Basic Celestial Navigation is Hilariously Easy
I recently learned how to find North using the sky, and it turns out to be so easy that I feel stupid for not learning much earlier. Perhaps you would like to learn too! It could save your life in some unlikely scenario, I guess, but actually in everyday situations it's just pretty handy to be able to look at the sky for a couple of seconds and tell which direction you're facing.
Iâm not any kind of expert at this, I just learned how to do it recently. I donât know the proper names for things, and lots of the things I say are going to be technically not quite correct. But Iâm not trying to teach astronomy, I just want to quickly find which way Iâm facing, and these simplifications work very well for that.
Also, this whole thing will assume you're at a moderate northern latitude, which you very likely are if youâre in Europe or the USA.
Say youâve got a clear night with a half moon in the sky.
Getting a rough direction with the Moon is extremely easy - all you need to know is that the terminator (the line between light and dark) points south. So you just take that terminator line and mentally extend it down to where it hits the horizon, and that's approximately South.
Thatâs close enough for the girls I go out with.
If itâs not a perfect half moon, so the terminator line isn't straight, you just kind of take the direction that the curved line is facing, overall. You basically want the tangent at the middle of the terminator. Or if you prefer, you can draw the line connecting the top and the bottom of the terminator, which ends up being the same angle. These images show both options:
With a crescent moon this is even easier, you can just take the line between the âtips of the hornsâ of the crescent.
Extend that line down to the horizon, and thatâs approximately South.
Now, if the moon is very low in the sky, so the terminator is at a steep angle, then this method will be extra inaccurate. But that's ok, because if the moon is that low, you don't really need to trace the terminator line to find south, because you know that the Moon just rose or is about to set, so the moon itself is basically East or basically West, and you can tell which by looking at the terminator.
If that doesnât feel obvious, thatâs perhaps to be expected, and I should tell you about The Ecliptic.
Basically all the big stuff in the solar system is always on more or less the same plane. So the Sun, the Moon, and all the planets are always found close to a single line, which crosses the sky from East to West. If youâre near the equator it goes almost straight overhead, but at moderate northern latitudes, like you probably live at, itâs tilted over pretty far to the south. So the Sun and the Moon will rise more or less due East, be more or less due South at their highest point, and set more or less due West.
That means if the Moon is just above the horizon, like in this example, we can look at the angle of the terminator and see itâs tilted way over to the right, so we know South is to our left, and the Moon must be about to set in the West.
(In fact I got this from sky visualisation software, so itâs helpfully showing us where West is anyway)
Another way to think about this is: The Moon moves through the sky in a direction roughly perpendicular to the terminator. So we can take our terminator line and draw a line at right angles to that, and thatâs roughly the path the Moon is travelling along:
We can follow that line to the horizon and see where the Moon will set, i.e. basically due West. So the Moonâs terminator points South, and kind of acts like the vertical line of a T shape, with the arms of the T pointing towards East and West.
Hopefully you can convince yourself that all this makes sense by thinking about shining lights on basketballs and so on, but you can also just use the method without being able to visualise clearly why it works.
All of this is approximate and simplified - the Moonâs orbit is actually 5 degrees off the Ecliptic, and the Ecliptic is 23 degrees off the Celestial Equator, and so on. None of it really lines up and youâll often be off by like 40 degrees, but I donât care. This method is extremely quick and it's usually good enough, especially in cities laid out on a grid, where you only need to get it to within 90 degrees.
And if you want more accuracy, this is still a good thing to start with, because if you can quickly find roughly-South, that makes it much quicker and easier to look roughly-North and locate the stars necessary to pin down exactly-North.
Since the Sun follows basically the same path as the Moon, you can also get rough orientation from its location. If itâs low in the sky, it just rose or is just about to set, so you know itâs East or West. You canât tell which by looking at the Sun itself, since it doesnât have a neat terminator line like the Moon (and you shouldnât really look at it anyway), but the neat thing about the Sun is that, unlike the Moon, it always rises in the morning (East) and sets in the evening (West). If itâs the middle of the day itâll be to the South.
But also, donât forget about the Moon during daytime! Itâs often out during the day. And in fact, itâs often not out at night! Or sometimes itâs out, but itâs Full or New and you donât have a terminator line to use. You canât rely on just the Moon, even for coarse orientation. So letâs talk about stars.
Bold Orion, Mighty Hunter
Orion is a lovely constellation thatâs very distinctive and easy to spot, and you can use it to find roughly-South very quickly, just like you can with the Moon.
Heâs meant to be a hunter, with a belt of three stars, and a sword hanging from it. You had to make your own fun, back in those days.
Itâs a sword, ok? He has a sword on his belt.
The point is, the sword points South just like the Moonâs terminator line does. So if you canât see the Moon, maybe you can use Orionâs sword.
Now we have two ways to quickly find roughly-South, and therefore roughly-North. Letâs get more accurate.
The sky provides us with an improbably convenient star called Polaris, which we call The North Star, because itâs almost perfectly over the North Pole.
This location is unique, in that it makes Polaris essentially the single static point that the whole rest of the sky rotates around. For a given latitude on Earth, Polaris is always exactly the same height above the horizon, in exactly the same direction, and that direction is North, to within less than a degree.
Itâs also really bright for its region of the sky - thereâs no brighter star within like 30 degrees of it.
Polaris is so extremely convenient that I consider it to be (very weak) evidence of a loving god. Finding the exact strength of that evidence, in bits, is left as an exercise for the reader.
So Polaris is an A+ gold star, now all we need is a way to find it.
First off, because itâs always at exactly the same height above the horizon, once youâve found it a few times it gets easier, because you know where it is vertically. If youâve also used the Moon or Orion to narrow your search down to like a quarter of the sky horizontally, you can often spot Polaris very quickly.
But it helps a lot to be able to find it using constellations.
This is the one everyone knows, the Big Bear. People also call it The Big Dipper, The Drinking Gourd, Big Spoon Energy, or The Plough.
Hereâs an image from WaddenSky:
To find Polaris, you find the Big Dipper, take the line between the two stars on the end, and extend that line up by about 5 times its length, and thereâs Polaris. Easy enough.
The main thing that can make this tricky is, as I said, Polaris is the fixed spot that the whole night sky rotates around, so the Big Dipper could be dipping bigly all over the place. It could be like itâs shown there, it could be almost directly overhead and upside down, it could even be partly below the horizon, and invisible behind the trees:
So, you donât want to put all your eggs in the Big Frying Pan.
Cassiopeia is basically just as good as The Big Dipper when it comes to finding Polaris. She looks like a big M, or a big 3, or W, or E (ÎŁ, really), depending on what orientation she feels like right now. And the nice thing about her is, sheâs on pretty much the opposite side from The Big Dipper, so if you canât see one, you can see the other. Learn how to spot both, and youâre set.
To find Polaris from Cassiopeia, you basically draw a line between the two ends of the zigzag, and make a right triangle twice as long, from the bottom of the 3 (or the right side of the M, and so on).
Thatâs really all there is to it: find roughly-South using the Moon or Orion, use that to look roughly-North until you either see Polaris itself, or find it using Ursa Major or Cassiopeia, and now youâve got North to within one degree. Usually takes something like 5 seconds.
And there you have it! Now you can find your bearings quickly and easily using just the sky, no matter what!
Unless itâs cloudy. If it's cloudy youâre fucked.
