Who was the first person to write about the British Isles?

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The British Isles, tucked away in the northwest of Europe, has been inhabited by humans since Paleolithic times, but the people who lived there didn't develop a writing system until much later, and the first local account of the isles did not appear until Anglo-Saxon times, around the seventh century A.D.
So who was the first person to write about the British Isles and describe its inhabitants? To find out, we need to look to the south — to the Mediterranean world of the ancient Greeks.

A Greek mariner named Pytheas made the first recorded voyage to the British Isles in the fourth century B.C.
He circumnavigated the island of Britain, explored the northern lands of Europe and was the first to describe the Celtic tribes of Britain, the midnight sun, dramatic tidal shifts and polar ice. When he returned home, he wrote an account called "On the Ocean" ("Peri tou Okeanou" in Greek) that circulated widely throughout the ancient world and was read, discussed and debated by scholars for centuries.

Little is known about Pytheas. He was a citizen of Massalia, a Greek colony in what is now Marseilles in southern France, and it is uncertain whether he was a merchant or simply a gentleman scientist. "We can judge from his writings that Pytheas had a scientific education," Barry Cunliffe told Live Science. Cunliffe is an emeritus professor of European archaeology at the University of Oxford and author of "The Extraordinary Voyage of Pytheas the Greek" (Walker & Company, 2002).

Pytheas made a series of astronomical calculations of latitude during this journey with a device called a gnomon, which was an instrument similar to a modern-day sundial.
He accurately estimated the circumference of the British Isles — that is, the distance around the islands of what is now Great Britain and Ireland — placing it at approximately 4,000 miles, according to the Encyclopedia Britannica.

Pytheas wrote "On the Ocean" once he returned to Massalia. Until the writings of Tacitus and Julius Caesar some 300 years later, "On the Ocean" was likely the only source of information about Britain and the northern latitudes for most of the world, Cunliffe told Live Science. There were likely copies of Pytheas's work in the great libraries of Pergamum in what is now Turkey; Rhodes, Greece; and Alexandria, Egypt.

Unfortunately, "On the Ocean" has not survived. Only fragments of it remain, paraphrased or excerpted in the writings of other classical writers such as Strabo, Polybius, Timaeus, Eratosthenes, Diodorus Siculus and Pliny the Elder.
But the fragments we have are significant, Cunliffe said, as they contain a multitude of astronomical, geographic, biological, oceanographic and ethnological observations that have considerable scientific and anthropological significance.

full article:
https://www.livescience.com/first-western-description-british-isles