| T O P I C R E V I E W |
| PaulSKemp |
Posted - 23 Dec 2005 : 15:48:17
I posed a question in a Midnight's Mask discussion on my blog (http://www.livejournal.com/users/paulskemp/) and thought I'd do the same here.
We know that Selune's Tears are a multitude of various-sized asteroids stuck in a Trojan point around Toril. According to the FRCS, Selune orbits Toril at a distance of only about 20K miles, yet it appears about the same size in the sky as Earth's moon, which is about 231K miles from earth, on average. That means that Selune is about 1/10th the size of Luna, giving it a diameter of about 210 miles.
Here is an interesting thought experiment.
Figure out the respective mass of Luna and Selune (Luna is available online at many sites, within a reasonable margin of error). Then, given their respective distances from their parent bodies, apply the inverse square law to their respective gravitational effect and determine which exerts more gravitational force on its parent body. Ballpark figures are fine. I'm genuinely curious. My suspicion is that Selune, notwithstanding its vastly smaller mass, exerts as much or more gravitational effect on Toril than Luna does on earth. |
| 26 L A T E S T R E P L I E S (Newest First) |
| Antareana |
Posted - 24 Dec 2005 : 14:01:27
the influence of the Sun on Earth's tides is only 1/3 of Lunas. As you said correctly, distance is the factor here. At such large distances, even if you have a huge gravitational effect, you have to compare the radius of Earth (because the oceans are on Earth's Surface) to the distance Sun-Earth. The gravitational force of the Sun (or Luna) works against the force of Earth (and the gravitational effect on Earth's surface depends on your distance to the core).
About the period, I will ask my prof when holidays are over ^^
All that I got was that the mass of the smaller object is irrelevant for the period, if it is only small enough (sattelite)
For now we have only the choices:
1) Selune is 1/10 the diameter of Luna (200miles), 20000 miles in distance and it's gravitational force on Toril is 1/2 of that of Luna on Earth. The period topic is yet to discuss 
2) Selune has the diameter and distance of Luna and therefore it's period. The tides would be the same as that on Earth, only slightly influenced by the shards
However, I highly doubt that Selune has the same gravity as Toril ^^ |
| PaulSKemp |
Posted - 24 Dec 2005 : 11:35:39
quote:
Originally posted by Wooly Rupert
Even ignoring the Spelljammer info, the other points remain: Selūne is, according to the FRCS, approximately 2000 miles in diameter, and it takes 30 days to orbit the planet.
Luna is 2160 miles in diameter, and takes 28.25 days (I think that's the number) to orbit the Earth.
And the FRCS says how Selūne appears in the sky, which we can readily compare to how Luna appears.
So, if the two moons are the same size, appear about the same size in the sky, and have approximately the same orbital period, then we can assume that it's the same distance from the planet as our own moon.
Agreed. The FRCS must have bad information. But is it wrong as to distance or diameter? Can Selune have the appropriate orbital period if it is at 20K miles distant and say 200 miles in diameter? Or is that size and orbital period impossible (without magic), such that we can assume the entry is incorrect as to distance? |
| Thelonius |
Posted - 24 Dec 2005 : 10:43:30
Wow, a very interesting one! I think thi should be one of the "tomes" here. Thank you very much for this one Sage. |
| The Sage |
Posted - 24 Dec 2005 : 09:13:17
quote:
Originally posted by Thelonius
An interesting topic, indeed. I thought the "outer" features of the realms were the same as in the real world. That makes me think, what other differences of this kind are bewteen them both? Closest sun, more planets in their "solar sistem"? Where can this info be found?
Paul Westermeyer presented some rather intriguing udpates for the Realmspace accessory on the very first SJML many years ago.
They've been compiled (and updated again) at spelljammer.org:- http://spelljammer.org/worlds/Realmspace/
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| Talanfir Swiftfeet |
Posted - 24 Dec 2005 : 08:38:08
quote:
Originally posted by Wooly Rupert
No asteroid belt.
Then how can we use the Meteor Swarm spell  ? Does it come from the Selunes tears or from the elemental plane of earth. |
| Wooly Rupert |
Posted - 24 Dec 2005 : 08:25:24
The original Spelljammer boxed set gave a very brief overview of Toril's solar system, and the Spelljammer supplement SJR2 Realmspace did a more complete breakdown.
