Hey there! I'm happy to help!
Meridians of longitude identify degrees East and West of the Prime Meridian. This line is located at 0° longitude and the East and West coordinates are based around it.
These lines are scientifically based because mathematics is a field of science and these lines are based on angles they create with the selected line. If you draw a line from the Prime Meridian to the center of the Earth and then you take a line from another point at the same latitude and connect it to the point in the center of the Earth, that angle is what is used to describe the given point on Earth's surface used to make the angle with the Prime Meridian.
Have a wonderful day and keep on learning! :D
