Challenge Questions

The two moons are orbiting at different speeds. One would appear to move the wrong way if it was the slower one, as the faster one catches up. This is an optical illusion because we would tend to use the other moon as the reference when judging the direction of motion of the one.

Phobos has an orbital period of about 7 hours 39 minutes which is considerably shorter than the 24hour and 40 minute martian day so it will move from west to east relative to the martian surface. Deimos has an orbital period of about 30 hours and 18 minutes so it will move from east to west relative to the martian surface.

Hey, Troy 36, I like that answer! Sounds like you've been there?

Has anyone read the discussion of Mar's moons in Gulliver's Travels? This was something like 100 years before their discovery! I don't recall the actual discussion.

Troy I'm with you, good answer

Hi Troy,

I think you got it right. A little research goes a long way. Question about your avatar. Is it a jet plane coming out of a maneuver in a moist sky, perhaps?

S

Thanks StandardsGuy.

My avatar is a close up picture of a flower in my wife's flower garden. I bought her a new camera for her birthday last year and was playing around with the close up settings. I just liked the way the picture turned out.

Good work Troy36. You have the right answer and the correct sidereal periods.

All periods and motions in this discussion are sidereal, all given values are provided by NASA.

Phobos has (according to NASA) a mean orbital altitude of 5981Km and an orbital period of .31891 Martian days or 7 hours and slightly more than 51 minutes (martian day=24.6229hr x .31891 day Phobos orbit). Diemos has a mean orbital altitude of 20062Km and an orbital period of 1.26244 Martian days or 31 hours and just more than 5 minutes.

So assuming the 24.6229 hour Martian sidereal day, 360dg/24.6229hr=14.6205dg/hr, thus Mars rotates slightly less than 15 degrees per hour. Which is to say that the stars appear to move through the Martian sky from east to west at a rate of 14.6205 degrees per hour.

Phobos moves through the Martian sky at a rate of 360dg/7.8525hr=49.5970dg/hr which, minus the sidereal rotation of Mars, 49.5970dg/hr-14.6205dg/hr, gives Phobos an apparent motion of 34.9765dg/hr West to East!

Diemos on the other hand, moves through the Martian sky at a rate of 360dg/31.0849hr=11.5812dg/hr which, also minus the sidereal rotation of Mars, 11.5812dg/hr-14.6205dg/hr gives Diemos an apparent motion of 3.0393dg/hr, east to west.

Thus yes, Diemos, like Earth's own moon, moves east to west, while Phobos moves west to east. Apparantly.

Thus yes, Diemos, like Earth's own moon, moves east to west, while Phobos moves west to east. Apparantly.

Visually apparant............this is where you believe your lieing eyes instead of the factual suported truth.

r1a@aol.com

Astronomical phenomena is measured in earth hours, minutes, days. For consistent units,

a day on Mars is 24.6229 earth hr or 24 hr 37 min

Phobos period is 0.31891 earth days or 7 hrs 39 min

Deimos period is 1.26244 earth days or 30 hr 18 min

Your conclusions are still correct

I agree with "Dr." "Troy" "Standard". With all the info available on the Net I couldn't say it better.

The simple answer is that one moon is orbiting lower and at more rpm (revolutions per month) than planet rotation and the other moon is higher and at a slower rpm than planet rotation.

Terry Kelpien

I think 'stanards guy' is the closest to making sense.'Troy' and 'Moose' are also correct in fact that Phobos ( being below synchronous orbit, therefore faster than Mars ) and Deimos ( being just outside synchronous orbit, and slower than Mars ), can and do co-exist in oppsoite orbit.First off, it is my understanding that Deimos and Phobos DO orbit opposite one another. So unless im wrong, the question seems to me hypothetical, but proposes "the two moons orbiting the same direction ", with the same visual orbiting affect of east to west/west to east. In which case 'Standards' first entry would be correct i believe, with the exception of any one moon being a focal or referance point.

Its a simple matter of Mars rotating faster than Deimos really

There's a good animation here:

http://en.wikipedia.org/wiki/Deimos_(moon)

Who needs to be smart anymore when you've got internet access...

corrected link:

http://en.wikipedia.org/wiki/Deimos_(moon)

GOSH WHY DOES IT NOT LIKE MY LINK *(&%^&(%^$^%

http://en.wikipedia.org/wiki/Deimos_%28moon%29

Who needs to be smart anymore when you've got internet access...

GOSH WHY DOES IT NOT LIKE MY LINK *(&%^&(%^$^%

Maybe we still need to be smart after all.

Isnt it silly if you have to look up something because you dont know what your ( someone ) is talking about though?

My God - 9am and only one coffee inside me and I think I have the answer!!

Mars' rotation is slightly quicker than that of one of the moons orbits making it appear to move 'in reverse', but slower than the orbit of the second moon...

Too early for the math bit though!!

Karl Green (aka Karlos Fandango)

In fact, both Phobos and Diemos orbit Mars in the normal direction, that is, counterclockwise when viewed from above the planet's north pole. Of all the planets, Neptune's Triton is the only major moon with a retrograde, or clockwise orbit. Each of the gas giants have a handful of small, irregular moons in retrograde orbits, but these are thought to be captured asteroids and comets. The illusory motion of Diemos moving from east to west is due simply to the fact that Diemos' orbital period is greater than Mars' rotational period, in the same way that Earth's moon appears to move from east to west in Earth's sky, even though over the course of a month, Luna does in fact proceed through the sky from west to east.

