Radiocarbon dating is used to determine the age of carbon-bearing material by measuring its levels of radiocarbon, the radioactive isotope carbon-14. Invented by Willard Libby in the late 1940s, it soon became a standard tool for archaeologists. Radiocarbon is constantly created in the atmosphere, when cosmic rays create free neutrons that hit nitrogen. Plants take in radiocarbon through photosynthesis, and animals eat the plants. After death, they stop exchanging carbon with the environment. Half of the radiocarbon decays every 5,730 years; the oldest dates that can be reliably estimated are around 50,000 years ago. The amount of radiocarbon in the atmosphere was reduced starting from the late 19th century by fossil fuels, which contain little radiocarbon, but nuclear weapons testing almost doubled levels by around 1965. Accelerator mass spectrometry (apparatus pictured) is the standard method used, which allows minute samples. Libby received the Nobel Prize in Chemistry in 1960.

Read more: https://en.wikipedia.org/wiki/Radiocarbon_dating

_______________________________ Today's selected anniversaries:

1916:

First World War: Russian troops won a victory in the Battle of Kostiuchnówka, with the Polish Legions playing a key role on the Austro-Hungarian side. https://en.wikipedia.org/wiki/Battle_of_Kostiuchn%C3%B3wka

1941:

Second World War: After a three-month siege, the Italian garrison of Saïo (in modern-day Ethiopia) surrendered to Belgian, British and Ethiopian troops. https://en.wikipedia.org/wiki/Siege_of_Sa%C3%AFo

1971:

After visiting several Asian communist countries, Romanian leader Nicolae Ceaușescu gave a speech on a number of neo-Stalinist and socialist-realist ideals, which became known as the July Theses. https://en.wikipedia.org/wiki/July_Theses

2013:

Gunmen attacked a secondary school in Mamudo, Yobe State, Nigeria, killing 42 people, mostly students. https://en.wikipedia.org/wiki/Yobe_State_school_shooting

_____________________________ Wiktionary's word of the day:

aphelion: (astronomy, also figuratively) The point in the elliptical orbit of a comet, planet, or other astronomical object, where it is farthest from the Sun. https://en.wiktionary.org/wiki/aphelion

___________________________ Wikiquote quote of the day:

Within the body there are billions of different particles. Similarly, there are many different thoughts and a variety of states of mind. It is wise to take a close look into the world of your mind and to make the distinction between beneficial and harmful states of mind. Once you can recognize the value of good states of mind, you can increase or foster them.   --Tenzin Gyatso, 14th Dalai Lama https://en.wikiquote.org/wiki/Tenzin_Gyatso,_14th_Dalai_Lama