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The Solar Eclipse Coincidence

The views expressed are those of the author and are not necessarily those of Scientific American.

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Annular eclipse (Credit: sancho_panza)

When the Sun is eclipsed by the Moon this Sunday, for many observers across much of the world it will be temporarily replaced by a beautiful ring of fire – a brilliant annulus of stellar plasma just peeking out around the dark lunar disk. This doesn’t always happen, partial solar eclipses merely trim away a chunk of the solar disk, and true total eclipses perfectly blank out the visible surface of the Sun. It’s all a matter of alignment between Sun-Moon-Earth and our mutual orbital gymnastics.

It is an interesting coincidence that the Moon should so nearly perfectly blot out the Sun, since there is really no physical reason why this has be the case. The Moon happens to be about 400 times smaller than the Sun, but the Sun happens to be about 400 times further from the Earth than the Moon is. So simple geometry tells us that the apparent disk of the Moon is almost exactly the size of the apparent disk of the Sun. Of course this match is not always quite the same, the Earth orbits the Sun in a modestly non-circular, elliptical, path and so our nearest and furthest distances (perihelion and aphelion) differ by about 3.3%. And the Moon’s orbit has a roughly 10% difference between its near and far point to us, so the precise degree of total solar eclipse will vary a little as the apparent sizes of Sun and Moon vary. This Sunday the distance variations conspire to make the Moon appear 94.4% the size of the Sun.

However, on longer timescales the Earth-Moon system is not static. Tidally driven evolution of the orbits and spins of these two bodies results in a number of things. First, as we know well, the Moon’s spin rate is matched to its orbital period so that it always has the same face to the Earth (except for some small librational wobbles). Second, because the Earth’s spin is faster than the Moon’s orbit, the tidal bulge raised on the Earth pulls on the lagging moon, gradually raising its orbit and slowing our day. Every year the Moon’s orbit grows by some 3.8 centimeters and our day lengthens by about 0.000015 seconds.

At this present rate, in about 50 million years the Moon will never completely eclipse the Sun, it will simply appear too small on the sky. This orbital evolution also implies that total solar eclipses in the distant past would have been just that – completely obliterating the Sun from view. It is very likely that a scientifically minded Tyrannosaurus Rex never got to see the circle of fire, or Bailey’s Beads in an eclipse.

So is there some great significance to the fact that we humans just happen to exist at a time when the Moon and Sun appear almost identically large in our skies? Nope, we’re just landing in a window of opportunity that’s probably about 100 million years wide, nothing obviously special, just rather good luck.

The eclipse path, from one side of the planet to the other (Credit: NASA)



Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. jtdwyer 5:25 am 05/19/2012

    Perhaps the greater coincidence is that humanity just happened to develop during the rather brief period of time when the moon so neatly fits just within the Sun’s disc at its full eclipse, that we are present to observe the Sun’s corona. Earlier in the Earth’s history the moon would have totally obscured the Sun at its maximal eclipse, and eventually the receding moon will appear much smaller than the Sun even at its maximal eclipse.

    Of course, Earth’s now rather stable rotational and tidal conditions resulting from the specific proximity of the single, massive moon’s orbit may be a critical factor in the development of complex life and human intellect – perhaps that’s the greatest coincidence…

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  2. 2. HMS003 10:46 am 05/19/2012

    “So is there some great significance to the fact that we humans just happen to exist at a time when the Moon and Sun appear almost identically large in our skies?”

    I think this statement highlights some general misconceptions people have. The correct statement may be that environment generated correlations with the observed position of the moon have significantly more relevance than human generated correlations, however the generation of human correlations is highly dependent on the particular observed position of the moon.

    Certainly by now we should be able to make the statement that a 100 million year window is sufficiently long to impact evolutionary processes.

    Where people get confused is with the particular weighting scheme to place on the correlations. If we would choose to place more weight on the human generated correlations (flip the heirarchy) then the particular position of the moon has a huge significance and is actually dictated to a large degree by our specific existence as humans.

    This point of view is unsettling for a lot of people because it seems unnatural and it is certainly much easier to assume our own existence is of little weight. However, a priori there doesn’t seem to be a natural requirement that the hierarchy should have a particular direction.

