[HN Gopher] One more clue to the Moon's origin
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One more clue to the Moon's origin
Author : gmays
Score : 73 points
Date : 2022-08-12 13:34 UTC (9 hours ago)
(HTM) web link (ethz.ch)
(TXT) w3m dump (ethz.ch)
| cscurmudgeon wrote:
| > It was not until Galileo's time, however, that scientists
| really began study it
|
| It is 2022 and yet we see mildly racist falsehoods like this.
|
| https://en.m.wikipedia.org/wiki/Surya_Siddhanta
| the diameter of the moon as 2,400 miles (actual ~2,160)[6] and
| the distance between the moon and the earth to be 258,000
| miles[6] (now known to vary: 221,500-252,700 miles
| (356,500-406,700 kilometres).[11]
|
| https://en.m.wikipedia.org/wiki/Aryabhata
| Aryabhata also noted that the luminosity of the Moon and other
| planets is due to reflected sunlight.
| lioeters wrote:
| I didn't downvote you, but I think "implicit or unconscious
| ethno-centrism" might be a more suitable term than "mildly
| racist". At least it could have been more palatable to the HN
| hive mind.
|
| From the article:
|
| > Humankind has maintained an enduring fascination with the
| Moon. It was not until Galileo's time, however, that scientists
| really began study it.
|
| I'd agree with you that this statement ignores how scientists
| around the world, including European and "non-Western"
| cultures, have studied the moon - more or less scientifically
| and mathematically - for centuries before Galileo. (In the case
| of Aryabhata, a millenium earlier.)
| newfie_bullet wrote:
| Had never heard of these - thanks for sharing!
| jahewson wrote:
| I think they're referring to studies with telescopes, rather
| than math.
| [deleted]
| cscurmudgeon wrote:
| You can do science without telescopes.
|
| Studying orbits is not just math.
| deepdriver wrote:
| Ancient Greek study of the Moon predates the Surya Siddhanta by
| ~1000 years:
|
| https://www.smithsonianmag.com/science-nature/ancient-greek-...
| 2,500 years ago, Anaxagoras correctly determined that the rocky
| moon reflects light from the sun, allowing him to explain lunar
| phases and eclipses
| [deleted]
| nickelcitymario wrote:
| I'm not sure why this is getting downvoted... seems to be an
| accurate statement. The Surya Siddhanta predates Galileo by
| more than a millenia.
| moistly wrote:
| Because OP used a trigger word and HN instinctually downvotes
| that word instead of taking the more charitable action of
| benevolently and accurately interpreting OP as having meant
| "cultural bias."
| nickelcitymario wrote:
| Also fair, I think.
| sokoloff wrote:
| I did not downvote, but my racism detector's gain is not high
| enough to detect the racism here, which might be leading to
| downvotes.
|
| The earlier learnings are interesting; the accusation that
| racism has something to do with the omission is not.
| macintux wrote:
| Agreed. If someone argued that geology began in the 1800s
| and neglected to mention Eratosthenes' calculation of
| Earth's size 2000 years earlier, I'd hardly say they're
| showing an anti-Greek bias.
|
| In this case it's more likely either ignorance or a
| different definition of "study".
| nickelcitymario wrote:
| The article doesn't state that "astronomy" as a whole was
| invented by Galileo, but rather that the moon itself was
| never studied scientifically until his time. That's a
| patently false statement that requires dismissing the
| millennia of Indian observations and research on the
| moon. I'm pretty sure we could make similar arguments for
| the Middle East or Asia.
|
| It's like when they teach that everyone thought the world
| was flat back then. They didn't. That's a fairy tale.
| Some people thought that, but the idea that the Earth was
| in fact round was not a shocking finding to contemporary
| people.
|
| Likewise, geology as a field may have begun in the 1800s,
| but you wouldn't claim that no one tried to study any
| aspect of the earth before then. And if you did, I would
| describe that as being at least ignorant of Eratosthenes.
| (Actually, no I wouldn't, because I almost certainly
| wouldn't have thought of that precise example. But I
| digress.)
