Well, no, a new decade has not begun, no matter how much some media insist on it: 2020 is the last year of the second decade of the 21st century, which, by the way, began on January 1, 2001. In the case of the decades, the confusion has much to do with the widespread custom of naming them by the tens digits, as in the well-known expression “the Roaring 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 time unit of measurement (we do not say that we are in the CCII decade AD), this confusion is irrelevant, and it is even legal to call any ten-year period a decade, for example, from the end of the Second World War to the mid-fifties (although in this case it is more correct to use the term “decade”). But in the case of the centuries the error is inadmissible, and it is hard to believe that it still persists.
Numbers of the form abcabc are divisible by 1001, and 1001 = 11 x 13 x 7, which allows for an amazing “mathmagic” trick.
As for numbers of the abab form, such as 2020, they do not have any special properties. Those of the form abcabc do have it, since they are divisible by 1001, and 1001 = 11 x 13 x 7, which allows us to perform a surprising trick of matemagia:
One of those present is asked to write a three-digit number on a piece of paper and then pass the paper to another person, who then repeats the same three figures. A third person is asked to divide the resulting six-digit number by 11, 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 the 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 since it is then divided by 1001, the initial number is obtained again.
total eclipse and annular eclipse
As an example of the media oversight over the decades, the recent solar eclipse of December 26 was repeatedly announced as “the last eclipse of the decade”, a title that actually corresponds to the one that will occur on December 14, 2020.
How wide could a ring of fire be?
The one on December 26 was, by the way, an annular eclipse, named for 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? How wide could a ring of fire be?
Let us remember that, rounding off, the diameter of the Moon is 3,500 km, and its distance from Earth ranges between 360,000 km and 400,000 km. The diameter of the Sun is 1.4 million km, and its distance from Earth varies between 147 and 152 million km.
Carlo Frabetti is a writer and mathematician, member of the New York Academy of Sciences. He has published more than 50 popular science works for adults, children and young people, including Damn physics, damn math O The big game. He was a screenwriter for The Cristal ball