Other Foreign Skies

This post is a response to a question I got on my Ask Tsana page.

Sam Keola asked:

Love the views of Jupiter from Ganymede and Io. How large would it appear from Europa or Callisto? And how large exactly would the sun appear? (I know tiny as hell, but another lovely picture would be amazing.)

The mathematical answer to that is explained in this old post. And my first set of Jupiter images (Io and Ganymede's skies) can be found here.

Jupiter

This time around, I used a different image of Jupiter so if you're wondering why it's rotated relative to the old pictures, that's why. For the Jovian images, I've used the same starting image because in the year since I last did this, I haven't managed to take a more suitable photo. Such is life.

The original photo with a full moon in Earth's sky.

So. Europa is the second Galilean moon out from Jupiter. It's made mostly of ice, is the smallest of the Galilean moons and might harbour life in its subsurface liquid ocean. The diameter of Jupiter as it would appear in the Europan sky is almost 24 full moons across. Remember that Europa's sky wouldn't actually look blue either since it doesn't have an atmosphere but I don't have a decent night skyline to work with. I'll do a night version eventually.

The size Jupiter would appear in Europa's sky. Or in Earth's sky if you swapped it with Europa.

You might be wondering whether Jupiter would actually be oriented the way it appears in these images. Well it depends. The direction the bands run relative to the moon's horizon would depend on where on the moon you were. Close to the equator, the bands would be vertical (although if Jupiter was high in the sky, it would be pretty difficult to tell. Perhaps better to say east-west). If you were near a pole, they'd be horizontal as in these images. And remember, the Galilean moons are all tidally locked, so Jupiter would never move, just change how much of it was illuminated by the sun.

And Callisto, the most distant of the Galilean moons. Callisto's Jupiter would appear "only" about 8.5 full moons across.

The size Jupiter would appear from Callisto. If Callisto had an Earth-like atmosphere and gum trees.

The Sun
 
The second part of Sam's question was how large would the sun appear from Jupiter. Well, on Earth, the sun and the moon appear to be approximately the same size (there's a little bit of a difference when the sun is at its closest and the moon at its furthest and vice versa). So the sun from Earth is about one full moon in diameter.

From Jupiter (or its moons) the sun would appear about 0.4 full moons across which is a little bit less than a sixth of the area of the sun as seen from Earth (remember, the moon and sun seen from Earth are on average the same size).

I cheated a little bit with these next two sun photos. They're actually two separate photos and I made the sun smaller in one of them. The reason the rest of the photo looks darker for the Jovian sun is because I was fiddling with settings on my camera. And if you're wondering why I chose sunsets, it's because those (and sunrises) are pretty much the only kinds of photos where the disc of the sun is properly visible.

Ordinary sunset on Earth:

Sunset. A little bit more than half the sun is below the horizon.

Sunset if Earth was at the same distance as Jupiter (but yet still warm enough to have liquid water. And plants. By the way, with these two, it's probably clearer if you click on the images to enlarge and compare the sun side by side.

A more diminutive sun, less than a sixth of the area of Earth's.

And there you have it. Photoshopped images (well, actually, I used Pixelmator) depicting the sizes of Jupiter and the sun from the Galilean moons and the Jovian system, respectively.

Foreign Skies: Daytime

When I wrote my first post on this blog, I wanted to include some photoshopped images of to-scale Jupiter hanging in the sky as it would above Ganymede and Io. At the time, I didn't have any good photos of the moon to compare with and paste over and I didn't want to steal something from Google Image Search so I put it off. I still don't have any good, cloudless photos of the moon at night with a suitably urban back drop. Instead, I decided to put my (very average) photoshopping skills to use and make a daytime Jupiter in the sky.

A caveat: Ganymede and Io both lack atmospheres that even remotely resemble Earth's. As a result, you wouldn't get a blue sky on either, even under a biodome of some sort (there wouldn't be enough air in the dome to have that effect). Instead the sky would be black outside of the giant orb of Jupiter (which, when full, would definitely be bright enough to make it hard to see stars).

So you shouldn't treat these images as "what the sky would look like on Ganymede/Io" more as an indication of how large Jupiter would be in the Earth's sky if you put it in Ganymede's/Io's place.

First! The original photo with the Earth's natural moon left in (taken with my phone, so it could be awesomer, but it served its purpose):

Unlike Earth, Ganymede and Io are unlikely ever have gum trees. Just saying. (Click to enlarge.)

For the remaining compositions, this APOD image is the shot of Jupiter I used. Credit to NASA, ESA, and E. Karkoschka (U. Arizona).