|
| Is the issue that "racist" just seems too harsh of a
| label for what may be a simple error? One can definitely
| argue there is a western bias in claiming Galileo as the
| starting point for studying the moon. Maybe conflating
| "western bias" with "racism" is a step too far.
| JumpCrisscross wrote:
| > _a patently false statement that requires dismissing
| the millennia of Indian observations and research on the
| moon_
|
| Every culture observed and most rendered calculations on
| the moon's motion. Forecasting eclipses is a motif of
| early mathematics.
|
| > _geology as a field may have begun in the 1800s, but
| you wouldn 't claim that no one tried to study any aspect
| of the earth before then_
|
| "Studying any aspect" of something isn't scientifically
| analyzing it. To my knowledge, Siddhanta and Aryabhata
| calculated the motion of celestial bodies remarkably
| early. But many others independently repeated those
| calculations, albeit with varying accuracy, across the
| world and millennia. None of those methodically delved
| into lunar makeup and origins. The tools didn't exist.
| The scientific method didn't exist.
|
| Aryabhata _did_ comment on the moon 's luminosity. But
| that's a far cry from attempting to prove it. Which is
| fine. It doesn't diminish him or his work. But it fails
| as a counterpoint to the rigor and volume of post-
| Galilean research.
| nickelcitymario wrote:
| > Every culture observed and most rendered calculations
| on the moon. Calculating eclipses was similarly attempted
| and solved by practically every major civilization.
|
| Just to be clear, I'm not suggesting it was only India. I
| was saying there's something western-centric about dating
| things to Galileo. However...
|
| > Nobody, however, methodically delved into lunar makeup
| and origins in a way remotely scientific, in part because
| the scientific method is a relatively recent invention.
|
| Fair point. If you're saying that anything prior to the
| scientific method (or that doesn't utilize the scientific
| method) doesn't count as science, I can see this argument
| and am happy to concede the point.
| JumpCrisscross wrote:
| > _anything prior to the scientific method (or that doesn
| 't utilize the scientific method) doesn't count as
| science, I can see this argument_
|
| Not quite arguing this. The article says it wasn't until
| Galileo that "scientists really [began] to study" the
| moon. Within the context of the moon's origin, it's a
| legitimate statement.
|
| Ancient astronomers weren't following the scientific
| method, but I wouldn't dismiss them as not being
| scientists. If someone said scientists didn't really
| start studying subatomic particles until the 19th
| century, it would be asinine to claim offense on the
| basis of Lucretius having mentioned indivisible building
| blocks of reality.
|
| Put another way, the same comment would have been
| stronger without the accusation of racism. (Qualifying
| with "mild" does nothing. Calling someone mildly idiotic
| isn't somehow less provocative than saying they're an
| idiot.)
| missedthecue wrote:
| Scientists believe the moon was formed by a collision into planet
| earth by another celestial body. Why didn't:
|
| 1. The other celestial body leave a very obvious elemental makeup
| in the collision point on earth? We can detect other asteroids,
| but this one was 100% identical to Earth's crust?
|
| 2. Why didn't the debris left by the collision form a ring around
| earth like Saturn? Instead it all coalesced into one compact body
| and left zero debris in a ring around earth.
|
| I am not an astrophysicist but these seem like obvious questions
| with no clear answers online after my quick Google search.
| sixbrx wrote:
| Re #1, one really cool thing is that the remains of Theia (the
| other body that collided with Earth) _may_ be visible in a big
| way in Earth 's mantle still, if recent study is correct. See
| https://skyandtelescope.org/astronomy-news/bits-of-theia-mig...
| ajross wrote:
| > but this one was 100% identical to Earth's crust?
|
| Given the size of this impact, the crust[1] didn't really
| survive. Most of the mantle material ended up vaporized and
| either falling back down or coalescing onto a single fragment,
| becoming the moon.
|
| It was a collision "of" the planet, not "on" the planet.
|
| [1] Which in any case wouldn't have survived this long anyway.
| The differentiation process that produces the crust we're
| standing on takes billions of years. There aren't any surface
| rocks that old, and wouldn't be even if the collision hadn't
| happened.
