Cidri's Moon

[BobP]

Originally Posted by thrash

I'm pretty sure someone would have noticed the enormous orb in the sky by now.

Maybe it's masked by a powerful illusion placed by the Mnoren. And who would think to disbelieve the sky?

[TippetsTX]

Any astrophysicists in the house? Assuming Cidri is a planetary sphere the size of Jupiter (just for the sake of argument), how far would the orbit of a moon the size of Earth need to be for it to appear at a similar observable size to our own moon?

[hcobb]

Note that Cidri has one G so putting Luna in a 60% higher orbit works.

[TippetsTX]

Originally Posted by hcobb

Note that Cidri has one G so putting Luna in a 60% higher orbit works.

I don't know, sometimes I think the extreme thrown and missile weapon range penalties could be indicative of a higher gravity constant. ;)

[Shostak]

Originally Posted by TippetsTX

Any astrophysicists in the house? Assuming Cidri is a planetary sphere the size of Jupiter (just for the sake of argument), how far would the orbit of a moon the size of Earth need to be for it to appear at a similar observable size to our own moon?

Given the plethora of magical phenomena, I don't sweat this particular part of unrealism. Cidri is big, and so is its moon. EZPZ

[Anomylous]

Originally Posted by TippetsTX

Any astrophysicists in the house?

Not me, but I happen to know one. He says, via e-mail:

Cool thread!

Apparent diameter scales linearly with distance and true diameter, so from the center of Cidri (or from any point where the “E-Moon” is on the horizon),

384,400 km (Earth to Moon) * (6,378 km Earth radius / 1,737 km Moon radius) = 1,411,458.4 km (Cidri to E-Moon).

If Cidri were same radius as Jupiter, that’d be 69,911 km, which is a pretty small fraction of the distance to the E-Moon. You could add that if you wanted the E-Moon to look the same size from the sub-E-Moon point (when it was straight overhead).

Jupiter’s Galilean satellites are an interesting comparison. I threw in the E-Moon as well and calculated angular size for each (doubling radius to get angle from edge-to-edge) and period in days.

sat: Distance(km) / Radius(km) / apparent size (degres) / Period (days)
—— —— —— ——— ——
Io: 421,700 / 1821 / 0.494 / 1.77
Europa: 671,034 / 1561 / 0.267 / 3.55
Ganymede: 1,070,412 / 2631 / 0.282 / 7.15
E-Moon: 1,411,458 / 6378 / 0.518 / 10.83
Callisto: 1,882,709 / 2410 / 0.147 / 16.69

So the E-moon would look only *slightly* bigger than Io actually looks from Jupiter. But it would be much bigger than the others.

Jupiter surface gravity is 2.53 G even though it’s mostly hydrogen; to tune that down to 1 G, it’d have to be mostly hollow, so it’s mass was 1/2.5 times as great as it actually is. That would also make the orbit periods longer; that goes like 1/ Sqrt(mass) so if you assume a Jupiter-sized Cidri with 1 g at the surface, multiply all those orbit periods (above) by 1.59. That still would give the E-moon a “month" only 17 days long, although Callisto would be getting almost to 28 days in that case.

All numbers above from wikipedia articles on Jupiter and Galilean satellites.

—————
I don’t have an account, but I’m fine if you post this.

(...dude, just make an account already. Whatever, at least we get to hear his legit professional opinion on the matter.)

[TippetsTX]

Very cool!

(and my version of Cidri is mostly hollow, BTW)

[Shadekeep]

Cidri has the same mass as Earth and therefore the same gravity, because beneath the surface it is 90% dungeon. ;)

Seriously, I think almost anything can be put down to Mnoren planetary engineering. With the science and magic of all those parallel Earths to draw on, it's not a stretch to imagine that the physics of the world could accommodate a huge surface and still have Earth-normal gravity.

Incidentally, if your version of Cidri doesn't have a Moon, it also doesn't have significant tides. The Sun does produce tidal effects, but nowhere at the magnitude of the Moon. Multiple moons would make for some complex tidal calculus, depending on the sizes and orbits of each.

And if you want to open another can of worms, feel free to debate whether or not Cidri orbits a class G2V star like Earth does. One presumes it does, since that's what the life is adapted for and Cidri likely occupies a space where a parallel Earth would have been. But it could orbit a red giant, or a blue dwarf, or multiple suns, in some campaigns.

[Shostak]

Or maybe the sun (and everything else) orbits Cidri.

[Shadekeep]

Originally Posted by Shostak

Or maybe the sun (and everything else) orbits Cidri.

Ha! What if the Mnoren were such medievalists that they build a flat world in the center of a Ptolemaic universe model? A flat planet could certainly resolve the "land area versus gravity" argument. Hopefully there's the equivalent of scrith on the bottom.