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Making the Stars Your Map

Nick Nelson smiles as he wears protective glasses and looks at a solar eclipse among a large crowd at Chico State.

If you got lost with no access to GPS, would you be able to figure out where to go to get home?

For hundreds of years, navigators used celestial bodies to guide their paths. Your phone can do that for you now, sure—but in a pinch, using the sun, moon, and stars as your map is a pretty cool skill to have.

Chico State professor Nick Nelson tells us how it’s done.


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Nick Nelson smiles as he wears protective glasses and looks at a solar eclipse among a large crowd at Chico State.
Chico State professor Nick Nelson, seen here enjoying the 2017 solar eclipse, enjoys a sense of wonder about celestial bodies and how they can help remind us of our place in the universe—and help us find our way home. (Jason Halley/University Photographer)


(SOUNDBITE OF PROFESSOR NICK NELSON): We don’t say, “Oh, go down to the hall to the east, and take a north.” And so, as a society, maybe we’ve forgotten that a little bit.


TRAVIS SOUDERS, HOST: This is Out of Curiosity, a podcast driven by the wonder of lifelong learning from California State University, Chico.

KATE POST, HOST: Welcome to Out of Curiosity. I’m Kate Post.

SOUDERS: And I’m Travis Souders.

POST: On today’s show, we have Chico State physics professor Nick Nelson, who specializes in astrophysics. Hey, Travis—have you ever gotten lost before?


SOUDERS: About 10 years ago, I was driving back to town from San Francisco and decided to take a shortcut. And I turned left when I should’ve turned right, and didn’t know that at the time, and it was really late at night, and this was before the age of, just 10 years ago, really reliable GPS or even cellular service. So I didn’t have a good GPS, I didn’t have a good mobile map on my phone, and I was lost. And with a little bit of ingenuity and probably more luck, I was able to find my way home.

POST: How did you figure it out?

SOUDERS: So, I was able to actually use the stars to do that, which is kind of like a good nerd brag, but it was a very useful tool, and I think it’s one we don’t really use anymore because we don’t have to. But as Professor Nelson’s going to show us on our show today, there’s still some application to that. And even where there’s not, it’s still a pretty good tool to have.

NELSON: Well, the most obvious thing is the thing we all still know really well, which is the sun rises in the east and sets in the west. Same is true for the moon. And there are other sorts of things—the fact that we’re in the Northern Hemisphere means that moss grows on the north sides of trees, because the sun is always on the southern side of the sky—there’s some things like that that tell you a little bit about direction. If you want to use stars to figure out which way things are going, they do exactly the same thing the sun does, in that they all move from east to west.

SOUDERS: Most people are familiar with at least one or two constellations, and one of those in particular is a really good tool to help get you home.

NELSON: The thing that almost everybody can find is the Big Dipper. We’ve all probably traced out the Big Dipper, and of course the Big Dipper points at the North Star. The North Star, if you’ve ever spent a night outside, you start to realize that even though everything else is rotating overhead, the North Star doesn’t. The North Star is at the north rotational pole of the earth, which means that it doesn’t move in the sky, it just sort of sits there. On any given night, you can always find it, and it’s always pointing north. The same’s true in the Southern Hemisphere, you have the Southern Cross, which if you’ve ever been to the Southern Hemisphere, serves the exact same purpose. It sits over the southern rotational pole of the earth.

SOUDERS: So, we’re lost in the North American woods here; we’ve found a clearing, and there’s the North Star—is that then the direction, we move toward that star, we know we’re going north?

NELSON: Yeah, so the North Star will be—the point that is on the horizon that is closest to the North Star is north. So you have some rough idea of at least which way you’re pointed.

POST: So it’s relatively easy to use the stars to figure out where you are north and south. But as Professor Nelson says, it’s much trickier to figure out where you are east and west.

NELSON: This was a problem where ships would get lost; they would know where they were north-south, but they wouldn’t know where they were east-west, so it was really hard to tell how much further you had to go, for example, of if you had missed the island you were trying to get to. It turns out you can do the exact same thing that you did to determine your latitude, where you are north-south, to get east-west, if you know what time of day it is. Because just like the sun always comes up, right, in the east, if you know what time it is, and you know what time sunrise happens, then you know where you are. It’s the same effect that causes time zones, right? The sun rises an hour later each 24th of the earth you go around, and so if you know what time it is when the sun comes up, you know where you are, and you can do the same thing with stars. The problem is knowing what time it is.

POST: And in order to know what time it is, you need a reliable clock, which was a hard thing to do back in the old days—or, for those of us who have a dead cell phone. But as professor Nelson says, our early astronomers had already figured out some solutions for this.

NELSON: There’s actually a fun little thing that Galileo actually came up with. Galileo figured out—well, he observed—that Jupiter has moons. There are four of them that you can see easily if you have a clear night and a good pair of binoculars, you can see the four what are are called Galilean moons of Jupiter. And Jupiter actually has dozens of moons, but these are the four big ones and they’re quite bright if you get a clear night and dark skies and a decent pair of binoculars, you can see them, and Galileo could see them. And the four moons of Jupiter all orbit Jupiter with specific periods, so they take a specific amount of time to orbit. And so you have an astronomical clock. The position of the moons of Jupiter can tell you what time it is.


