Monday 28 September 2015

Blood Moon: Recalling Eratosthenes


Although the Observatory at Greenwich still be-straddles the globe as the origin of longitude, and thus of Time itself, London long ago ceased to be a good vantage point for examining the heavens. Anthracite coal conquered the world, but it besmirched London’s skies, and then the Clean Air Act coincided with ever-stronger electric street lighting, so light replaced soot as the celestial pollutant, fading distant stars. Of more moment, British skies are always covered with clouds, so astronomy is well-nigh impossible. And yet, and yet, last night the London sky was sparkling clear, so the whole supermoon lunar eclipse was visible for the seven stages from first to final contact.

At this numinous and transient moment I sleepily tried to explain to myself what was happening. The earth was travelling round the sun, but not at a speed sufficient to account for what I was observing. The earth was rotating on its axis, but rotation does not cause shadow. The moon was apparently fixed in the sky, yet it was slowly falling into a shadow caused by the nearest heavenly body, the Earth, and the atmosphere of that home planet was causing selective filtering of light-waves, taking out the shorter ones, and leaving the red.

If Eratosthenes, the third-ever Chief Librarian of the great library at Alexandria, had been standing beside me at my London window, he would have been the perfect teacher, had he not, as would have been more likely, been using the event to avoid chit-chat and make his own observations and calculations.

Astronomical events were the first and greatest of puzzles faced by our ancestors, the stuff of creation myths, superstitions, rituals and eventually sceptical surmise: the dawn of science. Astronomy required a leap of understanding: that the all too solid earth on which we stand might also be just one hurtling dot among the many visible (and invisible) one in the skies. Tycho Brache, the last of the naked-eye astronomers, was chronicling the regressive paths of the planets, but could not fully agree with Copernicus’ interpretation of those wandering planets in terms of the Earth’s own orbit round the sun.  The Copernican shift of perspective was a leap of Piagetian proportions, in which an observant maturing child eventually understands that what they see, and what a doll placed in an assembled mini-landscape sees, are not one an the same thing. The ego-centric perspective of early childhood is attenuated by a growing appreciation of the perspectives of other minds. It is similar in intellectual status to a growing child noticing that kindergarten children are becoming smaller, but eventually realizing that is only because he has grown bigger, not because new generations have shrunk.

How well do contemporary citizens understand astronomy? I should add, understand it without looking it up and repeating it, only to fall into ignorance again? If the Flynn Effect is real, then it will be far easier for average persons to understand eclipses, night and day, summer and winter. I do not have up to date data on pass rates, but here is an interesting finding from a random survey of British adults in 1992, which is 122 years after the 1870 Education Act and 472 years after the publication of Copernicus’ De revolutionibus orbium coelestium in 1543.

John Durant, Geoffrey Evans and Geoffrey Thomas. (1992) Public understanding of science in Britain: the role of medicine in the popular representation of science. Public Understand. Sci. 1 161-182.



So, 30% of British adults thought that the observed passage of the sun in the sky meant that our star obligingly whizzed round us. 16% imagined it did so once a day, as per visual observation of the same apparent phenomenon. More recent findings gratefully received.

If I concentrate on matters involving events observable to our ancestors, and avoid calendrical calculations, science education websites show that popular misconceptions still include the following: that the phases of the moon are caused by the moon going into and out of the earth’s shadow; that the moon has a side which is in perpetual darkness; that the moon does not rotate; that the phases of the moon are completed in exactly the number of days it takes to completes its orbit of the earth; that the moon is somehow larger on the horizon than when it is high in the sky; that the four seasons are the result of the changing distance from the sun; and that heavier objects fall faster than lighter objects.

I know that we no longer live on the land, and therefore are more distant from nature, and from the peregrinations of the sun and moon. I know that ignorance about astronomy is very largely a matter of education, but it is likely also that some education has been given but was not retained, because egocentric observation is deemed sufficient by many people. I know that I can make errors, and that many people make errors in simple Newtonian physics (imagining that if you drop an object when running it will describe an arc backwards to the ground, not forwards to the ground).  I know all this, but if we were really getting brighter over the last two centuries we would be able to work out much of this for ourselves, as Eratosthenes worked out the circumference of the world when he heard a casual remark about a well to the south of Alexandria where on one particular day of the year the sun shone down to the very bottom of the well.

Just for amusement, here are some Northern hemisphere university graduates explaining the astronomical causes of seasonal variations.

Here are some popular misconceptions tracked down and explained.

So, these were some thoughts whilst watching the super-moon lunar eclipse last night, among the blazing street lights of urban London. If by some magical process Eratosthenes had stood next to me I am sure that I would have best served him only by listening to him attentively.


  1. Re: "taking out the longer ones, and leaving the red."

    Taking out the shorter ones.

  2. Thanks. You and one other on Twitter noticed my error.

  3. Here's a video, "A Private Universe," of Harvard students and working class high school students both explaining that it's colder in winter because the earth is further from the son:

    Both demographics give the same wrong answer, but the Harvard students are vastly more self-confident and delighted with their own cleverness.

  4. “So, 30% of British adults thought that the observed passage of the sun in the sky meant that our star obligingly whizzed round us. 16% imagined it did so once a day, as per visual observation of the same apparent phenomenon.”

    16% thought the earth took a day to go round the sun.

  5. Yes, correct, all this whizzing about has confused me.

  6. I disappointed several people in my childhood by taking next-to-no interest in astronomy. I think it was my revulsion from the stupid names of the constellations that put me off. On the other hand, the round earth notion seemed second nature to me - anyone who lives by the sea ought to find it intuitive, and anyway the moon looked round so why not the earth?

    If I remember correctly, there was a way of indirectly observing the sun that revealed its roundness too, but I can't remember the trick of it.