A question from PCC(E)

I’m admitting my ignorance by asking this, but if I don’t ask I won’t learn. Here is my question:

“Why, when there’s a solar eclipse, is the Moon exactly big enough to blot out the Sun? Does this have something to do with the laws of physics as manifested in orbits, or is it just an accident?”

Clear answers appreciated.


  1. Neil
    Posted August 21, 2017 at 6:17 pm | Permalink

    just a coincidence

  2. George
    Posted August 21, 2017 at 6:17 pm | Permalink

    It is a fluke that is explained well here:

  3. Posted August 21, 2017 at 6:17 pm | Permalink


  4. John Hamill
    Posted August 21, 2017 at 6:19 pm | Permalink

    Yes. It’s just a coincidence that the ratio of the size of the moon and sun, is the same as the ratio of the distances between the objects. I believe it wasn’t always so. The moon used to be closer to the earth and is becoming more distant over time. I think. 🙂

    • Posted August 21, 2017 at 6:23 pm | Permalink

      Oops, too late with my comment.

    • Posted August 21, 2017 at 7:21 pm | Permalink

      I was amazed to hear this very explanation given by a science head on MSNBC as they watched the eclipse march across the country. They even noted that the moon is in an orbit that is very slowly moving away from its current earth orbit and how the eclipse will likely look to viewers about 2500 years from now.

      The exchange was excellent until Ali Valshi open his idiotic trap and offered his opinion on why the clouds were clearing in the center of the country long enough for it to be observed: “Looks like a higher power had something to do with it.”

    • eric
      Posted August 21, 2017 at 7:56 pm | Permalink

      IIRC it’s not actually “the same,” the (apparent) sizes are a few percent different.

      Surprising coincidences where humans unconsciously themselves a lot of leeway in what counts as “the same” aren’t really very surprising. 🙂

      • gravelinspector-Aidan
        Posted August 22, 2017 at 6:39 am | Permalink

        IIRC it’s not actually “the same,” the (apparent) sizes are a few percent different.

        But the range to the Moon during an eclipse also varies by several percent (“Kepler and his damned ellipses” – quoth Ptolemy), and the range to the Sun by about 1%. Whether the shadow falls on an equatorial path or a near-polar path would make about 6000km difference and so 0.006/(0.363, 0.384, 0.406) (perigee, semi-major axis, apogee) or ~1.5% difference. Which is why you get a mix of annular eclipses, total eclipses and non-eclipses instead of a totality every month.
        You’d have to be a pretty astute observer with leisure time on your hands to notice an eclipse of less than 90% obscuration, if you didn’t know it was coming.

  5. Posted August 21, 2017 at 6:22 pm | Permalink

    I think the moon did a much better job a few hundred million years ago when it was closer to the earth.

  6. ThyroidPlanet
    Posted August 21, 2017 at 6:23 pm | Permalink

    Exactly my thoughts this afternoon – huh, the moon is almost the same apparent size, and I bet it’s pure coincidence.

  7. Posted August 21, 2017 at 6:24 pm | Permalink

    A happy accident.


    • Posted August 21, 2017 at 9:46 pm | Permalink

      Or… maybe it is *photoshopped*! 😀

      • John Frum
        Posted August 22, 2017 at 4:01 pm | Permalink

        What if g*d used something like photoshop to perform miracles.
        There’s a book in that.

  8. Mike Anderson
    Posted August 21, 2017 at 6:24 pm | Permalink

    The sun and the moon appear to be the same size because the sun is 400 times larger but 400 times farther away.

    This is clear proof of an intelligent designer.

    Also: the moon’s orbit is moving away from the earth by about 4 centimeters a year. This is so when we colonize the moon and start building houses there, and effectively increase the diameter of the moon, it will maintain its apparent size as viewed from earth. God thought of everything.

    • Greg Geisler
      Posted August 21, 2017 at 7:19 pm | Permalink

      SO good! You beat me to it. Nice job, Mike!

    • infiniteimprobabilit
      Posted August 21, 2017 at 10:06 pm | Permalink

      Beat me to it too. Damn.

      Of course, we didn’t see your little eclipse in NZ. Instead, we had our own eclipse. The Sun was eclipsed by the Earth. Lasted 12 hours. Happens every day.


