My aunt is an award-winning German translator, whose translations include Hitler’s Bunker: The Final Days of the Third Reich and Europa, Europa. While we were together, she was working on a book called Lexicon des Unwissens, not unlike a German version of Schott's miscellanies, those perennial bathroom-reading favorites. Why in this day and age of Wikipedia, I queried, would anyone want or need such a book? Well, she answered, it's precisely because you don't know what you don't know and until you know to look it up, well......
This is why I thought that someone might find it as useful as I did to know the difference between a Clementine and a Satsuma (also sometimes spelled Tsatsuma). Mandarins are now in season and they are deliciously sweet, seedless and easy to peel. I also like the challenge of having to consume the entire wooden crate-full as they are generally sold in bulk. I've included the links to Wikipedia but there's lots more on the web about both.
The short answer is: Clementines are the variety of mandarins from North Africa. The varietal is about 100 years old. Satsumas are the variety of mandarins from Asia and Asia Minor/Turkey and are more than 700 years old. The largest producer of mandarins in the world is China.
So what's a mandarin? Well, according to Wikipedia:
The Mandarin orange or mandarin is a small citrus tree (Citrus reticulata) with fruit resembling the orange. The fruit is oblate, rather than spherical, and roughly resembles a pumpkin in shape. Mandarin oranges are usually eaten plain, or in fruit salads. Specifically reddish orange mandarin cultivars can be marketed as tangerines, but this is not a botanical classification. The mandarin has many names, some of which actually refer to crosses between the mandarin and another citrus fruit. Most canned mandarins are of the satsuma variety, of which there are over 200 cultivars. Satsumas are known as mikan in Japan. One of the more well-known satsuma cultivars is the "Owari", which ripens during the late fall season in the Northern Hemisphere. Clementines, however, have displaced satsumas in many markets, and are becoming the most important commercial mandarin variety. The tangor, which is also called the temple orange, is a cross between the mandarin and the common orange. Its thin rind is easy to peel; and its pale orange pulp is spicy, full-flavored, and tart. The rangpur is a cross between the mandarin and the lemon
You don't know you don't know until you know.

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