[HN Gopher] Phobos's orbit and its apparent motion across the Ma...
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Phobos's orbit and its apparent motion across the Martian sky
Author : azeemba
Score : 91 points
Date : 2023-02-26 16:33 UTC (6 hours ago)
(HTM) web link (azeemba.com)
(TXT) w3m dump (azeemba.com)
| Ilverin wrote:
| Jupiter has some moons that really do go the "wrong" way: the
| Carme, Ananke, and Pasiphae groups.
| bagels wrote:
| Neptune's moon Triton as well:
| https://en.wikipedia.org/wiki/Triton_(moon)
| olivermarks wrote:
| 'wrong' doesn't exist in the cosmos.
|
| Perfection does.
|
| I recommend 'the little book of coincidence in the solar system'
|
| https://woodenbooks.com/index.php?id_cms=8&controller=cms#!L...
| patrickdavey wrote:
| That's no moon
| PicassoCTs wrote:
| Fun fact.. we could slow down phobos, sending it crashing into
| mars.. it would be human possible, with todays technologies right
| now.. we even could pick a spot.. and start with this smash a
| terraforming program.
| vkou wrote:
| 1. We have human technologies to impact 625m/s of delta-V to a
| moon that weighs 10,000,000,000,000 tonnes? [1]
|
| 2. What _exactly_ would that accomplish?
|
| [1] Should 'only' take 2 * 10^21 joules of energy[2].
|
| [2] Humanity's annual energy consumption is 5.8 * 10^20 joules.
| mk_stjames wrote:
| This is a whole lotta words and graphics to explain something
| that literally takes two sentences:
|
| Phobos orbits Mars faster than Mars rotates itself, while Deimos
| rotates slower than Mars. Thus from the ground their apparent
| motion is opposing in the sky; Deimos rises in the east and sets
| in the west, while Phobos does the opposite.
|
| Done.
| ali7388 wrote:
| This
|
| Earth has many artificial satellites that do that.
| addaon wrote:
| There's not that many satellites outside of GEO, and even
| those that go beyond are usually not circularized (e.g.
| Molniya orbits), so tend to have shorter periods. Unless you
| count things in GEO graveyard orbits (which rise very, very
| slowly in the west -- their orbit is just slightly less than
| synchronous), I can't think of a significant population of
| artificial satellites that orbit slower than earth rotates.
| jakelazaroff wrote:
| I thought it was an interesting article. Also, your comment
| breaks the guidelines:
|
| _> Please don 't post shallow dismissals, especially of other
| people's work. A good critical comment teaches us something._
|
| https://news.ycombinator.com/newsguidelines.html
| hooverd wrote:
| It's dismissive, but I wouldn't call it shallow...
| Kranar wrote:
| Are you claiming that OP, who posted a fantastic summary of
| the article, is being shallow?
|
| The guidelines are good for people to take into account on an
| individual basis, but they should not be used to police
| comments made by others.
| RedNifre wrote:
| It's a good comment because it summarizes the article better
| than the title does. Every article with a misleading title
| should have a top comment like that.
| dang wrote:
| Ok, we've replaced the title with a substring of the
| subtitle that is presumably less misleading.
|
| (in keeping with " _Please use the original title, unless
| it is misleading or linkbait_ " -
| https://news.ycombinator.com/newsguidelines.html)
| cjbgkagh wrote:
| I think that's shallow dismissal as in disagreement without
| substance, I.e. not adding much to an argument. Not dismissal
| as in a brief summary so you don't need to read the rest.
| hn_throwaway_99 wrote:
| Totally agree. I read through it and thought "this is not a
| hard concept".
|
| That said, I did think the last graphic in the article was
| really interesting, showing that with a highly elliptical orbit
| it's possible for a moon to both rise and set on the same side.
| The article could have been greatly improved by just showing
| those three planet/moon graphics and explaining the 3 possible
| states for each.
| kurthr wrote:
| There are satellites that use this (highly elliptical orbits)
| to spend most of their time (near perihelion) over a target
| area.
|
| One useful tool for thinking about orbits is that the swept
| area per time is constant so the farther out in orbit you are
| the lower the proportional angular velocity.
| dehrmann wrote:
| Reminds me of a flight I had from the Nordics to California in
| winter. Because of the latitude, the plane was flying faster
| than the rotation of the earth. The flight left in the morning,
| so I got to watch the sun set in the east.
| swayvil wrote:
| To capture our attention obviously.
|
| What nobody is mentioning is that the moon is actually
| _unnaturally round_.
| bagels wrote:
| Unnaturally?
| twojacobtwo wrote:
| Aren't all 'heavenly bodies' essentially perfectly spherical by
| default (assuming they are formed molten or from fine grained
| constituents)? As I understand it, only the effects of massive
| external gravity wells or high spin rates make things not
| round.
