Number 15


= 3 × 5

= 1 + 2 + 3 + 4 + 5
A triangular number.

Rugby Union is played with teams of 15 players.

15 is the constant of a 3 × 3 magic square.

Under British law, when you reach the age of 15 –

'15' on a bottle of sun lotion is its sun protection factor. Sunscreen products have factors like 4, 8, 15 and 20. The higher the number, the more protection from the sun you get.

The number gives you a rough idea of how long you can lie in the sun without burning -
'Safe time' with sunscreen =
'Safe time' without sunscreen
× Sun protection factor

For example, if you have fair skin, you may be able to lie without burning for 20 minutes in the hottest midday sun in the UK (try hard to remember this if you can). But if you wear a sun lotion marked '15' the safe time is roughly 15 times greater –
15 × 20 minutes = 300 minutes
– or five hours. Of course you have to be careful to make sure the sunscreen does not rub off during this time.

The factor number measures how well the sunscreen blocks out the ultraviolet B (UVB) rays of the sun. These rays tan the skin but they also cause burning and are a major cause of skin cancer.

A pattern made from 15 squares
A pattern made from 15 squares.

A crystal anniversary celebrates 15 years. The most common cause of celebration is 15 years of marriage, but almost any event can be celebrated in this way, from the reign of a queen, to 15 successful years of trading by a pickle factory. There are named anniversaries for every year from 1 to 15 –

1 year Cotton
2 years Paper
3 years Leather
4 years Fruit or flowers
5 years Wood
6 years Sugar
7 years Copper or wool
8 years Bronze or pottery
9 years Pottery or willow
10 years Tin
11 years Steel
12 years Silk or linen
13 years Lace
14 years Ivory
15 years Crystal

But once we reach 15 years there is a gap and the next recognised anniversary is China at 20 years. It seems sad that once a couple achieves 15 years of marriage, the system assumes that they have nothing really worth celebrating for another five years. With so many wonderful new materials available today to fill the gap, it should not be difficult to put things right. Perhaps the list should continue: 16 PVC, 17 Chipboard, 18 Plutonium...

'Fifteen men on a dead man's chest' is one of those lines from sea shanties that don't make much sense. Could it be a commentary on the violent nature of rugby football?

Number 15