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>
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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>
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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>
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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>