    • Posted August 22, 2017 at 7:23 am | Permalink

      Of course the moon was closer in the past & is moving away from the earth about 1cm a year I think…

    • Posted August 23, 2017 at 3:20 am | Permalink

      Maybe by the time we colonise the Moon, Dog will have intelligently redesigned us so that we don’t have to find out what it’s like long-term to have haemorrhoids in a low-gravity environment. Or dyslexia.

  9. Posted August 21, 2017 at 6:25 pm | Permalink

    But it’s a pretty amazing coincidence, don’t you think? And just like everything else, it seems, it’s proof of God :-).


    • Larry Smith
      Posted August 21, 2017 at 6:35 pm | Permalink

      What a depressing article and a boatload of scary comments! People, we are lucky to have this site and the opportunity for rational discourse.

      • merilee
        Posted August 21, 2017 at 6:45 pm | Permalink

        I wish I hadn’t looked at the comments…The article was bad enough.

        • rickflick
          Posted August 21, 2017 at 6:56 pm | Permalink

          I think most of the comments are from Deepak Chopra’s fans.

    • prinzler
      Posted August 22, 2017 at 11:51 am | Permalink

      Ouch, what an article! My highlight:

      “Of course what this all meant was simply that these immemorially ancient and vast objects, though as different in size as a single BB and a super gigantic beach ball — one that was over six feet in diameter — would from our perspective here on Earth seem almost precisely the same size. So if they ever just happened to align in the sky, they would match up perfectly. Not almost perfectly. But perfectly, and bizarrely so. ”

      Wait, what? The ratios do not match perfectly, and you even said that it seems “almost precisely” the same size.

      “Almost precisely” ≠ perfectly.


    • alexander
      Posted August 22, 2017 at 12:57 pm | Permalink

      Interesting. This insight allows you to actually play god. It is known that the Moon increases the size of its orbit around Earth because the Earth-Moon system loses energy in the form of friction in the tides on Earth that are caused by the Moon movíng around Earth. Now, there are electricity generating plants that use generators driven by the flow of water in and out of estuaries, using turbines (we should call this Lunar power). This obviously increases the energy dissipation caused by the action of the Moon, causing the tides. If you are connected to such a power system, and you switch on a light, you increase the energy dissipation caused by the tide, and you move the Moon farther in its orbit for perhaps a nanometer or less. Isn’t that great?

  10. Posted August 21, 2017 at 6:27 pm | Permalink

    A coincidence, and one which will diminish over the future eons as the moon is slowly moving away from the earth and thus will be “smaller” than the sun’s disk.

  11. Posted August 21, 2017 at 6:28 pm | Permalink

    Created by God of course, for our enjoyment and future apocalypses. 😃

    • Posted August 21, 2017 at 6:30 pm | Permalink

      Sorry, couldn’t resist

  12. George
    Posted August 21, 2017 at 6:29 pm | Permalink

    There was a lot of eclipse activity near PCC(e)’s office –

    I just woke up from a nap. Tequila sunrises were being served at the eclipse party I was at. Mostly overcast in the Chicago suburbs but we did get glimpses including at the 87% maximum. Not sure if it was the liquor but the light seemed funkier and the shadows more interesting after the max.

    Looks like Carbondale, IL will again be the place to be in 2024.

  13. Posted August 21, 2017 at 6:31 pm | Permalink

    The moon isn’t always big enough to blot out all of the sun. When that happens it is called an annular eclipse.

    • Luis Servin
      Posted August 21, 2017 at 6:46 pm | Permalink

      That’s right. I’m lucky enough to have watched two total eclipses and one annular eclipse. In the case of the annular eclipse it didn’t become nearly as dark as in the total ones, and the “ring” of sun around the moon was enough for allowing lots of light.

    • Posted August 21, 2017 at 7:34 pm | Permalink

      Beat me to it, and I missed this one. See below also.

  14. winc39
    Posted August 21, 2017 at 6:31 pm | Permalink

    It is just an accident. However the moon does not always just cover the son. There is enough variation in orbits so that sometimes there is a so called “annular eclipse”. Then there is a bright ring in the sky.

