Never be lost again: Navigating with stars

Knowing where you are can seem a given to us. Nevertheless, we need to remember that before the time of GPS and Google Maps, people still needed to know where they were. Did they just give up? No, some brilliant minds found innovative ways to use what they had. Since we always have the night sky wherever we are, they predicably looked up it. Today, the art of star navigation is all but lost. For many of us, we would be utterly lost without our cellphones. Let‘s try to fix that, shall we?

No tools, no worries

Let‘s begin with approximate positioning techniques that require nothing but a good eye. Humans have known for centuries that stars move in the night sky but also that this movement is predictable. As such, almost every civilization has found ways to regroup stars in constellations and use those for navigation. In all, about 58 stars are what‘s called “navigational stars“ - stars used to know where you are. Nonetheless, it is no surprise that the most important star here is Polaris. Why? Well, it just so happens that Polaris is extremely close to Earth‘s true North - that is the axis of rotation of Earth. In that way, when the Earth turns, Polaris stays about at the same place. In fact, Polaris still moves around a bit, since the position of Earth through the year affects the position of all stars. Still, the North star always stays at less than a degree from the celestial North Pole. Quite a useful coïcidence. So, find Polaris and you‘ll at least know where North is.

Now, how do you find Polaris? We can use the most well-known constellations for that. Polaris is the tip of Ursa Minor (“Little Dipper“) - the famous sky spoon. More often, it is easier to find her partner Ursa Major (“Big Dipper“). If you find the Big Dipper, take the vertical section opposite to the “handle“, take the distance of that section and repeat it five times up to find Polaris. If you need more confirmation, use the fact that the Big Dipper and Cassiopeia (literally a “W“) are about the same distance on either side of the Little Dipper. Just keep in mind that in some cases the Big Dipper and Cassiopeia aren‘t visible in the night sky. Still, Polaris is quite a bright star that you should be able to pinpoint.

What about if you are in the Southern Hemisphere (where you can‘t see Polaris)? Well, we still have some tricks although less intuitive. In that case, navigators would first try to use the South Cross. The South Cross is a small constellation that is easy to find even though it is not directly on the celestial South Pole. Try to find the two pointer stars (two very bright stars close to the South Cross and pointing to it) to validate that you use the right cross. After that, a bit of calculation. Imagine a line going down the cross to the horizon. Then imagine another line going to the horizon perpendicular to the middle point of the pointer stars. Where both lines cross will be about South. More complex I know but still useful.

Alternatively, you can use Orion which is visible in both the Northern and Southern hemispheres. This constellation is a bit more complex (go look it up) but in truth, you just need to locate the belt (made of 3 very bright “stars“) to find it. If your belt seems to be in an hourglass-shaped body you found it. You might also check yourself by locating the very bright and red tainted Betelgeuse, making the tip of Orion‘s club. Anyway, where the right star of the belt (called Mintaka) rises is East and where it falls is West! You can also use Orion to find South. Take the two bright stars just below the belt and continue their line to the horizon. This will be about South.

Angling up your position

What if you want to know more exactly where you are? Well, the sky can help you get your latitude and longitude. If you take a standard world map, the latitude would be how much you are above or below the equator. Your longitude would be how much left or right you are on the map. For the longitude, we generally use the Greenwich Meridian (in Britain) - now better known as UTC (Coordinated Universal Time) - as a reference.

Anyway, for the latitude, you can use Polaris again! If you measure the angle between Polaris and the horizon, you will find your approximate latitude angle to the equator! According to J. W. Nowie (see my reference below), you can measure angles without any instrument. Just hold your hand above the horizon with your arm stretched out. The angular width of the little finger is about 1.5 degrees at arm's length. In the South, you could use the Sun directly (watch out for your eyes), but you would probably need a reference table for accurate measurement (see the next section).

The longitude can be harder since we don‘t have stars always pointing East or West like Polaris for North. The Earth spins at about 15 degrees per hour, making it dependent on time even if there was a star to do so directly. Here, we need a chronometer or a watch of some kind with UTC. If you can find at what UTC the sun reaches his zenith (apparent noon) at your location, you can find your longitude! As I said, the Earth spins at about 15 degrees per hour. In that way, if your noon happens 3 hours later than UTC noon, you are 45 degrees WEST of UTC. Keep in mind that accurate local and UTC is paramount. Any error in the order of one minute leads to an error of about one nautical mile (1.852 km).

19th century navigation masterclass and the maritime almanac

Now, what was the best precision you could have before GPS and the like? The answer would be in almanacs! Maritime almanacs contain latitude and longitude data depending on star angles and specific meridian times to Greenwich. Almanacs required accurate angle measurement, our hand isn‘t sufficient here. In that way, many tools were used including classics such as the sextan and the mariner compass. I‘ve given a reference with one such maritime almanac down below.

For latitude, almanacs generally use the sun. Measure the observed angle between the lower limb of the sun and the horizon. It is easier to be accurate with the limb than with its apparent center directly. Then, add 16 angle minutes to go back to its center (subtract 16 angle minutes if you used the upper limb). Add the correction for the dip of the horizon (about a couple of angle minutes, given in almanacs). Then another correction for refraction (about one angle minute). After that, subtract from 90 degrees the result and you will obtain the TRUE meridian zenith distance (in plain English: the true angle made by the sun at your noon). You refer one last time to correct for the sun‘s declination using the almanac again and you get a precise latitude! Almanacs allow you to use other stars or the moon. Keep in mind that if you use the moon, you need to consider an additional correction for the parallax effect (also given in the almanac and about a couple angle minutes).

Longitude will require latitude and, again, precise local and UTC. Here, it was typical to use the lunar distance method. Using a sextant, you measure the distance between the moon and another body. After the parallax correction, anyone on the surface of the earth looking at the same would see the same angle. With that distance and your local time in hand, you can then reference the almanac to find Greenwich time. After that, you can find longitude just like before (one hour is about 15 degrees). The corrections needed are the same as for latitude: semidiameter, dip of the horizon, refraction and parallax. Almanacs offered quite impressive precision. By the start of the 19th century, they were about one-quarter of a minute of arc.

Good bye star navigator!

Well, look at you know! You can now find your way quite efficiently on your own. No need to keep that cell phone now. I am always impressed by the ingeniousity of humankind. Look at what we found out with little lead at the start. In any case, I hope you learned something useful for your next expedition.

As for the rest, see you next time.

References

Norie, J. W. (1828). New and Complete Epitome of Practical Navigation.