Superstitious? 13 things you might not know about Friday the 13th
Unlucky for some? Friday the 13th - also known as Black Friday - has long been regarded an unlucky day.
Where did the fear come from... and what's it all about?
Should we all be on our guard?
As part of Science Foundation Ireland Science Week, Dr Richard J. Smith from the UCD School of Mathematics and Statistics has compiled a list of thirteen thrilling facts about the number thirteen:
Paraskevidekatriaphobia is a fear of Friday the 13th.
2. Calendar talk
According to the Gregorian calendar, the 13th day of the month is more likely to be a Friday than any other day. There are three occurrences of Friday the 13th in 2015: February, March and November. The next time this will happen is 2026.
3. Unlucky for some
Bad things have happened on this day - on Friday, 13th October 1307, the French King Philip IV ordered the arrest, torture and subsequent burning at the stake, of hundreds of the Knights Templar military order. Several modern authors, such as Dan Brown in his novel The Da Vinci Code, have succumbed to falsely associating the superstition with this grisly tale!
4. Horse tips
Allowing for ties, there are exactly 13 possible ways in which the three fastest horses in a horse race can finish.
5. Sinister figures
The number 13 is considered in several cultures as a somewhat sinister number, a bearer of ill portent, when compared to its much lauded neighbour 12 (of months of the year, signs of the Zodiac, sons of Jacob, apostles of Jesus and days of Christmas fame, to name a few).
6. Number facts
13 is a prime number, that is, a number greater than one that can only be divided by itself and one. For example, six is not a prime number, because it can be divided by two and three, as well as six and one. In fact, 13 belongs to the much more distinguished club of so-called Wilson primes, of which only three are known. If there is a fourth, it must be larger than 20 million million.
Prime numbers have fascinated mathematicians for millennia because they are the building blocks of all numbers, and nowadays they are used globally every second of every day - whenever a credit card transaction is made online, prime numbers are used by computers to keep those details safe from prying eyes.
7. Creepy crawlies
Many species of the cicada insect in the US have a life cycle of 13 or 17 years (another prime). Some believe that these prime number life cycles help them avoid predators which appear in their habitat at regular intervals.
8. Introducing Fibonacci
13 is also a member of the famous sequence of Fibonacci, introduced in the 13th century. This sequence begins with zero and one, and the next number in the sequence is always the sum of the two preceding it, so we have 0+1=1,1+1=2, 1+2=3, 2+3=5, 3+5=8 and so on. If you want to know why rabbits breed so quickly, look no further than the Fibonacci sequence.
Fibonacci is also responsible for introducing the decimal number system to Europe. Developed by Indian mathematicians centuries earlier, and adopted by the Arabs, it revolutionised European banking and business because arithmetic is much easier with decimals than with clunky Roman numerals.
9. Lucky for some!
In certain countries, such as Belgium, some employees get a "13th month" pay cheque, so in fact the number 13 can be quite a lucky number.
10. Horror flicks
There have been 12 films in the Friday the 13th horror franchise. Perhaps it's bad luck that a 13th is due for release in 2017?
Alas, the origin of the Friday the 13th superstition seems far more mundane - it appears to be not much more than a hundred years old, and might just be a combination of two things traditionally associated with ill fortune, namely Friday and the number 13.
12. Hotel trivia
Fear of the number 13 means hotels and hospitals often skip the 13th floor, and even airports quietly omit gate 13 sometimes.
Triskaidekaphobia is the fear of the number 13.