Next up, Jupiter as seen from Ganymede. I left the moon in for the first one, just so you can visually compare sizes better:

To scale, albeit physically unrealistic. I don't think Jupiter would quite be that colour either (particularly the edges wouldn't be so dark, but I couldn't fix it) and of course the lighting is all wrong. (Click to enlarge.)

Just Jupiter alone in the Ganymedian sky:

Jupiter in Earth's sky if Earth was in Ganymede's orbit around the gas giant. (Click to enlarge.)

And, last one, the size of Jupiter in Io's sky. Large doesn't really cover it.

Jupiter looming as though over the Io skyline. Of course, Io is even less likely than Ganymede to have apartment buildings on it, but shh! (Click to enlarge.)

And there you have it. Once I have a decent night skyline with the moon in it, I will repeat this but at night. It will be significantly awesomer. I just have to get some night photography in first.

EDIT: I have made some similar images for Jupiter from Europa and Callisto and the sun as seen from the Jovian system. See my new post here.

Foreign Skies

Living on Earth, we are used to seeing the moon in the sky. Other planets, real or imagined, are unlikely to have moons identical to the Earth (well, ok, the imagined ones might). Other moons will look different and will take up different amounts of the sky. Today, I calculated how big various celestial bodies would appear from the surface of other celestial bodies and compared this with the size of the moon as seen from Earth.

In case you want to try this yourself, I will briefly explain how I did it. First I found the angular diameter of the object in the sky using the equation below. For objects that are much smaller than the distance they are away (so if radius divided by diameter is less than about 0.1), you can also use the small angle approximation, in which you can ignore the tan part and the angular diameter is approximately 2r/d.

Here, r is the radius of the object in the sky and d is the distance it is away. Both r and d should be in the same units,  that is, both in km (or miles). If you have your calculator set to degrees, then α is the angular diameter of the object in the sky.

 Now, because angular diameter doesn't really mean much on an intuitive level, I then divided this angle by the angular diameter of the full moon which is about 0.5 degrees. This gives me the number of full moons you could line up across the middle of the object. If you're interested in the area of sky covered by the object (in units of the full moon), then just square this number.

The Earth seen from the moon

The Earth is a lot bigger than the moon, so naturally it would look bigger if you were standing on the moon. In fact, the Earth seen from the moon is 3.6 times the size of the full moon. APOD have a nice photo for us of this scenario.

Living on Ganymede

Ganymede is the largest moon of Jupiter, and the third furthest away of the four Galilean moons. Ganymede is about three times as far from Jupiter as the moon is from Earth. However, Jupiter is very large (the largest planet in the solar system, in fact) and if visible from Ganymede's surface, would dominate the sky. As in happens, Ganymede is tidally locked with Jupiter, putting it in a synchronous orbit and meaning that the same face is always turned towards Jupiter. Unlike our moon rising and setting in the sky, it you stood on a part of Ganymede facing Jupiter, the gas giant planet would not move in the sky at all. Also, you could use Jupiter to navigate (even if it was directly above you, although it would be a bit trickier in that case) because the bands in its atmosphere run from east to west (here is another nice photo from APOD, which also shows Ganymede in the distance).

So, standing on the surface of Ganymede, how big would Jupiter look? Very big. Fifteen full moons across big. It's a bit hard to imagine and if I find a nice photo of a full moon above a city or other recognisable landmark, I will photoshop Jupiter in and post it here. Jupiter looks so big that you could spread 225 full moons over it and only just cover it up. Yeah. That big.

I also worked out how big Io, the closest Galilean moon to Jupiter, would look from Ganymede. Io is covered in volcanoes and sulfur and looks yellow. When Io is lined up between Ganymede and Jupiter, it will be 0.6 times the size of the full moon; a bit more than half. I initially decided that when Io was on the far side of Jupiter, even if it wasn't behind the parent planet, it would be too small to be very exciting, but doing a quick calculation I found that it would be about a third of the size of the full moon, so probably still big enough to be noticeable.

Phobos for a bit of fun

For a bit of fun, I also worked out how big Mars would look if you were standing on Phobos, one of its asteroid-like moons. Before doing this calculation, I never really appreciated how close in Mars' moons orbited. Firstly, Phobos is only about two Martian radii above the surface of Mars. This means that the small angle approximating definitely won't work when calculating Mars' apparent size. So how big does Mars look? Its angular diameter is a whopping 140º. That's the width of 280 full moons lined up next to each other. In fact, Mars would take up nearly the whole sky on Phobos. I think that's big enough to feel like you're falling towards it, particularly since Phobos has a very puny gravitational field. Scary.