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

Question

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

Answer 1

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](http://www.efalk.org/Navigation/) on the subject a long time ago.

Before the accurate chronometer was invented (by [John Harrison](https://en.wikipedia.org/wiki/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.