Realmspace doesn't really have too many parallels with our own solar system. No asteroid belt. Most of the planets are inhabited. Most planets only have one or two moons. There's not a bunch of gas giants, like in our solar system... At least one planet is mostly a giant plant, and other is a flat disk of water with a spindle of rock sticking thru it... |
| Thelonius |
Posted - 24 Dec 2005 : 08:00:45
An interesting topic, indeed. I thought the "outer" features of the realms were the same as in the real world. That makes me think, what other differences of this kind are bewteen them both? Closest sun, more planets in their "solar sistem"? Where can this info be found? |
| Wooly Rupert |
Posted - 24 Dec 2005 : 01:41:59
Even ignoring the Spelljammer info, the other points remain: Selūne is, according to the FRCS, approximately 2000 miles in diameter, and it takes 30 days to orbit the planet.
Luna is 2160 miles in diameter, and takes 28.25 days (I think that's the number) to orbit the Earth.
And the FRCS says how Selūne appears in the sky, which we can readily compare to how Luna appears.
So, if the two moons are the same size, appear about the same size in the sky, and have approximately the same orbital period, then we can assume that it's the same distance from the planet as our own moon. |
| PaulSKemp |
Posted - 24 Dec 2005 : 00:31:08
quote:
Originally posted by Wooly Rupert
Realmspace also places Selūne a bit further out -- 183,000 miles. I know that FR lore trumps SJ lore, and that newer lore trumps older lore, but I'm inclined to favor that distance. Moving a large orbital body that much closer really changes all the math. How can it be the same apparent size, about the same diameter, have a similar orbital period, and yet be closer and thus orbiting faster?
And if it's so much closer, but similar in size, why does it not appear larger?
I'm inclined to say the FRCS is either incorrect or is reporting erroneous information.
Interesting, Wooly. I wasn't aware of the Spelljammer info. Maybe the FRCS does have bad information. Or maybe this is just one of those questions that needs to be chalked up to the fact that Toril is a magical place (I find that somewhat unsatisfying, however). |
| PaulSKemp |
Posted - 24 Dec 2005 : 00:26:10
quote:
Originally posted by Kentinal
The tidal effects of the moon and the sun are considered near equal last time I checked, because of the distance factor.
Everything I've read, including this, http://hyperphysics.phy-astr.gsu.edu/hbase/tide.html#stid indicates that the sun's effect on earth's tides is less than half that of the moon's, due to its distance (and the inverse square law). Is that incorrect?
Either way, if your calculation regarding Selune's effect on Toril's tides is correct (namely, that it is a mere 1/1,000th or so of Luna's effect on earth's tides), I find that interesting, since it means that Toril would experience little in the way of tides (as you observed, only the solar tidal effect would be felt, assuming the Tears have no effect).
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| Firhyanda |
Posted - 23 Dec 2005 : 23:33:15
Formula for orbital period and distance try this.
P= the cube root of mass times distance to the third power.
D= p squared divided by mass to the power of .33
These should give good approximations of orbital period and distance for any orbiting body. It looks like the influance of selune would be vary small but if there are mutiple bodys similer in size to selune in orbit the net effect could be large. |
| Antareana |
Posted - 23 Dec 2005 : 21:12:17
urrr. that complicates things...
IF Selune was nearly as far out in space as Luna, then of course it would be nearly as heavy AND as large in diameter. That would also speak for the 3 tendays period. But if it really was so, all further discussion would be meaningless and we could replace the datas of Selune with those of Luna.
IF you then add that Selune may have the same gravitation as Toril, it would mean that (for Selune IS smaller in size than Toril) that its density is much, much higer than Toril's resulting in a mass that nearly equals (only nearly because of the bigger radius that Toril has) that of Toril.
Now you'd have something really complex: a double-planet system like that of Pluto and Charon with a combined center of mass between the two objects
oh, and the radius is squared, not cubed ^^ |
| Kentinal |
Posted - 23 Dec 2005 : 20:48:19
quote:
Originally posted by PaulSKemp
Kentinal,
Mass is only one of the pertinent variables. The other is distance. As I noted above, Selune is 100 times closer to Toril than the moon is to the Earth. Hence my mention of the inverse square law. Consider that earth's sun is monumentally more massive than earth's moon, yet the moon has a much more noticeable effect on earth's tides. The reason: relative distance to earth.
The tidal effects of the moon and the sun are considered near equal last time I checked, because of the distance factor. Bringing a cubed factor to a square distance I did consider in my estimate. Staitionary obit is about 22258 miles above Earth, the mass of those of course small. Selune by the math would be in this condition if not retrograde. Mass does indeed matter a lot to the calculations, one needs to know the mass of both objects. Though I suspose if you want to consider Earth is about 80 times as massive as Lunar and garvataional force is about .16 on Lunar then Earth there can be refinements to the numbers. The about 1,000 times less should stand as a close to correct answer if such an item existed in real world. The satilite is too small to offer much gavatational effects, you of course will still have solar tides which I did not include in my calculations. Not sure where the 1/2 came from though that gavatitaional formular is almost correct.
