Astronomers say that a once-in-26-year eclipse, predicted to occur in January, probably happened months ago without anyone seeing it.

Alpha Comae Berenices, from red and blue plates of the Digitized Sky Survey.
Alpha Comae Berenices, from red and blue plates of the Digitized Sky Survey.

Photometrists around the world have been psyched for months about the once-in-26-year eclipse predicted for the edge-on binary star Alpha Comae Berenices. We wrote it up in the January Sky & Telescope, page 50: "Vigil for a Unique Stellar Eclipse." The eclipse was due to happen just about now.

Well, oops. Lead investigator Matthew Muterspaugh (Tennessee State University) announces that the prediction was wrong. Three measurements of the binary made about a century ago accidentally swapped the primary and secondary stars. As a result, the eclipse (if it happened at all) already happened a couple months ago.

Muterspaugh writes,

I have some bad news regarding Alpha Com and its predicted eclipse. After many difficulties in obtaining updated separation measurements, it has been discovered that the orbit model which predicted the time of the eclipse was in error due to 3 position measurements from ~100 years ago listing the binary position angle incorrectly by 180 degrees. Using these skewed the orbital model, despite the presence of over 600 additional measurements, in just enough of a manner to cause the timing prediction to be incorrect. The eclipse, if it did happen, was about two months ago (at which point, the system was only barely above the horizon at sunrise, so photometry would have been rather difficult anyways). I am working on writing up a formal report detailing the error, describing the events leading to the discovery and correction, presenting a corrected orbit, and the times of the eclipses. However, I wanted everyone to know this as soon as possible so that efforts might not be wasted.

Here's the full story.

Next chance? Late 2041.


Image of Anthony Barreiro

Anthony Barreiro

January 20, 2015 at 4:16 pm

Some of us might make it to 2041. Perhaps by then Sky and Telescope will be a direct neural download, updated every millisecond.

I've been looking at Alpha Comae through binoculars when the weather has been clear before dawn. I haven't noticed any changes. But at least now I know where it is in the sky!

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Image of Peter Rowen

Peter Rowen

January 20, 2015 at 10:03 pm

I just put it in my Google calendar. The New Horizons encounter has been in there for years, so it's time to plan a little further out.

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