Laura Erickson's For the Birds

Thursday, November 11, 2021

Top Ten Reasons to Relish the Number Seventy

Belted Kingfisher
A lot of times on my birthday, I look for the top ten reasons why the new number representing my age is a cool one. So here goes: 

1. Seventy is the smallest weird number—that is, a natural number that is not semiperfect but is abundant. In and of itself, that's a fun, weird fact that any former junior high math teacher couldn't resist explaining. So here goes:

Mathematically, a number is perfect if, when you add each one of its factors except itself, you get the number. Six is the smallest perfect number because its factors—1, 2, and 3—add up to 6. Twenty-eight is the next perfect number because its factors—1, 2, 4, 7, and 14—add up to 28. 

A number is abundant if, when you add each of its factors except the number itself, you get a sum larger than the number itself. The smallest abundant number is 12— its factors, 1, 2, 3, 4, and 6, add up to 16, which is larger than 12.

A number is semiperfect if it’s both abundant and you can add up some of its factors to get the number itself. That smallest abundant number, twelve, is also semiperfect because some of its factors, namely 1, 2, 3, and 6, add up to 12. 

Seventy is abundant—its factors—1, 2, 5, 7, 10, 14, and 35—add up to 74. But 70 is not semiperfect because no combination of those factors adds up to exactly 70. So that makes 70 a weird number. And 70 happens to be both the smallest weird number and the only weird number that is a possible age in a person’s lifetime. I doubt if Paul Simon was thinking about weird numbers and synonyms of "weird" when he wrote the song "Old Friends," with the line, “How terribly strange to be 70.” Well, maybe he did—he is the guy who also wrote the song "When Numbers Get Serious." And mathematically, that line in "Old Friends" is a true statement.   


2. And speaking of Paul Simon, my favorite songwriter of all time, the fact that he specifically named the age of seventy in one of his most beautiful songs is a Good Thing in and of itself. 

3. “Three score and ten,” or 70 years, is the nominal length of a human life in the Bible. Of course, that same Bible says Adam lived to be 930 and  Methuselah to 969, so in Biblical terms I may well have some time left. 


4. 70 is the atomic number of the rare earth metal ytterbium, named for the Swedish village Ytterby where Swiss chemist Jean Charles Galissard de Marignac discovered it.  Ytterbium is most useful in the extremely rare situation where you can use it in a game of Scrabble. 

Grandpa and me

5. Seventy is probably the age of the youngest people who remember when the name of Armistice Day was changed to Veterans Day, in 1954. My Grandpa fought in World War I, which ended at the armistice, on the 11th hour of the 11th day of the 11th month. In honor of my most beloved veteran, to this day I call my birthday Armistice Day.

6. U.S. copyrights now normally last for an author’s life plus 70 years. 

7. In Roman numerals, 70 is LXX, which is my initial plus a couple of kisses.   

Belted Kingfisher

8. The 70th bird on my lifelist is a wonderfully cool one, the Belted Kingfisher.

9. The 70th day of each year, March 11 (except in Leap Years, when it’s March 10), falls at the most joyous and hopeful time of the annual cycle, when spring migration is just starting to kick in. 

10. I started birding on March 2, 1975. Exactly 70 days later, May 11, was the first time I ever had at least 10 lifers in a single day, and among them were my very first warblers. 

Of course, since my birthday is 11/11, my top ten lists always includes eleven items. 

11. Most obviously, the alternative to turning 70 is not turning 70.

Belted Kingfisher

3 comments :

  1. What a delightful read. Happy birthday, Laura!

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  2. Happy Birthday! Today is mine, too. But I'm only 68!! Just a youngster!!

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  3. When I turned 60, I thought to myself "This is no big deal, nothing has changed." Same when I turned 65.

    But when I turned 70 (two years ago this month) I though to myself "This feels a bit weird."

    And now here you are explaining exactly why. I was brought up with recreational mathematics and I never heard of weird numbers before, but now I learned about them at the perfect time. Thanks!

    And yes, I'm turning 72 later this month and 72 is a semiperfect number as are all multiples of 6.

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