How do you navigate by the stars if the Earth’s rotation means they’re always moving?


I just finished my ten thousandth watch if Moana with my son, and I do not understand how you can travel by holding your hand up to the starry sky. It would make sense if the stars were stationary relative to your position, but they’re not. A star you measure at 10pm is in a completely different position by 2am. I understand the Disney version is an oversimplification, but how does the real thing work?

In: Earth Science

>A star you measure at 10pm is in a completely different position by 2am.

Except for one. Since the stars *turn* in a circle there’s one spot in the sky that doesn’t move— the center of the circle. In the northern hemisphere, just by coincidence, there’s a star almost exactly at that spot. A fairly bright one, too, called Polaris, at the end of the handle of the Little Dipper (the tail of Ursa Minor).

If you can see and identify Polaris you know that that direction is north, which lets you keep the boat pointing in the right direction. But also, if you can measure how far it is above the horizon, *you’re that far north from the equator,* in terms of degrees latitude.

If you’re in the southern hemisphere, it’s harder. There’s still a stationary spot in the night sky, but there’s no star right on top of it. You can do the same trick by finding the spot where an imaginary star *would* be (by recognizing the stars nearby), but it’s harder to get good measurements.

The Disney version wasn’t that simplified. Polynesians really did just use the hands.

The stars move in a very specific way in tandem with each other.

If X shifts to the west, Y will come from the East. These are setting points, so like when the sun sets, you know which way is west and the general time.

As you developed experience, you memorized sets of stars and could navigate that way.

Stars follow predictable patterns. A star moves across the sky over the course of a day, but if you know where it was at 2 AM last night, you know it’s going to be in pretty much the same spot at 2 AM this night.

Obviously stars move over the course of a season as well, but there are stars that are visible all year, called circumpolar stars, so those stars in particular are useful for navigation

Some very good explanations on how using a fixed point can give you latitude. In order to accurately get longitude, you’ll need an accurate time keeper (i.e. a watch). That’s actually what drove the race toward making an accurate clock.

Modern celestial navigation requires that you also know what time it is so you can look up the star’s position in the navigational almanac. I actually wrote a [tutorial]( on the subject a long time ago.

Before the accurate chronometer was invented (by [John Harrison](, it’s an interesting story), your options for telling the time were limited. You could note the Moon’s position against the background of the stars, but the math behind it was murder and it wasn’t very accurate. You could note the transitions of Jupiter’s moons, but you needed a telescope and a stable place to observe from, so it couldn’t be done at sea. On Cook’s first voyage to Australia, they used this method. It meant sending the ship’s astronomer to the nearest beach in a rowboat. This is part of why Cook’s maps were so good.

Without a way to tell time, there was really no way to get your longitude. The best you could do was to pick a star (or planet or the Sun), and note its elevation at its highest (or lowest) point in the sky. That, plus an almanac would tell you your latitude. Typically, ships navigating by that method would sail to the latitude of their destination and then just keep sailing due west or east until they reached land.

Of course, if you can see Polaris, it’s a lot easier since it’s less than a degree off of true north. That won’t work in the southern hemisphere, of course.

You were asking about the Polynesians, of course. Someone else would have to answer that. I think it basically consisted of paying attention to where on the horizon stars rose or set.

One of the things I love about Moana is that all the incidental stuff in the movie — the plants and animals, the architecture and culture, etc — is very well-researched*, so the deviations from actual Polynesia are deliberate screenwriting choices rather than mistakes. And that goes for the astronomy too.

In the movie, Moana is sighting by the particular stars that Europeans call Orion, and which the Hawaiians know as “Ka Hei-hei o na Keiki”, or “Cat’s Cradle” — yes, the kid’s game with string. The three stars in the middle we call “Orion’s belt”, and the Hawaiians call “Na Kao”, or “The Darts”.

These stars are distinctive: they lie along the celestial equator, so they always rise in the east and set in the west, no matter where you are on Earth. So first, you can tell what direction you’re heading by spotting them near the horizon. But even better, the *angle they make with the horizon* can tell you what latitude you’re at — more or less. You can see Moana using her hand to check that angle.

[Here’s]( a quick demonstration I put together with planetarium software. The [first image]( shows Orion setting from Hawaii, the [second image]( is from Tahiti, and the [third image]( is from northern New Zealand. As you go south, the three stars are more vertical, perpendicular to the horizon as they set.

The only real simplification Disney made in this shot is that Orion isn’t the only constellation the Polynesians used. They had a number of “star lines” — chains of constellations and star patterns that formed lines in the sky, which could be used to identify direction or latitude at any time of year, even when the sky was partly cloudy. Here’s more info:

* I grew up in Hawaii and took a bunch of Hawaiiana classes in school, but I’m not a native. So take my opinion that Moana is “well-researched” with a grain of salt.