Monthly Archives: March 2016

Direction of star rise / set

I’ve been off-and-on interested in Polynesian navigation. Just went to Hawai’i where I saw the Bishop’s planetarium presentation on it, which wasn’t great but made visual what I’ve read.

The key thing that I didn’t understand there was that the Hawaiian latitude, the star Procyon always rises due east and sets due west. Or within 6° of that, at least. That seemed like magic to me! But it turns out all stars have the property that they rise and set at the exact same place on the horizon every day. Ie: the azimuth at star rise is constant for a specific latitude. Except the sun of course, which wobbles with the seasons. I’d naively thought the whole sky wobbled the same way which shows how unobservant I am. (Why doesn’t it wobble? Does it wobble a tiny amount, except the distance to the stars is so great it’s not visible?)

The Hawaiian star compass is a navigation tool that uses this property to determine latitude. To grossly simplify, it’s a list of stars and the azimuth they rise/set at for the specific latitude of Hawai’i. If you’re in Hawai’i and want to sail northeast, find where Castor rises in the sky and sail to that point. Can also be used to travel to a specific latitude. Anywhere in the world, if you want to get to Hawai’i sail due north or south until Procyon is rising due east, then sail due east or west until you hit land. Admittedly that’s not perfect, but it captures the idea. Polynesian navigators knew which stars corresponded to which islands, and could use that knowledge to navigate to different islands’ latitude. Pretty amazing.

Reading all this reminds me you can also use the time of star rise / set as a way to determine longitude. That time depends on the day of the year and your longitude. Using this in practice requires an accurate clock and a good database, something humanity only accomplished in the 18th century. It’s much easier when the star you reference is the sun, at noon.

Of course Polaris is the absolute best star for navigation, always pointing due north all night at a fixed altitude. But it’s only useful if you can see it, which means you have to be in the Northern Hemisphere. And ideally not too far north, it’s hard to make out direction when it’s too high. Also it only has the nearly-true-north property from 500-3000AD. So these alternatives are handy. (Also TIL Polaris is a Cepheid variable.)

(I had thought this blog was totally dead, I seem to have lost all interest in using my telescope and also let my Sky & Telescope subscription lapse. But here I am.)