  15. Posted August 21, 2017 at 6:37 pm | Permalink

    “Why, when there’s a solar eclipse, is the Moon exactly big enough to blot out the Sun”

    Well first of all it isn’t. If it were total eclipses would last just a moment, not minutes. That being said the fact that’s it’s not considerably larger or smaller, as other have mentioned, is just a coincidence.

  16. George
    Posted August 21, 2017 at 6:51 pm | Permalink

    Amazing that the orange fool can even screw up an eclipse:

  17. Posted August 21, 2017 at 6:55 pm | Permalink

    An amazing and hilarious full-page ad from Chiquita Bananas in the New York Times today, claiming credit for the eclipse. Much of the copy is on their corporate website too.

  18. Posted August 21, 2017 at 7:02 pm | Permalink

    Well, first of all, it’s not. The Moon’s orbit is sufficiently elliptical to cause the phenomenon of an annular eclipse if it happens that the moon is near its perigee at the time of the eclipse. Second of all, it wasn’t always in the past (when the Moon was closer), and it won’t be in the future (as the Moon recedes). For various reasons (such as tidal friction) the Moon is receding from the Earth at about 38mm/year. Eventually it will be so far away that its angular diameter will be less than the Sun’s at every point in its orbit and there will be no more total solar eclipses.

  19. rickflick
    Posted August 21, 2017 at 7:09 pm | Permalink

    Another lunar coincidence is it’s large size and closeness to the Earth. When life formed about 3.8 billion years ago, the moon was much closer and the tidal effects were much stronger and more frequent. There is speculation that it had a good deal to do with the way life evolved.

  20. Posted August 21, 2017 at 7:11 pm | Permalink

    I saw today that the moon is 400x as large as the sun but is 400x closer to Earth. This makes it look to us like the moon covers the sun during the eclipse.

    Is this right?

  21. Stephen Barnard
    Posted August 21, 2017 at 7:11 pm | Permalink

    On a different topic, I wonder how important the unusually large moon was to the origin and evolution of life on Earth. Tides are obviously important, but the moon also strips off atmosphere from Earth. If there were no moon the atmosphere would be thicker and the biosphere would be very different.

    • Stephen Barnard
      Posted August 21, 2017 at 7:12 pm | Permalink


    • Posted August 21, 2017 at 7:43 pm | Permalink

      Some of the effects that get discussed. The first one is most likely true. The others are possibly true but I don’t know enough to judge.
      1. The moon has obviously protected us from some large impacts, and some of those would have altered the course of evolution.
      2. The orbiting moon stabilizes the axis of the earths’ tilt. Without it, the earth could have a greater tilt, and we would have more extreme seasons as a result. That would have a huge effect on evolutionary history over time, I expect.
      3. After the moons’ violent formation, the earth was bombarded for a time from leftover debris. This has enriched the upper levels of our crust with metals. This has been crucial for humans emerging from the stone age.

      • Stephen Barnard
        Posted August 21, 2017 at 7:52 pm | Permalink

        Interesting. You’ve thought about this.

      • Gregory Kusnick
        Posted August 21, 2017 at 8:29 pm | Permalink

        I’m not sure #1 is obvious. Clearly the moon has taken some large impacts, but so has the Earth, and in terms of angular area, the moon blocks only a very tiny fraction of the sky. So I don’t think we can credit it with any significant shielding effect on that score.

        Moreover, a case can be made that the moon’s mass creates a gravitational lensing effect that magnifies the Earth’s impact cross-section. In other words, it’s possible that the moon has deflected more impactors toward Earth than away from it. I’m just guessing here, and don’t have numbers to back this up, but I don’t think it’s something that can obviously be ruled out.

        I’m not saying that everything would be the same down to the smallest detail if the frequency of impacts had been marginally different. To the extent that Gouldian contingency holds, we should expect a somewhat different mix of species. But I don’t think the general conditions of evolution or the evolvability of the global ecosystem would be substantially altered by tweaking the impact rate slightly up or down.

      • Nobody Special
        Posted August 21, 2017 at 8:59 pm | Permalink

        re. your example #2; as I understand it, the tilt of the Earth varies (wobbles) over time, although the timescale for change of millions of years is so vast we don’t see the change. Without the stabilising influence of the moon the degree of wobble will become larger and changes will be far quicker, making climatic and tidal conditions on Earth highly unstable and erratic.