F=GMmr/R^3
The R cubed of course greatest factor.
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| Wooly Rupert |
Posted - 23 Dec 2005 : 20:32:46
As I recall from Spelljammer lore, Selūne has normal gravity. It's been a long time since I did anything with physics, but does that not imply that it has some serious mass?
Realmspace also places Selūne a bit further out -- 183,000 miles. I know that FR lore trumps SJ lore, and that newer lore trumps older lore, but I'm inclined to favor that distance. Moving a large orbital body that much closer really changes all the math. How can it be the same apparent size, about the same diameter, have a similar orbital period, and yet be closer and thus orbiting faster?
And if it's so much closer, but similar in size, why does it not appear larger?
I'm inclined to say the FRCS is either incorrect or is reporting erroneous information. |
| Antareana |
Posted - 23 Dec 2005 : 20:27:19
Nope, it doesn't have to be much faster. Because of the much smaller mass of Selune the gravitational force that works against the tangential force is much smaller.
Maybe Selune would have to be faster, but...
well, I could calculate that, too ^^ |
| PaulSKemp |
Posted - 23 Dec 2005 : 20:15:03
quote:
Originally posted by Kentinal
quote:
Originally posted by Talanfir Swiftfeet
This got me thinking about another question. If Selune is closer than the Moon does this mean that a month in Toril is shorter than a month in Earth (it should, otherwise it would mean that Selune is traveling much slower around Toril than the Moon is around Earth, and Selune would start falling towards Toril unless there's some magic involved).
Well magic is involved, a momth is 3 tendays and a month is orbit time of the moon.
The gavatational effect of Selune will be 1/1088th (more or less) then that of Lunar has on the earth, because of its much smaller mass (this infered that density of the two moons are the same) there will be very little todal effect. A 5 foot tide would become about a .05 inch tide. Also as indicated in RW a satilite that close would need to travel much faster to avoid crashing into the planet, unless of course the mass of Toril varys greatly from the mass of Earth.
Kentinal,
Mass is only one of the pertinent variables. The other is distance. As I noted above, Selune is 100 times closer to Toril than the moon is to the Earth. Hence my mention of the inverse square law. Consider that earth's sun is monumentally more massive than earth's moon, yet the moon has a much more noticeable effect on earth's tides. The reason: relative distance to earth. |
| Kentinal |
Posted - 23 Dec 2005 : 19:35:42
quote:
Originally posted by Talanfir Swiftfeet
This got me thinking about another question. If Selune is closer than the Moon does this mean that a month in Toril is shorter than a month in Earth (it should, otherwise it would mean that Selune is traveling much slower around Toril than the Moon is around Earth, and Selune would start falling towards Toril unless there's some magic involved).
Well magic is involved, a momth is 3 tendays and a month is orbit time of the moon.
The gavatational effect of Selune will be 1/1088th (more or less) then that of Lunar has on the earth, because of its much smaller mass (this infered that density of the two moons are the same) there will be very little todal effect. A 5 foot tide would become about a .05 inch tide. Also as indicated in RW a satilite that close would need to travel much faster to avoid crashing into the planet, unless of course the mass of Toril varys greatly from the mass of Earth. |
| Talanfir Swiftfeet |
Posted - 23 Dec 2005 : 19:19:15
This got me thinking about another question. If Selune is closer than the Moon does this mean that a month in Toril is shorter than a month in Earth (it should, otherwise it would mean that Selune is traveling much slower around Toril than the Moon is around Earth, and Selune would start falling towards Toril unless there's some magic involved). |
| Antareana |
Posted - 23 Dec 2005 : 19:02:28
well, it might take some time, but I might be able to calculate this, though it might be a bit vague (since I don't know the formula for Seulne's ellipsoid movement)
So theoretical Physics do at last have their usefulness ^^
The influence of the shards on the tides depends on their size and their position in relation to Selune. if they are quite huge and a quarter circle behind or before Selune (they have nearly the same orbit if I'm correct) then they might minimize the tides.
If they are close to her (or in direct opposition) then their and Selune's gravitational effect would sum up.
For the calculation:
First you have to say that Selune looks as big as Luna, though it is 10 times nearer to Toril. That means that it's diameter and radius is 10 times smaller than Luna's.
The result of this is a 1000 times smaller volume and mass (Mass and volume are proportional)
simplifying our calculation, we assume that Luna's and Selune's orbits are perfect circles and no ellipsoids and that Toril and Earth are not much influenced by their moons.