| changoplatanero wrote:
| Some scientists believe that we can detect the place the moon
| hit the earth at.
| https://www.science.org/content/article/remains-impact-creat...
| cossatot wrote:
| This isn't 'the place the moon hit', it's material from the
| colliding object (Theia) that has been added to the Earth's
| mantle. The mantle convects, and this material is a little
| more dense, and has pooled up at the bottom of the mantle,
| but because the mantle convects (the material circulates)
| there is no real reason to believe that these bodies are at
| the location of collision, especially considering that much
| of the mantle (and the parts of Theia that stayed behind)
| would have been reduced to flying bits of lava in orbit
| around the core, before coalescing.
| sokoloff wrote:
| It would be the place the earth was hit by an
| extraterrestrial body, which resulted in the creation of the
| moon, not the earth being hit by the moon.
| g6762736 wrote:
| One of the hypotheses to explain the Large low-shear-velocity
| provinces at the Earth Mantle-Core boundary is that they are
| remnants of the impactor that lead to the formation of the
| Moon: https://en.wikipedia.org/wiki/Large_low-shear-
| velocity_provi...
| towaway15463 wrote:
| If the body was big enough it would have completely erased the
| earth's crust, after cooling again our current crust would be a
| mixture of both bodies.
|
| Having an outsized moon might also degrade the orbit of any
| material that would have been left over after coalescing into
| the earth and moon.
| causi wrote:
| _1. The other celestial body leave a very obvious elemental
| makeup in the collision point on earth? We can detect other
| asteroids, but this one was 100% identical to Earth 's crust?_
|
| Earth's crust was rubble and magma. Sort of like how you
| couldn't find an impact point if you threw a water balloon into
| a can of paint.
|
| _Why didn 't the debris left by the collision form a ring
| around earth like Saturn?_
|
| It did. Earth had very impressive rings for thousands of years
| after the impact.
| missedthecue wrote:
| Ah ok first point makes perfect sense.
|
| How do we know we had rings around earth for so long? And
| where did they go?
| vvilliamperez wrote:
| Rings of space debris coalesce into small celestial bodies
| over time. That's one theory of how moons form.
| Nux wrote:
| Gravity had them I guess. Saturn's going to lose the rings
| as well, and "soon" (hundreds or thousands of years).
| balfirevic wrote:
| Did you mean hundreds or thounsands of _millions_ of
| years?
| Nux wrote:
| Yes, my bad
| moffkalast wrote:
| About 100-300 million years, and they only formed
| sometime in the last 100 million. A blink of an eye from
| a cosmic standpoint.
|
| For comparison, the dinosaurs existed before than the
| rings did.
| bmurphy1976 wrote:
| Saturn also has conveniently placed moons that help keep
| the rings locked in place.
| causi wrote:
| The short answer is "math". The material either coalesced
| into the moon or eventually fell back to earth. It's
| actually a pretty short lifetime for the rings, compared to
| ring systems like Saturn. Earth has several disadvantages
| when it comes to maintaining a set of rings. It doesn't
| have gravity strong enough for tidal forces to tear
| orbiting bodies apart at far remove from the planet and it
| doesn't have "shepherd moons" to keep ring material in a
| stable orbit.
| verisimi wrote:
| Could maths be used to prove the exact opposite, to show
| why the earth still has rings?
|
| I know it doesn't, but I say you can use maths to prove
| whatever you like.
|
| So, 'maths' as an answer, is not satisfactory to me.
|
| AFAIK, all this discussion is in the realms of
| hypothesis.
| toiletfuneral wrote:
| causi wrote:
| _AFAIK, all this discussion is in the realms of
| hypothesis._
|
| It is. It's just currently the best hypothesis, and it's
| refined to better match observation over time. For
| example, the impact with Theia was originally thought to
| be a glancing blow by a planet spinning similarly to
| earth, but more recent simulations indicate it was a
| head-on collision and that Theia had relatively little
| angular momentum. This hypothesis does make predictions
| about the moon's core that are falsifiable but it may be
| some time before we figure out a way to make the
| necessary measurements.