And Galileo actually worked this out, and as long as you could observe those moons regularly and carefully, you could actually tell what time it was, and hence, if you knew what time it was and you knew what time the sun came up, you could figure out where you were on the planet. So, the path the sun takes across the sky, if you’ve noticed, is also the one the moon takes. That’s also the one Jupiter takes, and Mercury, and Venus, and all of these planets—they appear in this plane that goes east to west across the sky. So that tells you sort of a line across the sky to look at, figuring out which one’s Jupiter is usually pretty easy, because it’s the bright one. The brightest planet in the sky is Venus, but Venus you can only see near the horizon, either at sunrise or sunset, depending on the season. But Jupiter, since it’s further out, you can see it at any time. So the Jupiter trick only works half the time, when Jupiter is up at night. If Jupiter is up during the day, you’re out of luck, which is one of the shortcomings of Galileo’s fun little trick. But as long as you can regularly check what time it is with some kind of reliable source, right, if you’re an explorer, if you can go somewhere that has a clock that you trust, then you’re sort of good for a few months while Jupiter’s up in the sky and you can observe it.

SOUDERS: Using this method of using Jupiter’s moons is really handy, but there might be a problem if you don’t have a couple of days to track the moons of Jupiter, or if it’s daytime.

NELSON: The sun’s always in the southern part of the sky in our part of the Northern Hemisphere here in Northern California. So that can give you a rough idea of which way is south, and if you know one cardinal direction, you can figure out the other ones—and you can always just remember the Never Eat Shredded Wheat mnemonic that tells you the ordering of the directions—but that tells you roughly which way south is. You know, one of the interesting things about what time we live in, is that we don’t have to rely on understanding north, south, east and west very often. We are often in buildings, we’re often, you know, other places where it doesn’t matter. We don’t say, oh you just go down the hall to the east and take a north. And so as a society, maybe we’ve forgotten that a little bit.

SOUDERS: Even though we think of navigation using the stars as a really ancient kind of method, there are still some modern applications to it. Modern sailors and pilots still have to learn celestial navigation as a backup in case their technology doesn’t work, and it’s still a very practical skill for any of us to have in situations just like that.

NELSON: I think there’s definitely some practical utility in just being able to orient yourself, to know which way you’re pointed and what might be around you. I think there’s also something that’s just, there’s a connection to where you are that I think we miss out on because we don’t have to worry about it so much. There’s something that tells us where we are and where we’re going. There’s just some aspect, or sort of wonder, about understanding why it is that the world behaves the way it does.

SOUDERS: Practical application or not, the universe is such a massive place and there’s so much stuff to learn that learning just for the sake of learning is worth it. It’s that curiosity that drives us even from young ages that makes things so interesting for the rest of our lives.

NELSON: It’s a useful exercise whether or not it has any practical value, whether or not you ever plan to be on a boat or drive a plane or go backpacking somewhere. This idea of just understanding how the world around you works is something worth doing. I can remember when I was a kid, we had, my dad had a telescope and we’d pull it out and look at stars and try to figure out how things worked in the sky and why they did what they did. Certainly played a big role in me wanting to study that sort of thing. The other thing that was really amazing to me is that we can actually predict what the sky’s gonna do. We can predict where the moon’s going to be, or what time the sun is gonna rise, or any number of things like that. I’m dating myself a bit here, but I can remember Halley’s Comet in the 80s, the idea that we could predict when that thing was going to show up, and how bright it was gonna get, and how its brightness was gonna change.

POST: Professor Nelson talked about how there’s some commonality in all of us witnessing these celestial events at the same time. When the whole world is watching an eclipse happen at the same time, you’re part of a common experience. We should just be interested in the stars because it’s interesting.

NELSON: It’s worth doing at some point in your life, you know, drag your kids out of bed, get up in the middle of the night and go look at one of these things. I remember we had the eclipse, the lunar eclipse, but you know, you get up in the middle of the night and go drag yourself on top of some hill and it’s cold and it’s windy, and you actually get to experience this sort of dramatic change that happens in the sky. Those experiences are, they really give you a sense of sort of the scale of things around us, right? Those sorts of scales are often lost on us, and so yeah, it’s absolutely worth looking into some of those events and observing some of those things.

POST: So Travis, next time you go on a road trip, what are you gonna do to prepare?

SOUDERS: The first thing I’m gonna do is make sure I pack an analog watch so I can tell what time it is, but the other is to also understand that yes, you can use the stars to help navigate and find your way if you’re lost. It’s still just a good skill to have because it connects us with our universe.

POST: And also, we should just take time to look up at the stars because it’s worth doing, and it helps us reorient our sense of place in this world.

SOUDERS: Thank you to our guest today, professor Nick Nelson. This has been Out of Curiosity. I’m Travis Souders.

POST: And I’m Kate Post. This show is produced by University Communications, with sound editing help from student Hayden Duncan. Make sure to never miss an episode by subscribing wherever you listen to podcasts.