This is trivial. One moon is above a geostationary orbit (whatever tha is do Mars) and the other is below the geostationary orbit. I have neither the time nor inclination to look up the relative masses of Mars and it's moons, but the calculations would be trivial with that data.

I tried to register, but had to certify I had read and agreed to a set of rules, but couldn't find the rules. Great website!

gsmith404@suddenlink.net

well ive never been into the cosmos trajectories, but it sounds like the moon that appears to be traveling backwards is actually traveling in the same direction but slower than the mars orbit time around its axis.

thus the "alien" eye as a fixed origin is turning faster than the moon around the same axis..

Right, they move in the same direction but the orbits have different angles. Hence, when you see those huge objects approach they seem to move in the opposite directions without a visual change in size. Nikolay

The solar day oon Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds.

From the surface of Mars, the motions of Phobos and Deimos appear very different from that of our own moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outside synchronous orbit—where the orbital period would match the planet's period of rotation—rises as expected in the east but very slowly. Despite the 30 hour orbit of Deimos, it takes 2.7 days to set in the west as it slowly falls behind the rotation of Mars, then just as long again to rise.

Because Phobos' orbit is below synchronous altitude, the tidal forces from the planet Mars will gradually lower its orbit. In about 50 million years it will either crash into Mars' surface or break up into a ring structure around the planet.

It is not well understood how or when Mars came to capture its two moons. Both have circular orbits, very near the equator, which is very unusual in itself for captured objects. Phobos's unstable orbit would seem to point towards a relatively recent capture. There is no known mechanism for an airless Mars to capture a lone asteroid, so it is likely that a third body was involved—however, asteroids as large as Phobos and Deimos are rare, and binaries rarer still, outside of the asteroid belt.

Best Regards RS

Mars rotates in the same direction as the orbital directions of Phobos and Deimos. the rotational period of Mars is 24 hours, 37 minutes, or 24.62 hours. Phobos makes 1 revolution around Mars every 7 hours and 51 minutes, and would appear to the observer on Mars to be moving from West to East. Deimos on the other hand, makes one revolution around Mars every 31 hours and 5 minutes, thereby appearing to move from east to west.

M of mars=6.4x(10to the 23)kg

Mars day is about 24 hours, so R for a areosychronous orbir will be about 20,000km.

With rotation and orbits all in the same direction, Phobos, with R=9,000km, will have a period less than 24 hours, about three orbits a day and so will appear to revolve twice a day. It will move in the oppocite direction of the sun. Deimos, with R=23,000, will have a period longer than a day, about 1.3 orbits a day and so will appear to move a revolution in the direction the sun goes about ounce every three days. Thus they appear to move oppocite of each other. Thanks to Google I didn't do much real work! Ron

The term I'm waiting to hear is "retrograde motion" and I thing the coffeeless god guy came closest.

The difference in relative motion is due to the fact that mars is also rotating. The rotation speed is somewhere between the translational speed of the two moons.

Too many brewskys

We could always leave this for another sol. July 20th 2007 perhaps (I'll have to check what that's relative to) ?

How can anyone make such a meal of this?
#2 (by Troy36) is simple, precise, correct, and clear. Read, mark and inwardly digest. Then, perhaps, think of something amusing to add...

Spoilsport

I just happened across this site. I'm not sure I want to join. I don't know about engineering, but I do know the answer to this question, and math is NOT required. It's a "trick" question. The answer is simple: The two moons are on opposite sides of the planet, both moving in the same orbital direction. Deimos, moving east to west, is moving behind Mars in its orbit, while Phobos' orbit is taking it in front of Mars, from left to right. This, each moon appears to be moving toward the planet and each other!

Hi Guest ,

Things are not always as they appear. Have a look at past questions and CR4 in general . You may be surprised at what you find. Explore the site a little before you reach conclusions. That's all I can suggest , but you may find the time taken to look around worthwhile. Certainly revisit to see the 'answer' when it appears. I'm just a casual user here , but it's the best place I know on the internet.

Kris.

A solemn, moving precession?

Can I calculate anything from the information given in the question ?

"Alas poor jorrie ! I sued him no ratio."

Can I have one point at least for cringe-inducing humour ?

You are allowed to look up additional information... Surely the information in the question only needs to be complete to the extent that users can come to sensible conclusions if they take the trouble to acquire any necessary additional information. Even that was not needed here to visualise the basic process. It's just as well Martians aren't very tall.

Fyz

Points will be deducted for cringes - and points mean prizes (Adapted from Humphrey Littleton - now returning to radio4)

If there's no numbers I'm not playing. I'm having trouble trying to parse that long sentence of yours Fyz.. Little rocks orbiting a spinning planet , all at different rates does not amuse enough. Even matters of perspective do not inspire me to draw anything. The question seems 'bereft of life'.

I refuse to bring additional facts to the question . There must be more to this than 'think of common sense answers' , or 'go learn about Mars'. I chucked in bad Shakespeare to help ER's blog. Nobody seems interested in a 'sol' or Mars Day , so I shall disappear to sulk until the answer is posted. I may need points as I still can't find the Stiletto's I told ER about.

...and giving people silly things to do.

That's just what we need - an antidote!

...Yes , quite right . A change is as good as a rest.

ps - I do not wear Prada , or stilettos. Usually.