        • rickflick
          Posted August 21, 2017 at 9:40 pm | Permalink

          I wonder if it’s known that the wobble came about from the impact that created the moon.

          • Gregory Kusnick
            Posted August 21, 2017 at 9:50 pm | Permalink

            The impact may have altered the axial tilt at the time. But ongoing variations in the tilt, it seems to me, would have to be the result of forces in play now, such as the gravitational influence of Jupiter and other planets, and changes in mass distribution due to Ice Ages, erosion, and the motion of tectonic plates.

            • rickflick
              Posted August 22, 2017 at 8:18 pm | Permalink


            • infiniteimprobabilit
              Posted August 22, 2017 at 11:01 pm | Permalink

              The earth is wobbly? Why does that not surprise me.

              Also slightly pear-shaped.


              • Posted August 23, 2017 at 11:28 am | Permalink

                Also from what I understand smoother than a regulation pool ball.

              • infiniteimprobabilit
                Posted August 23, 2017 at 8:43 pm | Permalink

                A pool ball is rougher? That would explain why I always miss the pockets. It’s the little bumps!


        • Torbjörn Larsson
          Posted August 22, 2017 at 11:33 am | Permalink

          As I noted in my longish comment, there has been a period of confusion here since early models had an error. (Actually an inherited programming error, I think.) They resulted in variations on much shorter time scales than today’s, where axis tilt is mainly rotationally stabilized and chaotic changes are relatively rare. For instance, there is no anthropic selection in this since the longest period without dramatic tilt change is roughly as long as tetrapod land evolution.

          I dunno about smaller scale wobble, which is caused by all the gravitationally influences Gregory describes. Mars has no large stabilizing moon, but no plates and less erosion. I would not worry about it, Gale crater was long term habitable.

    • Posted August 22, 2017 at 4:52 am | Permalink

      Isaac Asimov covers this well in The Tragedy of the Moon.

      • Posted August 22, 2017 at 8:26 am | Permalink

        The collection or the article with that title? I was thinking of the companion piece, in the same collection, “The Triumph of the Moon” (excerpted here).


        • nicky
          Posted August 22, 2017 at 1:20 pm | Permalink

          Asimov was a brilliant story writer, as well as a scientist, we knew that, but clearly a very original thinker too. How fascinating his ideas about the relation of the moon and earthly life, how well argued. One of the greats in my pantheon.
          I herewith pledge I’ll try to read everything he wrote that I can find in this year, starting 22 of August 2017 a.d. or, as I would like to see, 158 a.D. (= anno Darwini, not of his birth, but of the publication of ‘On the Origin…’).

          • nicky
            Posted August 22, 2017 at 1:22 pm | Permalink

            Again, I flunked the ‘unbold’. Yes, again, my apologies. Is there not a way of editing after posting?

          • Posted August 22, 2017 at 2:22 pm | Permalink

            That’s potentially more than 200 books ahead of you!


            • Zetopan
              Posted August 24, 2017 at 9:57 pm | Permalink

              “That’s potentially more than 200 books ahead of you!”

              “Potentially”? Asimov authored 400 books,
              the last of which was published posthumously. In other words twice your “potential” estimate.

              • Posted August 25, 2017 at 8:20 am | Permalink

                Ah! 400 is more than 200, no?! I was basing my estimate only on the fact that he published a book called _Opus 200_. 😉


        • Posted August 23, 2017 at 4:14 am | Permalink

          Sorry, Ant, you are right. I was referring to the collection, which includes The Triumph of the Moon, which is the essay I was thinking of.

          • Posted August 23, 2017 at 4:42 am | Permalink

            Now worries, Colin. I bet you had (still have?) the 1975(?) Coronet edition.


            • Posted August 23, 2017 at 5:00 am | Permalink

              I may well have had that one, as I remember reading it back then. About ten house moves later it had disappeared, so I recently bought a nice used Abelard-Schuman 1974 one through Amazon, complete with dust cover with a moon with a great chunk out of it. Rather touchingly, the fly leaf bears the inscription “Dix, With love from Pam – Christmas 75“.

              Now that I’ve brought it out, I’m going to read it again. I particularly love The Week Excuse, in which he proposes a very simple perpetual calendar.

    • Torbjörn Larsson
      Posted August 22, 2017 at 11:25 am | Permalink

      There are two parts in this.

