We were going along fine there until we got almost to the end of the question.
That's where we found that you too are one of the huge number of people who
thinks that there's a standard set of "lines".
There isn't. Some maps and globes have some lines of constant latitude and
longitude printed on them, and some don't. The ones that do may have them
printed every 45 degrees, or every 30 degrees, or every 20, 15 or 10 degrees.
It's completely up to the publisher of the globe or the map. The mapping software
that I use will print a line every 2 seconds of longitude if I want them. That would
be 648,000 lines of longitude around the world.
If the International Date Line had not been zigged and zagged to avoid splitting
states, countries, or islands into two different calendar dates, it would have
exactly followed the meridian of 180 degrees longitude ... directly opposite the
Prime Meridian and 180 degrees away from it, both east and west.
180 of them.
The Midnight Meridian refers to the line of longitude directly opposite the Prime Meridian, marking the halfway point between the International Date Line and the Prime Meridian. It is where each calendar day begins.
There are two. They are the Greenwich Meridian and 180 degrees longitude, the originally defined International Date Line.
The line of longitude for a place that is halfway between the prime meridian and the 80th line of longitude and west of the prime meridian would be -40 degrees longitude. The prime meridian is at 0 degrees, and halfway between 0 and 80 is 40 degrees. Since the location is west of the prime meridian, the longitude would be measured as negative.
The line of longitude where one day ends and another begins is called the International Date Line. It roughly follows the 180° meridian in the Pacific Ocean and marks the transition between calendar days.
180°
180 degrees
180 degrees
180 of them.
The Midnight Meridian refers to the line of longitude directly opposite the Prime Meridian, marking the halfway point between the International Date Line and the Prime Meridian. It is where each calendar day begins.
Morocco
180 degrees of longitude.
90 degrees west longitude
There are two. They are the Greenwich Meridian and 180 degrees longitude, the originally defined International Date Line.
24 Hours. When you pass the international date line which is the meridian of 180 longitude, you add a day if going west or subtract a day going east.
The line of longitude for a place that is halfway between the prime meridian and the 80th line of longitude and west of the prime meridian would be -40 degrees longitude. The prime meridian is at 0 degrees, and halfway between 0 and 80 is 40 degrees. Since the location is west of the prime meridian, the longitude would be measured as negative.
If you take a globe or a map and draw a line through all the points that haveexactly the same longitude, the line you get is the meridian of that longitude.