The moons are lovely...

So, you look up at the night sky and see the moon. Lovely, isn't it? As in uffish thought I stood, I started thinking about the Jovian moons.

If you were standing on the surface of Jupiter* and looking up, what would the Jovians look like? I would assume that Ganymede would have a familiar disk, being larger than Mercury. But looking up at Io, Europa, and Ganymede together*, would you see different disks? Would there be a notable difference between Io and Europa as Io is closer but they are similar in size?

I guess my real question is that is it at least plausible with physics that you can look up and see three different sizes of moon, or would it be an interesting happenstance that the moons because of their size and resultant distance that they happen to look mostly the same, disk-wise?

Posted here since it's actually a detail for a game, and not a wholly random musing.

* Not on the surface-surface; on the visible cloud cover, hovering, in my orbital platform with an unobstructed view of the heavens. Roll with it.

* Yes, I know you can't see the three of them at the same time. Keep rollin'.

No ideas at all on any answers here, but kudos for an interesting train of thought.

There are mostly much further from Jupiter, than Luna is from Terra. Ganymede for example is three times further from Jupiter than our moon is from Earth. Also, it's a bit of an optical illusion how large the moon appears. The eye perceives it as larger than it is just because it is so bright. You get a good feel for the true size of the moon without any illusion during a lunar eclipse. As such, the crescents that they produce would appear much smaller than that of Luna.

I remember having seen pictures that simulated the view of the moons you are imagining, but I don't at all remember where I've seen them.

Angular size is inversely proportional to distance. So pulling numbers off the this site and doing rough calculations:

Io would appear slightly smaller than the moon appears from Earth
Europa and Ganymede would both appear roughly half the size of our moon
Callisto would be about a quarter size our moon

As they all have different periods, I imagine there would be times when you could see all of them at once.

Baron Opal II said:

So, you look up at the night sky and see the moon. Lovely, isn't it? As in uffish thought I stood, I started thinking about the Jovian moons.

If you were standing on the surface of Jupiter* and looking up, what would the Jovians look like? I would assume that Ganymede would have a familiar disk, being larger than Mercury.

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So, quick calculation:

From Jupiter, Ganymede takes up about 0.3 degrees of arc in the sky.

The Moon, seen from Earth, takes up about 0.5 degrees of arc.

So, Ganymede is a disk, but smaller than the moon.

I guess my real question is that is it at least plausible with physics that you can look up and see three different sizes of moon, or would it be an interesting happenstance that the moons because of their size and resultant distance that they happen to look mostly the same, disk-wise?

Click to expand...

It would be quite a coincidence that they all happen to look mostly the same. But, coincidences do happen. For example, we just happen to live at a time when the Sun and our own Moon just happen to be the same size in the sky.

Given the relatively large radius of Jupiter compared to the orbital radii of the innermost big moons (roughly 20%, I think?), I wonder if those moons would change appreciably in apparent size (and brightness) from moon-rise to zenith? Would Io appear to "fall" toward the observer as it rises in the sky, then recede back over the horizon? (On earth, we have the illusion of the Moon being huge when it's low near the horizon, and small when overhead. Would the visual effect be reversed on Jupiter?)

I also imagine the moons go in and out of Jupiter's shadow quite frequently, so lunar eclipses wouldn't be as special as they are for us. How often would "full" moons even occur compared to eclipses? Perhaps the full moons would be the special events?

Fun questions to think about....


(Also, the word "Io" is a compelling standalone argument in favor of serifed fonts, imho.)

Okay. So there isn't any particular correspondence or correlation between a moon's size, orbital distance, and apparent size from the planet's perspective. Good to know.

If the moon is 0.5 degrees of arc, I gather that 0.05-0.1 would be getting close to the point of seeing a disk rather than a star-like point.

Actually, due to the 14 harmony with the inner three moons, you can only see two of them at a time, as I understand it. When two would be above you, the third is at least a quarter orbit away. Callisto isn't in orbital resonance with the other moons, so she's doing her own thing. So, you could see three out of four at a time.

I take it that any such equations to determine disk size are readily found... Now, to repair the 3D printer!

Baron Opal II said:

Okay. So there isn't any particular correspondence or correlation between a moon's size, orbital distance, and apparent size from the planet's perspective. Good to know.

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Um... the apparent size is determined by the actual size and the orbital distance.

Other than the Roche Limit there is no particular correspondence between the moon's size and its orbital distance.

The Roche limit is a limit on the orbital radius of large smoons - if a large satellite gets too close to its parent body, the satellite will be ripped apart by tidal forces. You will not find very large moons very close to the planet, as they get ripped apart.

If the moon is 0.5 degrees of arc, I gather that 0.05-0.1 would be getting close to the point of seeing a disk rather than a star-like point.

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Human visual acuity can't generally make out any shape at under about 0.02 degrees - roughly the size of hte Intenational Space Station seen from the ground. Anything smaller than that is just a point. Larger than that, you go from a point to a dot, to a spot, to a disk. I don't know if there's an agreed upon limit when something gets called a "disk".

I take it that any such equations to determine disk size are readily found...

Click to expand...

To quote WIkipedia on Angular Diameter:




delta = 2arctan(d/2D)

Umbran said:

Um... the apparent size is determined by the actual size and the orbital distance.

Click to expand...

Right. I meant that it's individual, and moons of different masses won't arrange themselves so that they look the same. Smaller ones aren't necessarily going to move closer and large move farther away.

Thanks for the further details!

Not that this is relevant to Jovian moons, but it is relevant to D&D:


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.