      First, is the Moon unusually large? Every time “unusual” is flagged, it should be scrutinized with suspicion. Indeed, already our own system has the even more massive moon system of Pluto-Charon.

      If an event is too unusual we would not expect to see it. But modeling accretion a recent paper found that 0-3 (IIRC) sizable last impactors is to be expected during terrestrial planet formation, with a long tail up to the order of ten and a chance in ten (IIRC) to result in a moon from glancing impact. Hence neither Venus – no moon – nor Earth – one large moon – would be too unusual (i.e. unusual as in < 5 % likelihood).

      Second, the effects. The volatile and crust supply would be modified, the young Moon had a strong geodynamo field that interacted with Earth's and Earth atmosphere, there are axis tilt and tidal effects, there are impact flow effects, there are nocturnal possibilities, and possibly more effects I forgot to list. But as per above we cannot say that volatile and crust content as well as impact flows are much dissimilar from other terrestrials within a wide distribution of outcomes. Likely life would emerge anyway, and evolution is much too contingent to predict as other comments noted.

      As for axis, tidal and nocturnal effects, they are often inflated. Mars has twice the axis tilt of Earth, and it will change tilt and so climate chaotically. However the maximum time of any tilt era is as long as there have been tetrapod land life (up to 0.5 Gyrs) – early models had an error and predicted much shorter calm periods. Tidal effects would ease transition to land – water and food availability – under a short period; I do not see the alluded to importance in that shortening. Nocturnal life cope with darkness, and have evolved other senses to help with that.

      • nicky
        Posted August 22, 2017 at 1:38 pm | Permalink

        What always baffled me, with the moon being so relatively big and the distance so relatively great, that the centre of gravity of the Earth-Moon system is still within the Earth.
        It explains (in combination with the centripetal force) why we have a 12 hour tide cycle instead of a 24 hour one. A high tide when the moon is approximately on the opposite side. There is no other explanation for this, hence what we would call a proof.

        • Gregory Kusnick
          Posted August 22, 2017 at 2:07 pm | Permalink

          I don’t see what the center of mass has to do with it. The Earth’s gravity well distorts the moon into an ellipsoid, with bulges on both sides, and the moon’s gravity does the same to Earth, regardless of where the center of mass sits.

          To put it another way, the tidal force is the first derivative of the gravitational force, and it’s a smooth derivative that goes negative on the side away from the moon, hence the bulge on that side.

  22. Posted August 21, 2017 at 7:15 pm | Permalink

    A link with the moon 400x smaller than sun but 400x closer:


  23. Posted August 21, 2017 at 7:31 pm | Permalink

    As others said, this coincidence was not always so. And in the future it will also not be so since the moon is moving away from the earth.

    Since the moons orbit is not a perfect circle, at times the eclipse is an ‘annular’ eclipse, where the moon is a bit smaller, showing a ring of the sun all around it. I was fortunate to see that once many years ago. Not sure if these are rarer than total eclipses. It was cool, but a total eclipse would be wonderous to see, I think.

  24. Posted August 21, 2017 at 7:38 pm | Permalink

    It’s unique in the solar system.

    Pluto’s moon Charon would dwarf the sun in Pluto’s sky while Phobos, being smaller in Mars’s sky, would only cause annular eclipses – though twice a day!

    • Gregory Kusnick
      Posted August 21, 2017 at 8:38 pm | Permalink

      It may be unique to our solar system. The chances of this sort of coincidence obtaining elsewhere at this epoch are quite small.

      For that reason, there’s a school of thought that says the path of totality is the place to look for space aliens visiting Earth, because our eclipses would be counted among the natural wonders of the galaxy and should be a popular tourist destination not just for us but for any starfaring species.

      (There’s a character in Iain M. Banks’ Transition who advocates this view.)

      • Posted August 21, 2017 at 9:41 pm | Permalink

        I think some of the folks I met today might be space aliens.

        • Gregory Kusnick
          Posted August 21, 2017 at 9:43 pm | Permalink

          Look for the ones who never take off their eclipse glasses, or never come out of their RV.

