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I have an Earthlike moon orbiting a Saturnlike ringed planet at 1.8 million km. Neither the moon nor the giant are tidally locked.

How bright would Saturn appear to be in the sky? Would it hurt your eyes to look at, cause vision damage, or even be visible at all?

The star is a yellow dwarf comparable to the sun. The gas giant orbits at 150 million km. The albedo of the gas giant is 0.42 and the moon is basically Earth, in terms of size, atmosphere, and so on. I'm trying to determine how the gas giant would appear to humans living on the planet, what effects it would have, and in turn how it would be experienced from the surface.

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    $\begingroup$ Logically, somewhat less bright than looking at the ground when the sun's overhead. Unless, that's blindingly bright - can you fill us in on details like that and the giant's albedo, sun's brightness, anything we'd need to know about the atmosphere of your moon, the colour-reflectance profile of the giant. I think it would help to tell us what you're aiming for, what're you trying to achieve? $\endgroup$ Mar 17 at 4:51
  • $\begingroup$ The star is a yellow dwarf comparable to the sun. The gas giant orbits at 150 million km. The albedo of the gas giant is 0.42 and the moon is basically Earth, in terms of size, atmosphere, and so on. I'm trying to determine how the gas giant would appear to humans living on the planet, what effects it would have, and in turn how it would be experienced from the surface. $\endgroup$
    – www
    Mar 17 at 4:55
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    $\begingroup$ It appears that you might be seeking an explanation of albedo rather than presenting a worldbuilding problem, as far as I can tell. Could you clarify what you're after? Do you want it to be ridiculously bright, or not. How about heat during giant-up time? - Ridiculously hot? $\endgroup$ Mar 17 at 5:05
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    $\begingroup$ Note however, the angle of your gas giant from the moon's observer point would be pretty huge, 10 degrees (flat, not square) or more, as it depends on the planet's radius, and also its rings, if compared to Saturn's, would take another 3 times more. The planet won't be too bright, that's for sure, but its reflected light would cause enough effects to not call night "night" while it's on the sky. $\endgroup$
    – Vesper
    Mar 18 at 12:21

2 Answers 2

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We have a sun-like star and a planet at earth-like distance from the star, with paper-like albedo. Therefore looking at the planet when there aren't any clouds is like looking at an equivalent solid angle of paper on a sunny day. That would be about the size of a playing card held at arm's length, give or take some. Not harmful.

It would be painful to go from your eyes being fully adjusted to very-low-light conditions to looking straight at it, like suddenly looking at your phone at maximum brightness in the middle of the night. Amateur astronomers would want to wear sunglasses to stare at it for a long time. For overall light level, imagine you're on the couch across the room from a TV screen that is the only source of light in the room, and you look away from the TV at your friend.

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Quick answer: if you are okay with being out on your planet during the day, it should be fine to look at a gas giant.

The gas giant will look as bright as a matt light grey card with an albedo of 0.42 would on the surface of the planet on a clear day. The gas giant would be much bigger in the sky than our moon, and it reflects a lot more than the moon does, so their night may be a hundred times brighter than our night. It will provide a second source of bounced light that fills in the shadows a bit, but we would probably get more of that from the sky.

Ask a photographer who uses a white umbrella. They can show you how this works.

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