Mercury and the bandwagon effect


The study of the planet Mercury provides two examples of the bandwagon effect. In her new book Worlds Fantastic, Worlds Familiar, planetary astronomer Bonnie Buratti writes

The study of Mercury … illustrates one of the most confounding bugaboos of the scientific method: the bandwagon effect. Scientists are only human, and they impose their own prejudices and foregone conclusions on their experiments.

Around 1800, Johann Schroeter determined that Mercury had a rotational period of 24 hours. This view held for eight decades.

In the 1880’s, Giovanni Schiaparelli determined that Mercury was tidally locked, making one rotation on its axis for every orbits around the sun. This view also held for eight decades.

In 1965, radar measurements of Mercury showed that Mercury completes 3 rotations in every 2 orbits around the sun.

Studying Mercury is difficult since it is only visible near the horizon and around sunrise and sunset, i.e. when the sun’s light interferes. And it is understandable that someone would confuse a 3:2 resonance with tidal locking. Still, for two periods of eight decades each, astronomers looked at Mercury and concluded what they expected.

The difficulty of seeing Mercury objectively was compounded by two incorrect but satisfying metaphors. First that Mercury was like Earth, rotating every 24 hours, then that Mercury was like the moon, orbiting the sun the same way the moon orbits Earth.

Buratti mentions the famous Millikan oil drop experiment as another example of the bandwagon effect.

… Millikan’s value for the electron’s charge was slightly in error—he had used a wrong value for the viscosity of air. But future experimenters all seemed to get Millikan’s number. Having done the experiment myself I can see that they just picked those values that agreed with previous results.

Buratti explains that Millikan’s experiment is hard to do and “it is impossible to successfully do it without abandoning most data.” This is what I like to call acceptance-rejection modeling.

The name comes from the acceptance-rejection method of random number generation. For example, the obvious way to generate truncated normal random values is to generate (unrestricted) normal random values and simply throw out the ones that lie outside the interval we’d like to truncate to. This is inefficient if we’re truncating to a small interval, but it always works. We’re conforming our samples to a pre-determined distribution, which is OK when we do it intentionally. The problem comes when we do it unintentionally.

Photo of Mercury above via NASA

5 thoughts on “Mercury and the bandwagon effect

  1. Richard I. Cook

    It goes back even further: in 1796 the astronomer Maskelyne fired his assistant Kinnebrook because the latter’s observations did not match his own.

    There is a good deal of literature that addresses the underlying sources of this kind problem, eg. Peter Galison’s “How Experiments End” ( Particularly relevant are chapters 5 (“Theoretical and Experimental Cultures”) and 6 (“Scale, Complexity, and the End of Experiments”). Also relevant is the classic, “Laboratory Life: The Construction of Scientific Facts” ( by Bruno Latour.

  2. What’s more, the shoddy researchers making this type of mistake are not only not contributing anything to their field, they are doing damage.

  3. I am in my mid-40s. I have a Masters degree in Physics. As far back as I can remember ever having heard about the length of Mercury’s day (in particular I recall the orrery in my 2nd grade classroom), I have been under the impression it perpetually had the same face toward the sun. I even had this conversation with my own children just last week, when my third grade son told me that he learned in school that Mercury’s day was the same length as its year. I affirmed this to him, and tried to explain to hmi the dynamics of tidal locking in a way a third grader could understand.

    So now, in 2017, I discover that Mercury’s day has been known to be 3/2 of its year since 1965, that is, longer than my entire life. How has this fact not percolated through institutions of learning?

  4. Another example:

    In the ’20s, the number of human chromosomes was discovered to be 48, and various studies confirmed that number. Then thirty years later, a new study said that humans actually have 46 chromosomes, and looking back at the old studies, scientists only found 46 chromosomes. The bandwagon effect (or confirmation bias) in action.

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