Caoimhín's Content

Today, we are going to take a quick look at how you can figure out, and the limitations to figuring out, roughly where you are using the sky. Of course, we’re mostly going to be looking at the nighttime sky, so we will move into nighttime as usual, with a view from Ireland and some light pollution.

Just by looking around a clear night sky, we can find north pretty easily from here in the Northern Hemisphere by finding the Plough or the Big Dipper. If you are high enough on the Northern Hemisphere, then the Plough or the Big Dipper will always be above the horizon, it will be circumpolar, but below a certain latitude, the Plough will be under the horizon for at least part of the night, at least sometimes of the year. Thankfully, there are other constellations, like Cassiopeia, that can be used to find roughly where the North Star is. Of course, if you are seeing the sky with a lot of light pollution, there’s a good chance that you are at least not very far from civilization. As such, finding your way shouldn’t be too difficult, you’d likely be able to find roads or street signs or something nearby if the sky has a lot of light pollution. For a more useful view we will head out to the countryside, if you’re truly lost, the sky might not be absolutely perfectly dark, but it should have it a little bit less light pollution, which will show a sky a little bit more full of stars.

In any case, once you have found the North Star, one way that you can figure out roughly where you are is by looking at the angle between the North Star and the horizon. There’s a little angle measurement tool that we can use in Stellarium, but in real life a sextant and similar device is what you would use to figure the angle of something in the sky. We need to measure from the North Star as exactly as possible, I’m going to be a little bit off but that’s okay, down to the horizon in as straight a line as I can. I’m certainly going to be a little bit off, zooming in on the star will help make the measurement a little bit more exact. It is tricky just using a cursor in software.

The measurement I’m able to get is showing us 52 degrees, 3 minutes and 32.36 seconds of arc. These degrees of arc are how the 360 degree sphere of the sky is broken up, into degrees, minutes and seconds of arc. Stellarium is showing our location as Cork in Ireland and showing our exact location as 51.898 degrees north and 8 degrees west. So it is a little off, we’re off by a couple of minutes of arc. This is partly due to the horizon being a little bit obscured, and of course I haven’t lined up exactly on the North Star. If you were measuring this with sextant of some sort, then you would be able to line it up more accurately. Most sextants have a little telescope or sight on them that allows you to be a bit more accurate, it allows you to see exactly what star you want to take a look at. They usually have a weight to show what is straight down, in Stellarium we need to extend the measuring line down to the horizon. At a high enough level of magnification, the horizon should look pretty flat, and the baseline of the measurement tool should be straight underneath it. Even with all that lined up a little bit better it still is a little bit off. Without the proper tools, this kind of reckoning of your latitude can be a little bit tricky, but it does work from everywhere.

One drawback of Stellarium is that I can’t send myself to a random location without it essentially telling me where I’ve gone. If we go to a different location that’s not too far away to start with we can make a comparison. We’ll go down to somewhere in Spain, the North Star will be closer to the horizon because we are closer to the equator. You might also notice that the Plough or the Big Dipper is much harder to spot, we are seeing some of it, but a lot of it is now under the horizon. This is at least true early november nights. We still have Cassiopeia up above the North Star, roughly pointing us in the correct direction. These two constellations are pretty much on opposite sides of the sky to each other, so either they will both be on the horizon, or one will be above the horizon and one will be below. Of course, unless you’re in the Southern Hemisphere. We’ve now traveled to a different latitude, so let’s zoom in here and yet again, we will try to be as accurate as possible. From the North Star, or Polaris, and we’ll come all the way down to the horizon. It is dark and the landscape is dark, so finding the exact horizon with all of the blur and atmospheric distortion is a little bit tricky. In the end, the angle tool tells me 38 degrees, 18 minutes and 11.74 seconds of arc. Again, a little bit off, we’re down at 37.7467 degrees north. So we are a little bit off yet again, by almost a degree, but that is due to my own limitations, not being able to draw the line in exactly the right place.

Again, we will travel to another location closer to the equator, we’ll come down to North Africa. Then we’ll still see the North Star above the horizon, but at this time of the night, at this time of the year, the Plough is completely hidden under the horizon. We still have Cassiopeia up higher helping us to see roughly where we should be looking. We’ll take a closer look at Polaris again and we will measure down to the horizon. Where exactly the true horizon lies is a little bit blurry, but that gives us 19 degrees, 6 minutes, and a second or so of arc. If we take a look at the actual location we’re at 18.5 degrees north, so again, we’re off by about a degree and again that’s on me. With a proper sextant, with a proper measurement of where the horizon is then of course you’d have a much better idea. This kind of navigation was much more important out on the sea, where, of course, the horizon isn’t obscured by things like trees and buildings, it can be a little bit easier to see, especially on a clear night.

That is the North Star, and it is able to tell us where north is, and from there we can figure out south. By turning the whole way we’ll point south. Thankfully, Saturn’s in the sky, and I know that when Saturn’s at its highest, it’s going to be above the south. Saturn will be at its highest a bit before midnight, so there is a good chance that we’re looking pretty much south here. That’s the kind of thing that you need to be looking at the night sky regularly to build up that kind of familiarity, to know where Saturn is on a given night and know that it won’t have moved too much since the last time that you saw it.

However, the measurement of latitude isn’t too hard to do and it works in the Southern Hemisphere as well. Of course, you have to look for a different part of the sky. To see this, we’ll go to the south of South America. We’ve also moved far enough around the Earth that we are now looking at daytime once again, so we’ll move back into night time. We are still looking north, so we’re seeing objects and constellations like Taurus, the Pleiades, Orion, things that we’d normally see in our southern sky. Saturn as well, we’re now seeing around the northern sky. We want to look south here because we’re looking for the Southern Cross. The Southern Cross is along the path of the Milky Way, and roughly in the south. The Southern Cross, unlike the Pole Star, doesn’t actually rest directly above the South Pole, it more points at the South Pole. If we follow along the long line of the cross, the real celestial pole is somewhere past the end of it, in what seems to be a reasonably empty patch of the sky. There’s no South Star, it’s just a location where the long arm of the Southern Cross will point. If we move through time we should see that it appears as if the stars are rotating around this point, roughly. This is the same apparent rotation around the North Star from the Earth’s rotation that we see in the North.

Of course, in Stellarium, we have grids that can help us, showing the exact celestial South Pole, the pole of the sky. Then we can again do the same measurement again. Of course, thanks to the toolbar in Stellarium, I already know how many degrees latitude we are at, but by following the Southern Cross up into this empty space and drawing a straight line down to the horizon, we should get roughly the latitude. I’m a lot further off than in the Northern Hemisphere, because there isn’t a specific star to click on. I’m getting 42 degrees, but the location is saying 39 degrees, so off by about 3 degrees. That’s possibly because I clicked a little bit higher than the actual pole. Using the grid to be more exact I get roughly 38 and a half degrees. That’s again off by about almost a degree, so it is imperfect, but that is the principle of being able to tell at least your latitude just using the night sky.

Of course, if you’re out during the day, how high in the sky the Sun is during the day is going to vary significantly with the time of year as well as your location. Measuring the angle of the Sun in the sky can’t tell you your latitude. However, you can use the Sun to figure out where south is by figuring out when shadows are at their shortest. If you have a stick or a pole and you measure the shadow so that you can see when the Sun has reached its peak, the shortest shadow will be directly opposite the south. You can use the Sun to figure out where south is, but you can’t use it to figure out your latitude the way you can use the North Star to figure out your latitude.

Longitude is a whole different and much more complicated problem. Figuring it out almost always requires a watch, clock or chronometer in order for you to measure it, but that is something we’ll have to get into another time.

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