How can the Moon affect a star?

As things go in the universe, the Moon is a small thing, insignificant compared to a distant star.  Yet our astronomer found that it can have a definite influence on measures of the latter.

Occasionally our astronomer finds time to do some astronomical research.  We encourage it, if for no other reason than he seems happier afterward.  His current investigation showed something unexpected–which of course is what we always hope for.

His task at hand is to fit a smooth curve to some hundreds of measurements of the brightness of a certain star.  The difficulty (which makes it interesting) is that each of the measurements has something called “experimental error.”  This doesn’t mean that anyone has made a mistake!  It’s just that due to all sorts of small disturbances, turblence in the atmosphere or vibrations in the wires, no measurement is absolutely exact.  We prefer to call this unavoidable fuzziness experimental uncertainty, but the shorter word tends to win out.  At any rate, measurements of the same star at almost the same time never quite agree.  The smooth curve is meant to show what the star is actually doing, cleared of this noise.

(The data came, free, from the American Association of Variable Star Observers.  This organization has been doing what we now call “citizen science” and “crowdsourcing” for well over a century.  Their website is quite worth wandering through.)

In the old days it wasn’t hard to do, though often tedious.  You plotted all the points on graph paper, then with care and a French Curve, you sort of averaged out the dispersion.  It’s something the human eye-hand-brain system does well.  Of course these days our astronomer uses a computer and some fairly sophisticated number-crunching.  In order to get the curve to look plausible, he tried several different schemes (some with names like Legendre Polynomials and Savitsky-Golay Smoothing, which show how serious he is).  But there always remained a certain very small waviness, a regular tiny oscillation.  It seemed to repeat about every thirty days.

It was much too regular for this kind of star (this is where experience and scientific intuition come in).  But where could it come from?  Finally, he looked very carefully at the clouds of data points.  It seemed there were more, and then less, on a regular basis.  Checking the dates and a calendar, he found that there were fewer measurements at the Full Moon.  Well, amateur astronomers (the source of all these data) tend to avoid the Full Moon, when the sky is bright and it’s harder to see things.  This monthly variation in observers was translated by the software (which works faithfully but not intelligently) into a thirty-day oscillation in the average curve.  It is certainly present in the data–but not in the star.

The lessons from this episode mostly aren’t new, but they’re worth repeating.  Computers cannot lie, but neither can they use judgement.  Experience and intuition can help a lot.  And your data, especially if you’re an astronomer, pass through much before you get your hands on them.