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Phobos's orbit and its apparent motion across the Martian sky
- imafish 4y agoTIL Mars has a Moon that travels in the wrong direction. Then shortly after, I learnt it doesn’t.
- sdenton4 4y agoI feel like this would be best illustrated by a KSP mod.
- computerex 4y agoOr an Orbiter mod.
- patrickdavey 4y agoThat's no moon
- mk_stjames 4y agoThis 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 4y agoThis Earth has many artificial satellites that do that.
- addaon 4y agoThere'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 4y agoI 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 https://news.ycombinator.com/newsguidelines.html
- cjbgkagh 4y agoI 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.
- hooverd 4y agoIt's dismissive, but I wouldn't call it shallow...
- RedNifre 4y agoIt'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 4y agoOk, 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 https://news.ycombinator.com/newsguidelines.html)
- RedNifre 4y agoThank you.
- Kranar 4y agoAre 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.
- dehrmann 4y agoReminds 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.
- hn_throwaway_99 4y agoTotally 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 4y agoThere 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.
- mc32 4y agoThis can happen at a train station with multiple tracks or stationary car in a lot or multilane roadway. You're in the stationary one and the other train or cars move and if you're not thinking it can feel like you're going backwards.
- sebastianconcpt 4y agoWhat 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?!
- rstupek 4y agoIt's a phenomenon called tidally locked which is why Mercury exhibits the same thing with one side or mercury always facing the sun
- nick123567 4y agoSorry but this is incorrect! Mercury does not have one side that always faces the sun.
- johntb86 4y agoMercury is in a 3/2 spin–orbit resonance, where it rotates 3 times for every orbit.
- CompuHacker 4y agoTidal 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 4y agoThat’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?
- 4y ago
- olivermarks 4y ago'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#!LBC https://woodenbooks.com/index.php?id_cms=8&controller=cms#!L...
- hammock 4y agoIs this how Superman is able to reverse time by flying around the earth faster than it rotates itself?
- wunderland 4y agoDisappointing 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!
- Ilverin 4y agoJupiter has some moons that really do go the "wrong" way: the Carme, Ananke, and Pasiphae groups.
- bagels 4y agoNeptune's moon Triton as well: https://en.wikipedia.org/wiki/Triton_(moon) https://en.wikipedia.org/wiki/Triton_(moon)
- swayvil 4y agoTo capture our attention obviously. What nobody is mentioning is that the moon is actually unnaturally round.
- bagels 4y agoUnnaturally?
- twojacobtwo 4y agoAren'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.
- PicassoCTs 4y agoFun 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 4y ago1. 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.
- PicassoCTs 4y agoIts not about brute force, its about giving it a kick in the right direction, changing the already moribund orbit, into one which graces atmosphere now instead of in 10000 years. The rest it does on its own.. 2. At that scale, it guess reignite volcanic activity, freeze frozen water and atmosphere. Or create a snowball event, although one could argue that mars already experienced that.
- BMc2020 4y agoThere'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)