| whatshisface wrote:
| > _I say you can use maths to prove whatever you like._
|
| You're technically right, but only when compared against
| the exact way the comment you're replying to was phrased.
| Scientists don't just write down whatever equations give
| them the answers they feel like getting that day, they
| first figure out which equations give correct answers,
| and then use only those.
|
| The stability of ring systems can be worked out using the
| same old laws that Newton figured out, which describe
| inertia and gravity. They can then be tested in
| particular by observing rings around other planets, and
| in general by every observation that confirms Newton's
| laws.
| adamrezich wrote:
| > but I say you can use maths to prove whatever you like.
|
| > AFAIK, all this discussion is in the realms of
| hypothesis.
|
| it is unfortunate that you are being downvoted. I have
| been increasingly taking note of things that "science"
| says that the general public then takes for granted as
| fact, when they're entirely unfalsifiable, unprovable,
| and just conjecture. of course there is nothing wrong
| with conjecture, and especially nothing wrong with
| mathematically-backed conjecture. but there is a huge
| gulf between making conjectures about Earth possibly
| having had rings in the past and accepting as fact that
| Earth most certainly had rings in the past.
|
| it seems to me that a big part of this is the (at least
| contemporary) human "need" to be absolutely sure about
| everything. most people come away from reading e.g. this
| comments thread either "knowing" that Earth definitely
| had rings in the past, or rejecting the rhetoric here and
| believing Earth never had rings. few people allow
| themselves the "mental gray area" of not really believing
| or disbelieving something, instead leaving that "did the
| Earth ever have rings or not" factoid field in their
| mental database with the value of "idk, maybe?"
|
| once you start training yourself to embrace this "mental
| gray area" for unprovable (or merely unproven) facts, you
| start to notice a lot of things that many people take for
| granted as being absolutely True, when in fact there is
| ample possibility for them to be False. another example
| is the composition of the Earth's core--there's all sorts
| of ways we can inductively determine this, but ultimately
| we don't definitively know what the Truth of the matter
| is.
|
| this "lack of 'mental gray area'" phenomenon is what
| leads to many people today taking not just proven
| scientific research as gospel, but also all sorts of
| theories and conjectures and unproven hypotheses. how
| many ardently atheist "science-believers" literally
| believe in the existence of dark matter, because "the
| science says so" (even if that's not really the case)?
| awhile ago I posted a comment on this website about how
| an atheist friend of mine believed that a government
| policy definitely had no downsides or issues because a
| social "science" Study had been done saying no such
| issues existed, and he took this just as equally As
| Gospel as he would F=ma--it's _Science_ , Scientists say
| so, so therefore it must be True--as though the world
| were a computer simulation and Scientists are reading the
| assembly language and/or database entries that comprise
| the totality of the Universe verbatim. intuitively
| realizing that this explicitly is not and _cannot_ be
| True has really opened my eyes to the nature of how many
| (most?) people take lots of scientific conjecture and
| theory and social "science" as Accepted Fact, and how
| that's Probably Not The Best Way To Go About Things.
| pungentcomment wrote:
| Saying it' maths is simplifying it a bit. They use
| simulations. They can simulate how an object orbits and
| eventually fall down to earth. They can test those
| simulations everyday by predicting where a know object in
| orbit will be in the future. Multiply that by a billion
| and you can predict that earth's rings would have fall
| down eventually.
| ru552 wrote:
| The earth is littered with moon rocks. Said rocks may have
| formed the rings that have dissipated.
| Tagbert wrote:
| Those rocks identifiable as moon rocks on the surface of
| the earth are from ejected debris from the later meteor
| collisions that formed the craters that we can see on the
| moon. The early collision debris would have fallen on the
| molten surface of the earth and been absorbed.
| sfink wrote:
| My personal opinion, but informed by arguments from a number of
| "real" scientists: because that explanation is bogus. It never
| made much sense. A collision of a magnitude to break off a
| moon-sized chunk of mass would be enormous. And why wouldn't
| the chunk either escape or fall back down, rejoining the main
| mass? Do we have any evidence that such collisions ever result
| in significantly-sized orbiting bodies?
