Phobos's orbit and its apparent motion across the Martian sky | Hacker News Reader

Phobos's orbit and its apparent motion across the Martian sky

azeemba.com

96 pointsazeemba49 comments

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.

This

Earth has many artificial satellites that do that.

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.

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

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.

It's dismissive, but I wouldn't call it shallow...

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.

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)

Thank you.

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.

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.

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.

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.

This 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.

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!

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)

Jupiter has some moons that really do go the "wrong" way: the Carme, Ananke, and Pasiphae groups.

Neptune's moon Triton as well: https://en.wikipedia.org/wiki/Triton_(moon)

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?!

It's a phenomenon called tidally locked which is why Mercury exhibits the same thing with one side or mercury always facing the sun

Sorry but this is incorrect! Mercury does not have one side that always faces the sun.

Mercury is in a 3/2 spin–orbit resonance, where it rotates 3 times for every orbit.

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.

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?

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

Exactly :)

https://numbergenerator.org/randomnumbergenerator/1-10000000...

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.

Not to mention that it contains the digits of 1215 - the year of Magna Carta.

That's my lotto number, how did you get that?

> 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

Shooting from the hip… But I bet it’s actually not that crazy rare for moons and suns to be a similar size in the sky of world that could harbor life. Factors at play: habitatal zones of different stars, the size of those stars, and the distribution of moon sizes for planets that could harbor life.

Do you have a source to reference for that "fairly common"?

If it's common, then what commonality causes that outcome?

Tidal locking is common. The moon being visually the same size as the sun is not.

https://en.wikipedia.org/wiki/Tidal_locking

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.

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.

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…

Veritasium has a neat video on the general principle - https://youtu.be/t-_VPRCtiUg

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.

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.

Its 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.

To capture our attention obviously.

What nobody is mentioning is that the moon is actually unnaturally round.

Unnaturally?

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.

I feel like this would be best illustrated by a KSP mod.

Or an Orbiter mod.

TIL Mars has a Moon that travels in the wrong direction.

Then shortly after, I learnt it doesn’t.

That's no moon

Is this how Superman is able to reverse time by flying around the earth faster than it rotates itself?

'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...

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