17 July: Compass Day

Compass Day,also known as Wrong Way Corrigan Day because on this date in 1938, an aviator called Douglas Corrigan took off from New York, telling authorities he was going to Los Angeles, but 28 hours later, landed in Ireland. He claimed he was following the wrong end of the compass needle, but it was strongly suspected he'd done it on purpose. He'd wanted to fly the Atlantic but there had been restrictions on doing so. In honour of Compass day, here are 10 things you might not know about compasses:

1. For starters, the direction it is showing you is almost certainly wrong. Not as wrong as Wrong Way Corrigan's, but wrong enough that if you were going on a very long journey you could end up in the wrong place if you rely on a magnetic compass alone. A compass needle points at magnetic north, while maps relate everything to true north. The compass can be as much as 20 degrees off, depending where you are. Trained navigators are wise to this, of course, and know how to calculate the direction they should be going in.
2. The first compasses were made in China and were not used for navigation at all, but as fortune telling devices, and as a means of working out the best position of a building according to the principles of Feng Shui. There is an unproven theory that the Olmecs discovered the idea a millennium earlier than the Chinese did. The evidence suggests that they, too, would have used them for geomancy rather than navigation.
3. By the 11th century, the Chinese military had hit on the idea that the floating lodestone could be used for orienteering on land, and by the 12th century had started using it at sea. Their compasses would have consisted of a lodestone in the shape of a fish, floating in water. The earliest written references refer to it as a "south-pointing fish".
4. Before this new-fangled Chinese invention spread across the world, mariners relied on landmarks and celestial bodies to tell them where they were going. This was a problem if it was cloudy or misty, and so there was a distinct sea travel season in Europe, with only limited travel from October to April. By 1290, thanks to the compass, the sailing season was almost year round, and the age of discovery could begin.
5. Aside from the magnetic/true north problem, magnetic compasses have other limitations - large amounts of ferrous metal or Electric currents can disrupt them as well. So nowadays, large vessels usually use a gyrocompass, which finds true north by spinning very fast and using the rotation of the Earth. GPS is used as well although this is no good for Submarines which can't pick up the satellite signals underwater.
6. The diagram inside a compass which shows the direction points is called a compass rose, and the line that is aligned with the direction of travel and from which bearings are measured, is called the lubber line.
7. The liquid inside modern compasses is not Water, but lamp oil, mineral oil, white spirits, purified kerosene, or ethyl alcohol.
8. The cardinal directions are North, South, East and West, at 90° angles on the compass rose. Halfway between each of them are the The ordinal (or intercardinal) directions: Northeast (NE), Southeast (SE), Southwest (SW) and Northwest (NW). These eight directions are also known as the eight "principle winds": halfway between them are the half winds: North-northeast (NNE), East-northeast (ENE) and so on, and half way between these are the sixteen quarter winds: North by east (NbE), Northeast by north (NEbN), and so on. Incidentally, the 1959 Alfred Hitchcock film, North by Northwest, doesn't relate to a quarter wind direction but refers to travelling on Northwest Airlines.
9. In the middle ages, the eight principal directions were traditionally given more romantic sounding names: Tramontana (North); Greco or Bora (NE); Levante or Oriente (East); Scirocco or Exaloc (SE); Ostro or Mezzogiorno (South); Libeccio Garbino or Eissalot (SW); Ponente or Zephyrus (West); and Maestro or Mistral (NW).
10. In the 14th century, the Syrian astronomer and timekeeper Ibn al-Shatir invented a specialist compass incorporating a sundial, so it told you the time as well as which direction you were facing. This was particularly useful for Muslims as they could use it to tell them when to pray and which direction Mecca was in.