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Comment by ionwake | Hacker News Reader

is this a bad moment to mention how weird it is that the moon is perfectly positioned to create eclipses?

I think its pretty incomprehensible odds if you ask google.

But I could be wrong I know literally nothing about astronomy, it just always surprised me its not common, but it be cool if someone could explain that

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By the way, current Stellarium+ simulations indicate that the Moon is positioned to plow through occultations of the Beehive Cluster and Jupiter, albeit below the horizon for me, coming up in the first week of October. There will also be a very close encounter with Mars... which happens every month, anyway.

https://occultations.org/publications/rasc/2026/lunar26.pdf

I've personally been cataloging those notable stars which are close enough to ecliptic that they have a high probability of occultation, along with the planets that coincide at those times. And it's personally gratifying to watch the Moon occult most or all of the Pleiades, which is also a fairly common occurrence.

The Beehive Cluster is dimmer, and even though the Moon be a waning crescent, for those where it appears above the horizon, may be something exciting to think about!

The moon is 400 times smaller than the sun, but the sun is 400 times farther away, so they appear to be the same size. The solar eclipse we see, with the moon obscuring the sun but just allowing light around the edges the way it does, may be a very uncommon phenomena.

I think it's easier to understand if you consider the opposite case: how could the moon never be on the Sun-Earth ray? It would have to orbit Earth around the Sun-Earth ray. But the Sun-Earth ray changes direction as Earth orbits the sun, so the Moon's orbit would have to change with it. People in the northern hemisphere would not be able to see the moon at the same time as people in the southern hemisphere for significant portions of the Lunar cycle, and we would always see a half-moon.

That the Earth-Moon orbital axis is not incidental with the Sun-Earth orbital ray means we'd always, eventually, have solar and lunar eclipses.

Perhaps if Earth were in a tidally locked orbit with the Sun, the Moon could be in a tidally locked orbit with the Sun while also orbiting the Earth. But then half the planet would be frozen and the other half would be completely baked and we wouldn't be having this conversation.

it was my vague understanding that what is weird is the apparent size matching. since the moon is slowly moving away, eventually we'll no longer have total solar eclipses, just annular and partial.

The relative sizes are approximate, not perfect, but close enough that our monkey brains interpret it as more than a coincidence.

Idk, one could come up with a hypothesis that eclipses provide the first tangible validation of a system of mathematics.

If there were greater significance to it, I’d have thought the the visual sizes of the Sun and Moon would have been perfectly matched, rather than to within a few percent, give-or-take.

Also since the Sun-Earth distance varies and so does the Earth-Moon distance, the coverage of the Sun varies. We sometimes get annular eclipses where the Sun is not totally covered.

<distant growling low resonating voice that makes galaxies shake> Damn, those pesky IEEE-754 accumulation errors again.

It is never a bad moment to mention this. For planets with a single moon, the probability of such an occurrence has been estimated at 4% [1]. I like to think that this is what extraterrestrials would find most fascinating about our world.

[1] https://medium.com/@asorlik/the-probability-of-total-solar-e...

It’s also transient. In a few million years, the Moon will have drifted far enough away that it cannot fully eclipse the sun. Long ago, the Moon was so close you wouldn’t have been able to clearly see the Sun’s corona during a total eclipse.

Over 600 million years left! Complex life may be gone by then (see silicate weathering).

It’s more like, if the moon were much farther away then total solar eclipses wouldn’t happen at all. But the opposite is not true. If it were closer than total solar eclipses would be more common.

It is an interesting coincidence. But the statistician in me feels like calculating the probability is a bit spurious. For one, not all combinations of distance and size are equally likely, and running physical simulations to try and get decent Monte Carlo estimates of the odds feels a bit suspect - I imagine a decent chunk of what you’d be measuring is the influence of parameters whose values are empirically unknowable.

But, even more than that, it comes from the same place that requires me, when a cashier sees the total is exactly $50 and asks, “What are the chances of that?” to actively suppress the urge to say, “About the same as for $59.37.”

Im not trying to be mean but nothing you said made any sense, possibly because I wasnt clear.

I do appreciate your time and your feedback and Im not great at patience but Im going to try because I am dim in many subjects.

I meant an exact solar eclipse where the Sun and the moon seem to match perfectly.

As such the math odds are low.

Finally, if I ever went to a shop and I was charged a round number like $50, or in my case even $3 I would immediately assume I am being scammed, as that is rare given the context. ( which incidentally happened to me once in Barcelona due to a local spanish barman trying to charge me a tourist tax ).

But I am grumpy and it is late, please forgive coarseness of my reply.

It's simply a huge, beautiful coincidence. If the galaxy was teeming with life capable of interstellar travel, our planet would be famous for its perfect eclipses.

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