|
| Or maybe I'm just not understanding that "theory" well enough,
| because my current preferred hypothesis _could_ be described in
| much the same way. Let 's say there was a large collision, but
| it was when the Earth was hotter and more molten, and it added
| enough heat and instability (and perhaps angular momentum) that
| large globs were thrown off. Some escaped, some fell back down,
| and some were swept up by the largest glob, now the Moon.
|
| But either way, I think the answers to your objections are: (1)
| it was molten enough to meld together, OR it wasn't, in which
| case plate tectonics regularly replace the entirety of the
| crust (and underneath the magma layer is molten); and (2) the
| Moon cleared its orbit and the Earth is small enough to not
| support a large area to hold crud that makes up a ring? (All of
| this is guesswork.)
| hither_shores wrote:
| At these scales, there are no rigid bodies, and so no single
| "chunk". If the giant-impact account is correct, the moon
| coalesced out of ejecta in orbit.
|
| > And why wouldn't the chunk either escape or fall back down,
| rejoining the main mass?
|
| Some material likely did escape, but that takes much more
| energy than getting into orbit. You need to leave the surface
| of the Earth at about 11km/s to escape, but only 6km/s to
| reach lunar orbit.
| [deleted]
| cossatot wrote:
| >Or maybe I'm just not understanding that "theory" well
| enough
|
| From what you've written, I think you are misinterpreting the
| standard theory, which is much more like 'your idea' (the
| second paragraph) than your conceptualization of it in the
| first paragraph, i.e. there was no moon-sized solid chunk
| removed. I don't think any scientists working on this think
| that.
|
| Although there is some argument on the size of the impacting
| body (Theia), most/all the impact-origin theories hold that
| much of the planet was reduced to tiny liquid fragments
| and/or vaporized, including the material that became the
| Moon.
|
| Here is a video of a simulation of the the 'standard' theory:
| https://www.youtube.com/watch?v=wfImQOZp3hE
|
| Something that is important to keep in mind, but is non-
| intuitive, is that all of the Earth with the exception of the
| upper crust (say, the upper 10-30 km of the Earth, radius
| 6378 km) is hot enough to melt if it was not under the
| confining pressure of the weight above it. Remove material
| from above the lower crust or mantle, and it undergoes
| decompression melting. So even if the material that coalesced
| to become the moon was removed through gentle caresses,
| without producing heat due to friction, that material would
| melt, and so would the rock surrounding the area that it was
| removed from. So you don't need to place the collision
| happening in a context where the Earth was much hotter (which
| it probably was to some degree regardless). Add in the
| enormous friction caused by collision, and you are in a
| situation where much of the Earth and possibly Theia are
| reduced to tiny particles of liquid (lava) or gas (vaporized
| basically), though these would cool to basically volcanic
| glass very rapidly in space. Then the Moon and the Earth's
| mantle were reassembled from these tiny particles.
|
| Caveat: I have a PhD in geology but my research is not in
| this area and I don't follow it closely.
| GolfPopper wrote:
| It wouldn't necessarily "break off" anything. If the angular
| momentum of the post-impact planetary mass reaches or exceeds
| orbital velocity, and its temperature is high enough, you get
| a novel type of celestial body - a _synestia_ [1], a planet-
| sized hurricane of boiling rock. As the rock cools, magma
| rains inward re-fornming the Earth (Earth V2?) over decades,
| while everything far enough out (beyond the Roche limit [2])
| would coalesce to form the Moon.
|
| 1. https://en.wikipedia.org/wiki/Synestia 2.
| https://en.wikipedia.org/wiki/Roche_limit
| jbaber wrote:
| The hypothetical planet has a name (Theia).
|
| https://en.wikipedia.org/wiki/Theia_(planet)
| mvcalder wrote:
| I always thought of Tom Dooley as a Kingston Trio song. The
| article cites The Greatful Dead.
| JackFr wrote:
| It's a traditional folk song recorded by dozens of artists. The
| Kingston Trio are the only ones who charted with it.
| sammalloy wrote:
| Link for those who want to read about the fascinating history
| of the tune:
|
| https://en.wikipedia.org/wiki/Tom_Dooley_(song)
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