The Royal Observatory at Greenwich has been one of the most important skywatching sites in history – not so much for what it told us about the stars, but for its role right here on Earth. Its location marked the starting point for measuring longitude – the position east and west on the globe. It also marked the time standard for the entire world: Greenwich Mean Time.

The observatory was established on today’s date in 1675, by King Charles II. It was built on a hill near London, overlooking the Thames.

Greenwich was created to provide highly accurate maps of the stars, and to measure the motions of the Sun, Moon, and planets. The work was designed to help sailors determine their longitude. Establishing longitude at sea was extremely difficult – and dangerous; many ships crashed because their navigators didn’t know where they were.

The observations also played a key scientific role: they helped confirm that the motions of the Sun, Moon, and planets were governed by Isaac Newton’s laws of gravity.

In 1833, the observatory began a “time service.” It dropped a ball from a tall pole at precisely 1 p.m. That allowed mariners to set their clocks before they sailed. Greenwich later transmitted the time to the whole country by telegraph, then radio.

The observatory was moved in the 20th century, and closed in 1998. Today, the Greenwich site is a museum – preserving an important part of world history.

Script by Damond Benningfield