          • Stephen Barnard
            Posted August 21, 2017 at 10:01 pm | Permalink

            I have eight ISO compliant eclipse glasses I’m willing to sell at the going price. 🙂

          • gravelinspector-Aidan
            Posted August 22, 2017 at 7:30 am | Permalink

            I’d think the large rocket engines sticking out of the back of the RV might be a bit of a give away. (Not in the “free with ten packet tops” sense.)

    • George
      Posted August 21, 2017 at 9:06 pm | Permalink

      Not unique in our Solar System. Saturn’s moon Epimetheus can produce total eclipses which lat 0.6 seconds.

      • gravelinspector-Aidan
        Posted August 22, 2017 at 7:56 am | Permalink

        Damn. I was actually crunching the numbers by hand, but I’ve not entered Epimetheus into my bodge-fest. How many satellites does Saturn have today? Last time I saw a count it was in the high 60s or low 70s.
        Regardless, my estimator for “eclipse-ability” is angular size of largest satellite / angular size of Sun from planet.
        Planet, Satellite, Eclipse parameter
        Mercury, -, –
        Venus, -, –
        Earth, Moon, 1.0278
        Mars, – –
        Jupiter, Ganymede, 0.3631
        Saturn, Titan, 0.2302
        Uranus, Titania, 0.1339
        Neptune, Triton, 0.0406
        Pluto, Charon, 0.0017
        Charon, Pluto, 0.0034

        Oh, rude words, I forgot that WP will chew up table formatting. I’ll CSV it.
        Anyway, what you can see is that indeed the Moon and the Sun are very close in (angular) size, but beyond that the distance to the Sun rapidly becomes the dominating factor with the sun far smaller in size than the satellite. Strictly this is a measure for the probability of (small factor) annular eclipses. Which as you can see might be manageable with a bit of jiggery-pokery with Ganymede’s orbit. But further out, you’ll just switch the Sun off with little in the way of “interesting edge effects”. That said, the LOTA (Lunar Occultation Timing Association) produce detailed profile maps for when stars (other than the Sun) are occulted by the Moon, which helped to show the odd asymmetries in shape and centre of mass of the Moon in pre-Apollo days. Similar work recently suggests that next year’s fly-by of KBO 2014MU69 by the New Horizons could well be a fly-by of a binary KBO.

        I wonder – could you get far enough away from Ganymede (or one of the other Jovian satellites) to experience an annular eclipse (or totality with Bailey’s Beads) by standing on the surface of … Callisto? You know, I think you might.
        Damn it – no you won’t. Jupiter would get in the way. Or would it? Too close for my orbital mechanics to call.

        • Barney
          Posted August 22, 2017 at 9:08 am | Permalink

          By my calculation (wow, do I have my time priorities wrong …) the closest a satellite gets to being the right size/distance is Janus, orbiting Saturn. It’s at 151,500 km (Moon 384,400 km), radius 90.4 km (Moon 1737.4 km), and Saturn is an average 9.58 AU away from the Sun. So the ‘relative blotting size’, compared to a “1” for our Moon (sometimes full eclipse, sometimes annular) is 90.4/1737.4 * 384,400/151,500 * 9.58 = 1.27 . The orbit isn’t significantly eccentric, so it’ll always be full.

          On the other ‘side’, the one that is always annular, but blots out the most, is, I think, Amalthea on Jupiter – 0.53 of the ‘ideal’ eclipse size.

          • Barney
            Posted August 22, 2017 at 9:12 am | Permalink

            Oh, balls – I didn’t take into account the radius of the planet for if you’re on the ‘surface’ (a dubious concept for a gas giant – convention is level at with pressure is 1 atm). For close-in satellites of gas giants, that’s significant…

            • Barney
              Posted August 22, 2017 at 9:21 am | Permalink

              Recalculation for observation from ‘surface’, not centre, gives Amalthea a respectable 0.87, and throws Janus out to 2.06. Callisto becomes the ‘smallest over 1’, at 1.51.

              • Barney
                Posted August 22, 2017 at 9:46 am | Permalink

                Closest of all, I think, Pandora, orbiting Saturn – a very close factor of 1.03. With the eccentricity of Saturn around the Sun, that might even allow an annular eclipse, once in a blue moon …

            • Posted August 23, 2017 at 11:31 am | Permalink

              How far “down” is one ATM of pressure in Jupiter? (Relative to the standard of vacuum near earth.)

