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

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

#21

Post by permo-w »

iamphilrae wrote: post #14
sebastianconcpt wrote: post #8

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?!

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.…

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

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

#22

Post by kurthr »

hn_throwaway_99 wrote: post #18
mk_stjames wrote: post #5

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.

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.

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

#23

Post by nick123567 »

rstupek wrote: post #11
sebastianconcpt wrote: post #8

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.

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

#24

Post by johntb86 »

rstupek wrote: post #11
sebastianconcpt wrote: post #8

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

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

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

#25

Post by mock-possum »

sebastianconcpt wrote: post #8

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?!

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

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

#29

Post by Kranar »

jakelazaroff wrote: post #7
mk_stjames wrote: post #5

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.

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

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.

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