Category Archives: Uncategorized

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.)


astrophotography sites

The Feb 2015 issue of Sky and Telescope has a great article on very good amateur astrophotographers. (unlicensed copy here). Here’s a list of the links in the article.


Deep and Wide

Narrowband nebulae

Imaging for Science

Pushing the limits of resolution

Planetary feats


A fellow Reed alumni told me about iTelescope, a turnkey system for making your own astronomical images with rented time on scopes. Very simple and not too expensive. He had a nice 8 minute exposure of the Orion nebula, something pretty, for $22.


Max Ogden asked about Google Maps-like things for astronomy images, free alternatives to Google Sky. I foolishly said there were none. Happily the author of Chromoscope corrected me. does a nice job with different spectra. The source images are kind of low res though. It’s all on GitHub though and the image sets are downloadable, so it wouldn’t be hard to roll a different viewer.

I wonder what the state of the art in non-Mercator slippy maps are? Raster reprojection was a mess last I checked.

Here’s the Twitter conversation. I asked about other image sources, he referenced  and 

He also pointed me to aka

Observing programs

The July 2014 issue of Sky & Telescope has a couple of good articles on structured observing, going out at night with a specific plan to do something systematic. That seems like a good idea to me, would help me focus.

One of the articles specifically suggests the AstroLeague Observing Programs. It’s basically merit badges for astronomers, you record observations in a log and get a commemorative pin when you complete enough of a program. Well $30/yr members get the pin; cheapskates can just work from the website for free.

Some programs that look good for beginners: Binocular Messier, Constellation Hunter, Deep Sky Binocular, Universe Sampler, Messier Telescope.


Astrophotography without a telescope

Something I’m wondering about if it’s possible: getting some of the pleasure of amateur astrophotography without a telescope. Just start with raw image data captured from someone else and posted online, then do all the processing yourself. Sure it’s not as fun as making the exposure yourself, particularly the sense that somehow you didn’t acquire the image yourself. But my impression looking at astrophotos is that the real artistry and creativity is in the image processing anyway. Also maybe it makes sense to start with the software, since that’s much less of an investment than the telescope and mount and imaging system. If I don’t find hours of processing an image until it’s just right fun, it’s unlikely I’ll find it fun first spending hours and $$$$s acquiring an image, then more hours processing it until it’s just right.