              • Barney
                Posted August 24, 2017 at 3:42 am | Permalink

                You have to decide where you measure ‘down’ from, of course. This page from a retired astronomy lecturer says it’s perhaps 80 miles from the top of the ‘haze’ on Saturn to the level at which the pressure is 1 bar: http://cseligman.com/text/planets/saturnstructure.htm .

                But I imagine this has all had to be inferred from other measurements, and could be subject to revision – especially from the data the Cassini is now sending back during its “Grand Finale” of skimming Saturn’s atmosphere ever closer until, in 3 weeks’ time, it breaks up.

          • Gregory Kusnick
            Posted August 22, 2017 at 11:02 am | Permalink

            Meaning no disrespect to your calculations, it seems to me there are a couple of other factors that need to be taken into account to rival the Earth experience.

            How wide is the shadow compared to the depth of the atmosphere? If it’s very small, atmospheric scattering will wash out your view of the solar corona. Gas giants lose big here.

            How long does totality last? For tiny moons orbiting at tens of km/s, it’s measured in fractions of a second — nowhere near long enough for human eyes to adapt to the darkness and see anything interesting.

            Plus there’s the whole problem of where to stand. I guess ballooning might work.

          • gravelinspector-Aidan
            Posted August 23, 2017 at 7:54 am | Permalink

            Now would be a good time to point to “PhD”, Jorge Cham’s webcomic on the tribulations of being a PhD student. Theme : procrastination.
            There’s an eclipse strip up at the moment.

        • Torbjörn Larsson
          Posted August 22, 2017 at 11:39 am | Permalink

          I added Mars-Phobos at up to 0.75 angular size ratio, perhaps more at times,in a comment below. (It was certainly more historically, as Phobos is slowly approaching Mars.)

  25. Posted August 21, 2017 at 7:43 pm | Permalink

    Without this coincidence it would have been much harder to prove general relativity.

    • David Harper
      Posted August 21, 2017 at 9:15 pm | Permalink

      There are other experimental tests of general relativity, notably the perihelion advance of Mercury, which is measurably larger than what Newtonian theory predicts, and the change in the rate of passage of time in a strong gravitational field, which must be taken into account in GPS (satellite navigation) systems.

      The anomalous perihelion advance of Mercury was one of the examples which Einstein himself used as a demonstration of the correctness of general relativity.

      • Posted August 22, 2017 at 4:18 am | Permalink

        With Mercury we already knew about its orbit so it wasn’t a test of a prediction. The theory could have been fudged to match the observation; with the eclipse the observation confirmed the theory.

  26. GBJames
    Posted August 21, 2017 at 7:43 pm | Permalink

    Didn’t Zeus ordain it thus?

  27. ladyatheist
    Posted August 21, 2017 at 7:52 pm | Permalink

    For the same reason that a bee in your car is enormous but a bee in your bonnet is invisible.

  28. Mark R.
    Posted August 21, 2017 at 8:47 pm | Permalink

    The wax of the moon differed from the wane. Why is that? I saw the moon cover the sun and then when it retreated, the angles were not the same. I don’t think I’m making sense.

    • George
      Posted August 21, 2017 at 9:08 pm | Permalink

      Because the moon was moving. The wax was further west than the wane as you were looking at it. Earth was moving as well relative to the sun.

  29. Ullrich Fischer
    Posted August 21, 2017 at 9:11 pm | Permalink

    It actually is a co-incidence… and the only planet we know of in the known Universe which has this relationship between Moon size, distance, and star size and distance is the Earth. About 600 million years hence there will be a last eclipse as the moon is moving away from the earth at about 6 centimeters a year. After that, it will appear to be smaller than the Sun, so only partial eclipses will be possible. And no, this co-incidence is not proof that the Earth is special and designed for us by God. 🙂

    • gravelinspector-Aidan
      Posted August 22, 2017 at 7:58 am | Permalink

      It actually is a co-incidence… and the only planet we know of in the known Universe which has this relationship between Moon size, distance, and star size and distance is the Earth.

      See my number crunching up-thread concerning eclipses of the Sun by Ganymede, seen from Callisto.
      Wear your winter woollies.

    • Torbjörn Larsson
      Posted August 22, 2017 at 11:46 am | Permalink

      That is just because we know of only one exo-moon candidate thus far. Models says it is not uncommon, we would expect one Earth-Moon system every other planet or so (c.f. Venus vs Earth). Certainly every other system with habitable super-Earths.

