through Europe and Africa. The International Date Line also connects the north and south poles, but runs down the other side of the earth, through the Pacific Ocean. If you stand at any point on either one of them (except at the north or south pole), then you can't see any point on the other one.
That could either The Prime Meridian or the International Date Line.
Your thinking of the Prime Meridian, which terminates at the poles, and which connects to the International Date Line -- a non-straight line for political reasons, that essentially mirrors the PM, pole to pole.
A line running north and south/ up and down is called a vertical line.
A line of longitude.
equator
A geodesic line connecting the north and south poles.
Yes, every line of longitude runs from the North Pole to the South Pole.
No. It has two ends, at the north and south poles.
Those are "meridians of longitude".
the equator
I thought they were.
The difference is 180 degrees of longitude. They're two different [imaginary] lines on the earth's surface, directly opposite each other. The Prime Meridian connects the north and south poles and runs through Europe and Africa. The International Date Line also connects the north and south poles, but runs down the other side of the earth, through the Pacific Ocean. If you stand at any point on either one of them (except at the north or south pole), then you can't see any point on the other one.