Which capital city is closest to the prime meridian?
And the answer: London.
Like the equator, the prime meridian is an imaginary line that divides the Earth, but instead of running east and west, it runs north and south. The prime meridian divides the Eastern Hemisphere from the Western Hemisphere, and runs through Greenwich England, which is a borough of London. It's considered to be 0 degrees longitude.
While meridians are lines that humans create as points of reference, they serve an integral purpose in astronomy. Essentially, when a meridian is established, it serves as the "zero" reference line for astronomers – of course, any good graph has a reliable zero value. By use of meridians, astronomers can create a reliable map of the sky and its stars.
Since the late 19th century, the prime meridian has been officially located in Greenwich, England. Before Greenwich was established as the point of reference, local towns and cities kept their own time; there was no set rule or standard. As national and international rail lines began to take form, the need to standardize time became grew. Since the United States and 72% of the world's commerce had already based their time zone off of that of Greenwich, it was adopted as the official prime meridian of the world.
Before standardized time, coordinating travel was, well, tricky. Oftentimes, governments would publish maps running the prime meridian through their capital city. For example, a French map would locate Paris at 0 degrees longitude, while a Chinese map would use Beijing. Sometimes, local maps would center their town or city along the prime meridian!