      The historical coincidence is rougher to tell, but seeing how intelligence life appears when such a moon has worked its way out it would not be unique. Rare-ish to very rare, perhaps.

    • Posted August 23, 2017 at 11:35 am | Permalink

      Sounds like a good title for a science fiction story – “The Last Eclipse”.

  30. Kevin
    Posted August 21, 2017 at 9:31 pm | Permalink

    Seems an obvious case of Jungian synchronicity: https://en.wikipedia.org/wiki/Synchronicity
    A “meaningful coincidence” occurring with no causal relationship yet seeming to be meaningfully related. This would at first sight be a self-evident proof that God loves us, but on further examination is an irrefutable demonstration of the exact opposite.

  31. Serendipitydawg
    Posted August 22, 2017 at 3:59 am | Permalink

    Ceiling cat did it. As she bats the moon further away the eclipses will becom annular at best.

    We live in the best of times (though it would have been nice if she’d kept the clouds away in Cornwall for the only UK total eclipse in my lifetime).

  32. John Laughlin
    Posted August 22, 2017 at 5:05 am | Permalink

    The sun’s diameter is 400 times that of he moon and is about 400 times farther away from earth. That’s why the moon blocks out the sun in a total eclipse

    • Posted August 22, 2017 at 5:18 am | Permalink

      Yes, that much is clear but that wasn’t my question: why the match? The question has been answered, as you see.

      • rickflick
        Posted August 22, 2017 at 9:22 am | Permalink

        “Why” questions are the domain of sophisticated Theologians. 😉

  33. gravelinspector-Aidan
    Posted August 22, 2017 at 5:33 am | Permalink

    Odd. Comments not showing until “reply” clicked. I’ll try on the proper computer.

    • gravelinspector-Aidan
      Posted August 22, 2017 at 7:59 am | Permalink

      A new bizarrity of WP’s serving for mobile phones, I suspect.

  34. Posted August 22, 2017 at 6:27 am | Permalink

    Just Hank dicking with us to cause religious freaking out when eclipses occur. 🙂

  35. Torbjörn Larsson
    Posted August 22, 2017 at 10:56 am | Permalink

    It is just one of those accidents that come from statistical fishing. Historical accidents often hinges on system changes, here the Moon slowly drifting out due to increased tidal lock of Earth. (Eventually we will get stuck in the Pluto-Charon conformation of mutual synchronous rotation; https://en.wikipedia.org/wiki/Tidal_locking .) Another often mentioned historical coincidence is that the universe (just) enters the era of dark energy domination, when the universe expansion changes from deceleration of previous matter energy domination to acceleration.

    There has already been comments on similar eclipse accidents in the solar system. But to appreciate that it is not really unique I think it is noteworthy that already the closest planet with moons have appreciable annular eclipses. Phobos covering up to 3/4 of the solar radius have been imaged by rovers: https://en.wikipedia.org/wiki/Transit_of_Phobos_from_Mars . In this case Phobos is slowly approaching Mars (where it will rather soon end up), so we are lucking out from a better size match.

  36. Ann German
    Posted August 22, 2017 at 11:51 am | Permalink

    Proof of existence of flying spaghetti monster . . . .

  37. Zetopan
    Posted August 24, 2017 at 10:22 pm | Permalink

    Since the Earth’s Moon is in an elliptical orbit around the Earth, it’s current actual distance from the Earth varies between 225K and 252K miles. I mention this because there are willfully scientific illiterate people (creationists) who claim that the ratio between the Earth-Sun and Earth-Moon distances “exactly” match the ratios of the Sun to Moon diameters, respectively, hence a chief magician must have done it and no naturalistic explanation is even possible.

    In the distant past the Moon was much closer to the Earth and the length of the Earth’s day was also much shorter. Since the Earth rotates much faster than the Moon orbits around the Earth time-wise, tidal drag of the Moon on the Earth caused the Earth to slow down its rotation and the Moon to recede to a larger distance. The resulting Earth’s tidal action on the Moon’s rotation halted the Moon’s rotation long ago, such that the same side of the Moon currently always faces the Earth (this will not be true in the distant future).

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