    So from a certain vantage point the quoted remark is completely acceptable, but is certainly betrays a particular prejudice.

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  3. 3. Cognosium 5:25 pm 05/19/2012

    This, of course, inevitably raises the issue of “fine tuning”, the fitness of our planet, and perhaps the universe as a whole, for biology.

    There is a common misconception that “fine tuning of the universe for life” implies the existence of some kind of “creator” or “designer.”

    In fact nothing could be further from the truth.

    There is actually a great wealth of other evidence of seemingly inevitable directionality and “just right” conditions to be found downstream of the usually quoted dimensionless physical parameters, especially in such areas as geology, chemistry and biology.

    Most clearly observed in the way in which the the properties and timely abundances of the chemical elements and their compounds not only have allowed, but have made virtually inevitable the observed evolution of technology in the medium of the collective imagination of our species.

    This persistent and pervasive pattern is not to be ignored or swept under the mat by the very unparsimonious artifice of positing multiverses with infinitely varying physical properties. Nor by the tautologous anthropic platitude that these things must be so, or we would not be here to observe them

    Nor does it require for its interpretation “intelligent design” which is essentially a notion derived from the hearsay of superstitious mythology.

    Indeed, if we are to be objective, there are really no “designers, not even human ones.

    For it can be argued, with strong evidential support, that, except in the most trivial everyday sense, we do not invent or create artifacts or systems but that these are more properly viewed as having evolved within this collective imagination of our species.

    This being inherent in the machinery of nature.

    To quickly put this counter-intuitive view into focus, would you not agree that the following statement has a sound basis?

    We would have geometry without Euclid, calculus without Newton or Liebnitz, the camera without Johann Zahn, the cathode ray tube without JJ Thomson, relativity (and quantum mechanics) without Einstein, the digital computer without Turin, the Internet without Vinton Cerf.

    The list can. of course be extended indefinitely

    A broad evolutionary model of the kind outlined in “The Goldilocks Effect: What Has Serendipity Ever Done For Us?” will suffice to account for these patterns on a straightforward empirical basis. At the expense of swallowing a few human conceits!

    The book is avaiable as free download in e-book formats from the “Unusual Perspectives” website

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  4. 4. Dragonkill460 6:14 pm 05/19/2012

    so you say the moon is leaving the earth by about 3.8 centimeters, but as the orbit increases wouldn’t the effect of gravity decrease therefor sending the moon away at a farther pace?

    i personally love the moon. it is so amazing with the tides and lighting up the night sky and eclipses it is a mystery of epicness and perhaps we can plant a tracking system on the moon and as it goes farther from the earth it goes towards somewhere else and we can discover more of the universe

    cognosium, how can you say that there is no creator? what proof do you have? true we would have have geometry without Euclid, but only because someone else would have discovered it! people create, discover, explore everything we can, how could we have gotten the moon, in the Goldilocks zone, done anything he have done without some outside influence? its about 1 in 1,000,000,000that life started by itself, about 1 in 10,000,000 that it evolved, about 1 in 100,000,000 that it evolved into intelligent life, and very slim that we do not see evolution now

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  5. 5. Padgie 7:21 pm 05/19/2012

    The pressures of evolution and life fine will tune the participants to the environment. Should there be participants they may get to the level of thinking the environment was fine tuned to them. This level of conceit is what separates us from the common animals. Or maybe it is the ability to devise sophisticated arguments to suit what you have in mind.

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  6. 6. HMS003 2:05 pm 05/20/2012

    This has nothing to do with conceit or anthropic arguments. We have to accept there are solutions which contain consciousness and that there is a certain uniqueness to those solutions. If we posit some set of equations as constraints, we are also further reduced to consider that only solutions with certain types of observers can in any sense be considered “real”. In one space we consider probability amplitudes as what is defined for all time, in another we project all the potential combinations of observed outcomes.

