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# Ring of fire | Science

Well, no, a new decade has not begun, even if some media insist on it: 2020 is the last year of the second decade of the 21st century, which, incidentally, began on January 1, 2001. In the case of the decades, the confusion has a lot to do with the widespread custom of naming them by the numbers of the tens, as in the well-known expression “the happy twenties”, which seems to include all the years that have a 2 in that place, from 1920 to 1929 both inclusive.

Since the decade is not an official temporary unit of measure (we do not say that we are in the CCII decade AD), this confusion is unimportant, and it is even lawful to call any period of ten years a decade, for example, from the end of World War II in the mid-fifties (although in this case it is more correct to use the term “decade”). But in the case of centuries the error is inadmissible, and it is hard to believe that it still persists.

The numbers of the abcabc form are divisible by 1001, and 1001 = 11 x 13 x 7, which allows to perform a surprising “matemagia” trick

As for the numbers of the abab form, like 2020, they do not possess any special property. Yes, those of the abcabc form have it, since they are divisible by 1001, and 1001 = 11 x 13 x 7, which allows to perform a surprising trick of matemagia:

One of those present is asked to write a three-digit number on paper and then pass the paper to another person, who must then repeat the same three figures. A third person is asked to divide by 11 the resulting six-digit number, and the quotient will be divided by 13 by a fourth person, and the result will be divided by 7 by a fifth, which will pass the final result to matemago, which will guess the initial number (which is none other than the last quotient). Repeating the three figures is equivalent to multiplying the number by 1001, and as it is then divided by 1001, the initial number is obtained again.

### Total eclipse and annular eclipse

As an example of the media absenteeism over the decades, the recent solar eclipse on December 26 was repeatedly announced as “the last eclipse of the decade,” a title that actually corresponds to what will occur on December 14, 2020.

What maximum width could a ring of fire have?

That of December 26 was, by the way, an annular eclipse, named after the spectacular “ring of fire” that is seen around the lunar disk when its center coincides with that of the solar disk. Why is this ring seen in some eclipses and not in others? What maximum width could a ring of fire have?

Recall that, rounding, the diameter of the Moon is 3,500 km, and its distance to Earth ranges between 360,000 km and 400,000 km. The diameter of the Sun is 1.4 million km, and its distance to the Earth varies between 147 and 152 million kilometers.

Carlo Frabetti He is a writer and mathematician, a member of the New York Academy of Sciences. He has published more than 50 popular works for adults, children and young people, including Damn physics, damn math or The big game. He was a screenwriter of The Cristal ball

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