The Erythraean Sea is one of the names found in ancient cartography

Cartography

Cartography is the study and practice of making maps. Combining science, aesthetics, and technique, cartography builds on the premise that reality can be modeled in ways that communicate spatial information effectively.The fundamental problems of traditional cartography are to:*Set the map's...

. This name may have derived from the seasonal blooms of the red-coloured Trichodesmium erythraeum

Trichodesmium

Trichodesmium, also called sea sawdust, is a genus of filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters . Trichodesmium fixes atmospheric nitrogen into ammonium, usable also for other organisms...

near the water's surface, as the Greek "Ερυθρά" actually means "red".

In the third century, Flavius Philostratus made this comment: "And they say that the sea called Erythra or "red" is of a deep blue colour, but that it was so named, as I said before, from a King Erythras, who gave his own name to the sea in question."

Use of the name

The name 'Erythraean Sea' has been or is still used for the following places:

• As a name for the Red Sea
  Red Sea
  The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...
  
  , especially after the 19th century. The modern country of Eritrea
  Eritrea
  Eritrea , officially the State of Eritrea, is a country in the Horn of Africa. Eritrea derives it's name from the Greek word Erethria, meaning 'red land'. The capital is Asmara. It is bordered by Sudan in the west, Ethiopia in the south, and Djibouti in the southeast...
  
   was named after this ancient Greek name.
• In centuries past the name 'Erythraean Sea' was applied by cartographers to the whole NW part of the Indian Ocean
  Indian Ocean
  The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
  
  , mainly the area around Socotra
  Socotra
  Socotra , also spelt Soqotra, is a small archipelago of four islands in the Indian Ocean. The largest island, also called Socotra, is about 95% of the landmass of the archipelago. It lies some east of the Horn of Africa and south of the Arabian Peninsula. The island is very isolated and through...
  
  , between Cape Guardafui
  Cape Guardafui
  Cape Guardafui , also known as Ras Asir and historically as Aromata promontorium, is a headland in the northeastern Bari province of Somalia. Located in the autonomous Puntland region, it forms the geographical apex of the region commonly referred to as the Horn of Africa.-Location:Cape Guardafui...
  
   and the coast of Hadhramaut
  Hadhramaut
  Hadhramaut, Hadhramout, Hadramawt or Ḥaḍramūt is the formerly independent Qu'aiti state and sultanate encompassing a historical region of the south Arabian Peninsula along the Gulf of Aden in the Arabian Sea, extending eastwards from Yemen to the borders of the Dhofar region of Oman...
  
  . This name has now become obsolete and the name Gulf of Aden
  Gulf of Aden
  The Gulf of Aden is located in the Arabian Sea between Yemen, on the south coast of the Arabian Peninsula, and Somalia in the Horn of Africa. In the northwest, it connects with the Red Sea through the Bab-el-Mandeb strait, which is about 20 miles wide....
  
   is presently used, although for a smaller area. In maps where the NW Indian Ocean is named thus, the Red Sea appears as "Arabian Gulf". In the Periplus of the Erythraean Sea
  Periplus of the Erythraean Sea
  The Periplus of the Erythraean Sea or Periplus of the Red Sea is a Greco-Roman periplus, written in Greek, describing navigation and trading opportunities from Roman Egyptian ports like Berenice along the coast of the Red Sea, and others along Northeast Africa and India...
  
  , as well as in some ancient maps, the name of the sea refers to the whole area of the northwestern Indian Ocean, including the Arabian Sea
  Arabian Sea
  The Arabian Sea is a region of the Indian Ocean bounded on the east by India, on the north by Pakistan and Iran, on the west by the Arabian Peninsula, on the south, approximately, by a line between Cape Guardafui in northeastern Somalia and Kanyakumari in India...
  
  .
• In the opening sentences of Herodotus
  Herodotus
  Herodotus was an ancient Greek historian who was born in Halicarnassus, Caria and lived in the 5th century BC . He has been called the "Father of History", and was the first historian known to collect his materials systematically, test their accuracy to a certain extent and arrange them in a...
  
  's history he refers to the Phoenicians having come originally from the Erythraean Sea.
• The name "Erythraean Sea" was used as well to refer to some gulf
  Headlands and bays
  Headlands and bays are two related features of the coastal environment.- Geology and geography :Headlands and bays are often found on the same coastline. A bay is surrounded by land on three sides, whereas a headland is surrounded by water on three sides. Headlands are characterized by high,...
  
  s attached to the Indian Ocean, specifically, the Persian Gulf
  Persian Gulf
  The Persian Gulf, in Southwest Asia, is an extension of the Indian Ocean located between Iran and the Arabian Peninsula.The Persian Gulf was the focus of the 1980–1988 Iran-Iraq War, in which each side attacked the other's oil tankers...
  
  
• Nowadays the name is applied to a large dusky region on the surface of planet Mars
  Mars
  Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
  
  , known as Mare Erythraeum
  Mare Erythraeum
  Mare Erythraeum is a very large dark dusky region of Mars that can be viewed by even a small telescope. The name comes from the Latin for the Erythraean Sea, because it was originally thought to be a large sea of liquid water. It was included in Percival Lowell's 1895 map of Mars.-External links:*...
  
  .