    We are a little constrained by our definitions though. In order to have the projective space of observed outcomes, we must have something well defined as an observer. The observer provides the framework for the mutual exclusion of branches of outcomes. However, all outcomes remain self consistent with respect to what an observer sees. We have proven time and again that observer and observed are not separable. The appearance of a watch must imply a self consistent chain of events that can explain its existence starting with the simplest components. The situation would be no different than if a piece of alien technology was found in an archeological dig without any other information. The observance of such technology would prove absolutely the existence of aliens.

    There is nothing wrong with making a statement that humans are fundamentally unique in several respects. Certainly we do not have the ability to rule out exceptionally intelligent E.T.s, but we have enough evidence to say that the existence of such entities is exceedingly rare, which leads us to the conclusion that we should place more value to our own existence than some would choose to do.

    It should come as a shock that our distant cosmological horizon follows certain statistical laws. It should come as a bigger shock that even if we were to accelerate to the speed of light in any direction we would never reach the horizon, and it would still follow certain statistical laws. Combined with the fact that all particles have a dual wave interpretation, we have to contemplate the reality that every thing we see is a projective creation of whatever process our brain follows when accepting incoming frequency and intensity information. If our species eventually dies off, it is absolutely certain that barring some sort of artificial resurrection we would always exist in a place with a cosmological horizon exactly as it appears within the window of our current existence.

    Asking a question about whether the universe is custom built for use is not the issue, the issue is that it is increasingly impossible for us to be able to tell the difference. Invariably, a sufficient amount of processing must be able to derive the existence of the cosmological horizon, and this is possible because of a certain self consistent statistical determinism.

    What we then must contemplate is that since the universe has a horizon that follows a certain statistics, which may mean that time is emergent, there are certain laws that have an eternal existence. Otherwise we run into seemingly absurd questions about the statistical probability of the emergence of statistics (generally) from some other set of constraints, which is nonsensical. That we must assume that there exists something that extends beyond the mechanics of the universe is one possible candidate solution, although maybe not provable.

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  7. 7. HMS003 2:26 pm 05/20/2012

    As a follow on, this makes one wonder if a universe is defined as the composite of the all the unique solutions associated with each of the time concurrent independent observers.

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  8. 8. Plain-2009 1:52 am 05/22/2012

    Extremely interesting! Extremely interesting! So much that I would like to start studying Astronomy. I have absolutely no heavy reason to oppose the authors point of view but it seems to me that the fact that we are living in an era in which we see in the sky the size of the moon almost identical than the size of the sun is a lot more than good luck! And it has probably a lot more than superficial implication!

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  9. 9. Plain-2009 2:03 am 05/22/2012

    Was the Antikythera mechanism able to predict this yesterday eclipse or it only predicted eclipses seen in the Mediterranean area?

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  10. 10. quizzical 10:57 am 05/23/2012

    I am constantly amazed at how some folks’ super-egos assume that they know much more about all things than they actually do.

    This constant ruminating about the good luck that lies behind all the wonderful details of human existence convinces me that they are only armchair dreamers and not real scientists.

    The very existence of genetic codes proves the existence of an Intelligent Author. All codes are languages and all languages are codes. There is no such thing as any intelligent language or code that exists without an intelligent author.

    Folks can babble all they want to about life bubbling out of a warm ancient mud puddle, but that just shows their gross ignorance of what is really involved. Not only did the first living cells need to self-assemble, but also the code needed to be written that directs such self-assembly AND reproduction.

    So . . . Here is just a bit of evolutionary trivia:

    There are FEWER than 474 quadrillion seconds of time in 15 billion years. (any interested folks can probably handle that math)

    There are MORE than 1,152 quadrillion possible ways to arrange only 30 nucleotides in a DNA sequence. (4 nucleotides to the 30th power)

    Tell me again, how did all this massively complex and specifically designed and balanced biology ever develop by chance in the little time (3.5 to 4 billion years) that is hypothesized?

    The anti-God crowd is so allergic to simple reality that they will dream up endless nonsense to the contrary.
    I think they should stick to playing the lottery. The chances are much more favorable there!

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  11. 11. pbaylis 8:17 am 04/26/2013

    Yes, Caleb, I think we know what’s nagging in the back of your mind as you wrote this:-)

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