The Sea of Tranquility (Mare Tranquillitatis in Latin) is one of the most recognized features on the surface of Earth’s Moon. This vast, dark plain is situated prominently on the near side, meaning it constantly faces our planet and is visible to the naked eye. It forms a distinct patch within the northeastern quadrant of the visible lunar disc. This large, flat area is a remnant of the Moon’s ancient past, appearing darker than the surrounding terrain.
Defining the Lunar Mare
The term “mare” (Latin for “sea”) was given to these dark areas by early astronomers who mistook them for actual bodies of water. A lunar mare is a massive plain formed by ancient volcanic eruptions. These areas were created when large impact basins were flooded by molten material from the Moon’s interior billions of years ago.
The material that makes up the Sea of Tranquility is primarily basalt, a dense, dark-colored volcanic rock similar to what is found in ocean floors on Earth. This solidified lava is less reflective than the lighter, heavily cratered lunar highlands, causing the maria to appear dark when viewed from Earth. The mare material formed during the Upper Imbrian epoch, with samples suggesting a high metal content that may contribute to its slight bluish tint observed through specialized filters.
These immense volcanic plains cover about 16% of the Moon’s total surface area, with the overwhelming majority located on the near side. The Sea of Tranquility itself is one of the largest of these features, with a diameter of approximately 873 kilometers. The process of filling the original impact basin with numerous thin layers of lava flows resulted in the relatively smooth, flat surface that defines the mare.
Geographical Placement on the Moon
The Sea of Tranquility is located entirely on the Moon’s near side, making it a permanent fixture in the sky for Earth-based observers. Its central coordinates are generally cited near 8.5 degrees North latitude and 31.4 degrees East longitude. This position places it in the eastern hemisphere of the visible lunar face, near the Moon’s equator.
When looking at the full Moon, the Sea of Tranquility is discernible as a large, irregularly shaped dark patch. It is bordered by several other notable lunar features, including Mare Serenitatis (Sea of Serenity) to the northwest and Mare Fecunditatis (Sea of Fecundity) to the southeast. The boundaries of the basin are irregular because of the intersection of several other ancient impact basins. The total elevation difference across the smooth mare surface is surprisingly small, with less than a 500-meter variation between the highest and lowest points.
Historical Significance
The Sea of Tranquility gained its fame as the landing site for the Apollo 11 mission. On July 20, 1969, the Lunar Module Eagle touched down in the southwestern region of the mare, a location known globally as Tranquility Base. The specific landing coordinates for the first human visit to another celestial body were 0.67408 degrees North latitude and 23.47297 degrees East longitude.
The selection of this area was based on its relatively smooth and flat terrain, which offered a safe landing zone for the technology of the time. Astronaut Neil Armstrong took the first human steps onto the lunar surface at this spot, followed shortly by Buzz Aldrin.
The remnants of the Eagle’s descent stage, along with scientific instruments left behind, mark this precise location as a historical monument. To honor the crew, three small craters near the base were subsequently named Armstrong, Aldrin, and Collins.