| hammock wrote:
| Is this how Superman is able to reverse time by flying around the
| earth faster than it rotates itself?
| wunderland wrote:
| Disappointing that there wasn't an animation of both moons
| together, one rotating faster and one slower than Mars' own
| rotational speed. This was the short-but-sweet answer to the
| title question and it could have been explained in one graphic!
| imafish wrote:
| TIL Mars has a Moon that travels in the wrong direction.
|
| Then shortly after, I learnt it doesn't.
| sebastianconcpt wrote:
| What I find unsettling is that our Moon rotates at a the precise
| perfect speed to make us "see always the same side". From all the
| possible speed values in the universe, why precisely that one?!
| CompuHacker wrote:
| Tidal locking. All two-body systems stabilize in a similar
| configuration. The Moon isn't quite done yet, we still see IIRC
| 59% of the surface on average due to libration.
| iamphilrae wrote:
| That's actually fairly common. The really unsettling question
| is "why is the moon exactly the same size in the sky as the
| sun?". A little bit smaller and we wouldn't see full solar
| eclipses, a little bit bigger and we wouldn't see the suns
| corona during an eclipse. Another unsettling feature is that
| the moon is moving further away from the earth so in a few
| million years' time, we won't even have full solar eclipses. So
| now what are the chances that in the 4B years the earth's been
| around, we're alive at the relatively short time period that we
| can witness this earth/moon/sun relationship?
| permo-w wrote:
| chances are if weren't in that phase, there would be some
| other phenomena we'd be observing in awe and feeling special
| about it
| deanCommie wrote:
| Exactly :)
|
| https://numbergenerator.org/randomnumbergenerator/1-1000000
| 0...
|
| Wow! 197100522! (what i got on my first hit)
|
| Out of all the possible numbers, what are the odds? 1 in a
| billion! Two consecutive 0's! And two consecutive twos!
| What do you think they mean? And 5 in between? Well 5 times
| 2 is 10 which ends in a 0. But it starts with 1971, which
| is only 1 year off from the start of Unix time, which can't
| be a coincidence! etc.
| vkou wrote:
| Not to mention that it contains the digits of 1215 - the
| year of Magna Carta.
| quectophoton wrote:
| > The really unsettling question is "why is the moon exactly
| the same size in the sky as the sun?"
|
| Can't wait until the moment in the future where
| DISTANCE_FROM_EARTH_TO_MOON / DISTANCE_FROM_EARTH_TO_SUN =
| FINE_STRUCTURED_CONSTANT
| danuker wrote:
| https://en.wikipedia.org/wiki/Tidal_locking
| mabbo wrote:
| As others have already said, it's tidal locking.
|
| But to be more precise on what that means, on a long time
| scale, the rotation speed of one object orbiting another tends
| to either speed up or slow down until one side is always facing
| the larger object.
|
| Many other moons in the solar system do this with their parent
| planet, and Mercury does this with the sun.
|
| It's happening to Earth as well, albeit too slowly to matter.
| Scubabear68 wrote:
| I had no idea Mercury was tide locked to the sun. What a
| bizarre surface it must have with one half permanently
| pointed at the sun and the other perpetually dark.
|
| Edit: Google says it's not.
| mock-possum wrote:
| Veritasium has a neat video on the general principle -
| https://youtu.be/t-_VPRCtiUg
| jameshart wrote:
| Ganymede, Europa and Io orbit Jupiter in periods with precise
| 1:2:4 ratios. Pluto and Uranus have a 2:3 orbital period ratio.
|
| A superstitious person might suspect there were some cosmic
| hand tuning the celestial clockwork. Perhaps the motions of the
| planets has deep resonances with our own lives? You can see why
| astrology might have some appeal...
| rstupek wrote:
| It's a phenomenon called tidally locked which is why Mercury
| exhibits the same thing with one side or mercury always facing
| the sun
| nick123567 wrote:
| Sorry but this is incorrect! Mercury does not have one side
| that always faces the sun.
| johntb86 wrote:
| Mercury is in a 3/2 spin-orbit resonance, where it rotates 3
| times for every orbit.
| sdenton4 wrote:
| I feel like this would be best illustrated by a KSP mod.
| computerex wrote:
| Or an Orbiter mod.
| BMc2020 wrote:
| There's an old vignette (under 400 word) sci fi story about an
| astronaut who goes to mars. He says mars has two moons and the
| martians say no, there are three moons, Phobos, Deimos and
| Bottomos.
|
| There's a line of rocks on the ground that the martians always
| pause at, then step over quickly. The astronaut is about to ask
| them why when a puff of sand and dirt shoots out of a faraway
| cliff, then something shoots by just above ground level too fast
| to see. The Martians say see, that's the third moon.
|
| (the third moon was a tiny black hole orbiting mars just a foot
| off the ground)
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(page generated 2023-02-26 23:01 UTC)