How long would you have to spend flipping coins to get 76 heads in a row? Probably billions of years…

I know it’s a bit late, but 22 July (22/7) was Pi Approximation Day.

The fraction 22/7 is an approximation of pi, the mathematical constant that describes the ratio of a circle’s circumference to its diameter. Pi has an infinite number of non-repeating digits after the decimal point – see this site for the first million digits (or not…) Pi Day is 14 March (3.14), of course!

A fear of the number 13 is called triskaidekaphobia, and fear of Friday the 13th is known as paraskevidekatriaphobia or friggatriskaidekaphobia. In an extra for the phobics, this is the third one in 2012 and each was 13 weeks apart. Fear of Friday the 13th can become a debilitating obsessive disorder.

Is there anything in it? A paper in the *British Medical Journal* reported an increased risk of a transport accident on Friday the 13th – but this could be because people are more anxious, or there may be some other factor that has not been taken into account.

Mathematicians have modelled how snowflakes grow for the first time – but we still don’t know how they form in the first place.

Find out more about the science of snowflakes, and look at snowflake symmetry.

Pi Day is on 14 MarchÂ (3.14).

Pi has an infinite number of digits but has been calculated to 206,158,430,000 digits.

It takes the sun 365.242190 days to get round the sun (a solar year or tropical year). Because the calendar year is 365 days, that means we need to add an extra day every four years to keep the calendar year and the solar year in sync. In 1582, the calendar lost ten days – October 4th was followed by October 15th.

Leap years are years that are divisible by four, except… years that are also divisible by 100 are not leap years, unless they are also divisible by 400, when they are leap years. Got that?

If you tie 13 knots, all the same distance apart, in a piece of string, and make a triangle with knots at the corners and three gaps between knots on one side, four on the next and five on the third, you will have a right-angled triangle.

This works because of Pythagoras’ theorem – the square on the hypotenuse (the sloping side) is equal to the sum of the squares on the other two sides.