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In a message in the Jupiter's moons thread about compiling and using a simple database of almanac data, I mentioned that I left out a few important issues. And in a footnote, I wrote, " important issues... like star coordinates!"

For the folks who enjoy astronomical computation, if you take a sight of the star α Centauri, or Rigil Kentaurus as nautical almanacs named it and now adopted by the IAU as "official", what should you use for its coordinates? This is a double star with two bright components which can be distinguished at relatively low magnification. They orbit around each other in a bit less than eighty years. What you see through a sextant depends on your magnification. And what you are able to measure also depends on magnification. Should an almanac provide the coordinates of the center of light of the pair? Or the center of mass? Or just the coordinates of the brighter star, alpha Centauri A? There are no absolute answers here.

For a sense of scale, I'm attaching a standard orbital diagram (from here). Note that this is a "relative" orbit, so it treats the brighter star, A, as a fixed center with the fainter B component orbiting around it. In reality, A and B orbit their common center of mass with offsets directly proportional to the mass of each star. The star system's proper motion (against the background stars) over decades is a nearly straight line for the center of mass with A and B oscillating back and forth around that straight line. By the way, in this orbital diagram, the scales are in seconds of arc, but notice that they don't match. This is common in depictions of double star orbits. To picture its proper shape, you may want to stretch it vertically.

A bit of legend has developed around the use of double stars in celestial navigation. This appears to originate in statements from sources at the US Naval Observatory in documentation for the WDS (Washington Double Star) database. The WDS is (or has been) the primary, large database of double stars. The WDS is primarily of interest to professional astronomers as well as some private astronomers. The statement, attached image below, claims that the USNO needs to study double stars because of the issues for celestial navigation. Except for a handful of stars, like alpha Centauri, this is mostly a non-issue for normal sextant navigation. Advanced automated systems with higher angular resolution have a little more to worry about, but in that case, the question can surely be resolved with test sights. I suspect that the quote was really a slightly misguided attempt to explain (and excuse) why a US military department should be expending resources on maintaining a database that has no real practical value.

The quote below refers to double stars, including our friend Rigil Kentaurus, as "vermin of the sky". Now that's not very polite! Besides, I have heard that expression before, and it was normally applied to the endless millions of minor asteroids that still turn up as "noise" in deep surveys of faint stars. Those little rocks are the vermin, not the poor double stars! :)

Frank Reed

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