Earth and Toril have the same size, the same gravitation and therefore the same mass.
for the radius Earth-Luna and Toril-Selune we have to add the radius of both gravitational objects for gravitation is a "central force"
(radius Earth/Toril = 6378 km; Selune = 173 km; Luna = 1738 km)
oh, and we have to recalculate the radius into the metrical system ^^
then the distances between Luna and Earth are 3,27 * 10^8 meters and between Selune and Toril 3,84 * 10^7 meters (you see the influence of the added radii)
Getting our gravitational force then is pretty simple:
Gravitation = G * M * m/r² with G = 6,67*10^-11
That results in the following Forces:
Earth-Luna: 4*10^19 N
Toril-Selune: 2*10^19 N
So the Influence of Luna on Earth is two times that of Selune on Toril |
| PaulSKemp |
Posted - 23 Dec 2005 : 17:55:26
I was unclear in my initial post. Apologies.
What I'd like to know is whether Selune's relative gravitational effect on Toril is greater than Luna's relative gravitational effect on the Earth. In particular, I'm thinking of tidal forces. For that narrow question, I don't think we need to know the relative size of Toril to Earth because it doesn't factor into the calculations (it is true, however, that if Earth and Toril are the same size, Toril's gravitational pull on Selune would be substantially greater than Earth's gravitational pull on the moon; but that is a different question). |
| Kes_Alanadel |
Posted - 23 Dec 2005 : 17:45:39
quote:
Originally posted by PaulSKemp
I posed a question in a Midnight's Mask discussion on my blog (http://www.livejournal.com/users/paulskemp/) and thought I'd do the same here.
We know that Selune's Tears are a multitude of various-sized asteroids stuck in a Trojan point around Toril. According to the FRCS, Selune orbits Toril at a distance of only about 20K miles, yet it appears about the same size in the sky as Earth's moon, which is about 231K miles from earth, on average. That means that Selune is about 1/10th the size of Luna, giving it a diameter of about 210 miles.
Here is an interesting thought experiment.
Figure out the respective mass of Luna and Selune (Luna is available online at many sites, within a reasonable margin of error). Then, given their respective distances from their parent bodies, apply the inverse square law to their respective gravitational effect and determine which exerts more gravitational force on its parent body. Ballpark figures are fine. I'm genuinely curious. My suspicion is that Selune, notwithstanding its vastly smaller mass, exerts as much or more gravitational effect on Toril than Luna does on earth.
My goodness....figures this is the first thread I look at this morning, with not nearly enough coffee.
*Then realizes that it has been 20 years since she has had to use any physics or algebra or any other subject that might help with this, so has no idea how to even start an equation to figure it out.*
So I'll go with what Wooly said....42 *Goes to get Advil and more coffee*
~Kes |
| Kuje |
Posted - 23 Dec 2005 : 17:07:50
Ed has said in the past this:
"December 29, 2004: BrokenRulz, Ed hereafter makes answer to your question: How does Elminster celebrate the Yuletide/Soltice?
The axial tilt, size, and rotational period of Toril are close to our Earths but not identical. Solstices do occur, and at such times the Weave shines forth most brightly in the Realms."
So, I bet you that he could answer your query. :) |
| Wooly Rupert |
Posted - 23 Dec 2005 : 17:07:13
In one of the Ask Ed threads, I believe, there was a discussion about Toril's size, and notes about its orbit and such. |
| sleyvas |
Posted - 23 Dec 2005 : 17:04:18
Yeah, you'd have to make note of Toril's size (which in the old Gray Box they compared Faerun to North America and it ended up Faerun was pretty big)... but also, if it's 1/10th the size in a radius.... that means its 1/thousandth the size in mass. Given that, my suspicion would be the opposite, Toril probably influences Selune a good bit, which might be why its breaking up into shards. Of course, I have no idea of the formula you'd use. |
| Dhomal |
Posted - 23 Dec 2005 : 16:23:19
Hello-
Now Thats truly an interesting thought!
And - more imoportantly - I know we could find the answer. Darned if *I* know how though! 
Another point of consideration - the Tears themselves would probably help 'add mass' for the final calculation. Depending on just how many there are - this could be somewhat significant.
I truly would like the answer too.
BTW - do we know of the actual size of Toril? I know there have been global maps - but I was never sure if someone had done the calculations to see what size it is. If its not Earth-similar - then that would have to be taken into account.
Anyway - I'm glad to see someone thinking this early in the day! LOL
Dhomal |
| Wooly Rupert |
Posted - 23 Dec 2005 : 16:18:10
Uh... 42!  |
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