The moons are lovely...

[Umbran]

First graphic shows our Moon to the actual described sizes of Eberron's moons. Later on there is a graphic of their apparent size compared to our Moon. Zanatyr for example is roughtly half the size of Earth's moon, but would appear to be 9 times larger in comparison.

I think there's an issue in the calculations.

The radius of an orbit is typically given from center of the parent body to the center of the satellite.

The apparent size of the satellite is determined by the distance from the viewer, to the center of the satellite.

The "D" in the equations above, then, is the radius of the orbit minus the radius of the PARENT body.

So, we note that in the discussion of Zarantyr, we'd need to know the radius of Eberron, not the radius of Zarantyr.

Mind you, usually, in the real world, the orbital distance is very large compared to the radius of the Parent body. For example, the radius of the Moon's orbit is something like 60 times the radius of the Earth - if you just neglected the Radius of the Earth, your D would be off by less than 2%.

If you are looking for a ballpark - just use the Orbital radius, and never mind the rest.

 

[Beleriphon]

Mind you, usually, in the real world, the orbital distance is very large compared to the radius of the Parent body. For example, the radius of the Moon's orbit is something like 60 times the radius of the Earth - if you just neglected the Radius of the Earth, your D would be off by less than 2%.

If you are looking for a ballpark - just use the Orbital radius, and never mind the rest.

I recall correctly, the same blog has another post about the planet and just assumes it is more or less the same size as Earth.

 

[Umbran]

I recall correctly, the same blog has another post about the planet and just assumes it is more or less the same size as Earth.

Yes, but if you review the actual numbers above, they are pretty clearly using the satellite's radius.

 

[Beleriphon]

Yes, but if you review the actual numbers above, they are pretty clearly using the satellite's radius.

I'll have to take your word on it, I'm no astronomer.

 

[Umbran]

I'll have to take your word on it, I'm no astronomer.

It isn't a matter of astronomy. It is a matter of arithmetic.

Zarantyr, the first of Eberron’s moons, has a diameter of 1250 miles, or 2.0117x10^6 meters, while it has a mean distance from Eberron of 14,300 miles, or 2.3014x10^7 meters. The distance to the center of Zarantyr is then the mean distance plus half the diameter, which is 2.4019x10^7 meters. We can then plug our values into the equation.

He gives the diameter of Zarantyr. He gives a mean distance to Eberron.

If you take half the diameter of Zarantyr he gives, and add it to the mean distance to Eberron, you get the 2.4019x10^7 meters. That... includes a rounding error on his part as well.

Now, when I review these numbers in detail... woof!

So Zarantyr has a diameter of 1250 miles. Earth's Moon has a diameter of about 2160 miles. The Moon is on the order of twice as big across as Zarantyr.

Zarantyr is 14,300 miles from Eberron. That is absurdly close. Earth's Moon is 239,000 miles away. So, our Moon is nearly 20 times farther away from Earth as Zarantyr is from Eberron.

If I have much numbers right.... If we assume Eberron is Earth-mass, and Zarantyr has a density like our Moon...... Zarantyr is inside the Roche limit for the pair... and by normal physics would be ripped apart by tidal forces.

So, there's that....

 

[Beleriphon]

Zarantyr is 14,300 miles from Eberron. That is absurdly close. Earth's Moon is 239,000 miles away. So, our Moon is nearly 20 times farther away from Earth as Zarantyr is from Eberron.

If I have much numbers right.... If we assume Eberron is Earth-mass, and Zarantyr has a density like our Moon...... Zarantyr is inside the Roche limit for the pair... and by normal physics would be ripped apart by tidal forces.

So, there's that....

I know the numbers are straight from an article about Eberron's moons from WotC, so take that as you will. I think it's safe to assume that Eberron's moons are not the same density as our own.

 

[Umbran]

I know the numbers are straight from an article about Eberron's moons from WotC, so take that as you will. I think it's safe to assume that Eberron's moons are not the same density as our own.

Yeah, but it is several thousand miles inside the limit. 14,000 miles is really close. It is less than the circumference of the Earth! You have three basic optiosn for composition - ice, rock, or rock and iron. Even if the thing is made out of even ice... aside fromteh out-gassing from heating in the sun.... I think the tidal action on Eberron is apt to be devastating. No coastal cities for you!

It is less about density at that point, and more about how our laws of physics don't apply at all. Which is okay.

 

[Khelon Testudo]

Geez - what arc would Zarantyr subtend? Wouldn't it take up a large fraction of the sky?!

 

[Raduin711]

So what kind of effect would having multiple moons... or no moons... have on the ocean?

Would we even have tides without a moon?

And having multiple moons would make the tides unpredictable (or at least, less predictable) which would probably have some negative effects on maritime travel...

But this is all based on common wisdom which isn't always that trustworthy.

 

[doctorbadwolf]

Yeah, but it is several thousand miles inside the limit. 14,000 miles is really close. It is less than the circumference of the Earth! You have three basic optiosn for composition - ice, rock, or rock and iron. Even if the thing is made out of even ice... aside fromteh out-gassing from heating in the sun.... I think the tidal action on Eberron is apt to be devastating. No coastal cities for you!

It is less about density at that point, and more about how our laws of physics don't apply at all. Which is okay.

Clearly, it just needs the same treatment as Eberron’s population numbers. Add one or more zeros.