While they may look similar, navigational charts for boats are very different than street maps for cars. For starters, they’re divided up into a latitudinal and longitudinal grid. Latitude measures north and south. Longitude measures east and west. You’re smart people, you knew that already. But, let’s go a little deeper.

A nautical mile measures 1.15 of standard mile and is measured in knots meaning one knot is one nautical mile per hour. Each degree of latitude measures 60 nautical miles. Lines of latitude are horizontal parallel lines traveling east and west that start at 0º at the equator and go to 90º North or South depending on which pole you’re standing on. (Hopefully with a good jacket.) So add those up and you get 180 degrees and 10,800 nautical miles from pole to pole

Lines of longitude measure east to west and are vertical lines traveling north and south that connect at the poles. Unlike latitude lines that maintain equal spacing between, longitudinal lines get closer together the closer they get to the poles. But, since there are no east or west poles 0º longitude, or the Prime Meridian, is an arbitrary point on the map. For years, most of the major countries set the Prime Meridian as their own capital. That changed in 1884 when they all agreed that Greenwich, England would be the official global point marking 0º longitude and also set it as Coordinated Universal Time or UTC by which all 24 time zones are set. For early celestial navigators these three elements were critical for knowing where you were: Latitude, longitude and time.

The challenge facing early sailors was the open ocean. The challenge was knowing where they were when there was nothing but water on every horizon line. When there’s no reference point to work with every direction looks the same. Choose wrong and you end up lost at sea. Things aren’t bad. Your ship isn’t sinking. You just don’t know if you’re where you’re supposed to be or not. You’re just drifting and don’t know which way to go.

In fact, this is why for centuries all sailing was done along the shores. They didn’t have compasses and hadn’t figured out celestial navigation so they had to stay within sight of land to know where they’re going. It wasn’t until the Vikings first figured out how maintain a heading in open sea that anyone dared to sail in the open ocean. It’s not that anyone believed the world was flat, they just didn’t want to go into a place where everything was unknown with no way to know how to come back.

A compass, a chronograph and celestial navigation solved that problem. Because you’re smart people you know the sun rises, sets and follows a continually changing path across the sky as the year progresses. If you’re staying in the same place, this information can be used to create a calendar. But, if you’re a sailor you can use this information to determine your location at sea because exactly where on the horizon the sun is located in the sky at a specific date and time varies depending on where you are on the globe.

So, using a chronograph, that is synced with the Prime Meridian, to know exactly “when” noon is, a sailor can use a sextant to measure exactly “where” the sun is on the horizon at noon. Because the variation of the position of the sun is mathematically predictable, the sailor, with the help of some tables, can use that data to calculate his longitude and latitude and then plot that information on a chart.

And just like that, he knows where he is and can maintain or adjust his course accordingly.

And this ability is especially important when outside factors push him off course.

I’m sure I’ll find my metaphor on this subject eventually. It’s still cooking.

But before I get there I need to bring up one of the things sailors have to account for is something called set (direction) and drift (speed). It’s not just storms that can pull a sailor off course, sometimes it’s currents that he can’t see.

Let’s say you’re crossing in your sailboat from Florida to the Bahamas because you’ve finally had enough and those crystal-clear waters call to you. You’re going to have to cross the Gulf Stream to get there. The Gulf Stream is a huge current that travels north like a river between Florida and the Bahamas. Which means the current is going to pull you downstream (north) as you cross it. So, for the sake of easy math let’s say the Gulf Stream current is moving north (set) at five knots (drift). And let’s say that you’re sailing east at five knots. The moment you get into the current, no matter what your compass says, you’re not just traveling east, you’re actually traveling northeast. And let’s say the Gulf Stream is ten miles wide which means if you’re moving east at five knots, it’s going to take you two hours to cross the current. Which means that by the time you get across the Gulf Stream it’s going to have carried you ten miles north whether you wanted to go that way or not.

But if you know all that information before you ever attempt the crossing you can make a course correction to compensate so that you make landfall where you want and not ten miles north.

I feel like the metaphor is getting stronger.

Until tomorrow, I’ll leave you with this bit of wisdom from Dave Barry.

How to sail a sailboat according to Dave Barry

1. Figure out where you want to go.
2. Whichever way it is, do NOT aim the sailboat in that direction.
3. Aim the sailboat in some other direction.
4. Trust me, this is the way sailboaters do it.
5. They are heavy drinkers.