The Moon

[Guest]

The Moon is supposed to be at about 384,400 km from here. So when I look at the Moon outside, i am looking at an object about 384,400 km from where I am standing. I can see some of its features with my eyes.

Now, in Celestia, I place myself 384,400 km away from the Moon and I look. I see a small dot. I see no features at all, just one small dot, barely bigger than the stars around it.

- is that normal? Is that what the astronauts see when they look at the Moon from Earth's orbit?

- If so, what makes us see the Moon so clearly on the ground. Does the atmosphere act like a telescope and lets us see the Moon more clearly then from space at just about the same distance? That doesn't make sense, does it?

[Evil Dr Ganymede]

I think 'Field of view' is the problem here. The default FoV in Celestia is about 45 degrees, which (someone correct me if I'm wrong) is quite a wide-angle field of view. The Moon is about half a degree across, so it looks rather small there.

I don't know what the normal human field of view is, but you might want to fiddle around with it and make it smaller. Shift+left mouse button, drag to increase or decrease the FoV. That should make things look bigger and smaller.

[MrBatman]

I think 'Field of view' is the problem here. The default FoV in Celestia is about 45 degrees, which (someone correct me if I'm wrong) is quite a wide-angle field of view. The Moon is about half a degree across, so it looks rather small there.

I don't know what the normal human field of view is, but you might want to fiddle around with it and make it smaller. Shift+left mouse button, drag to increase or decrease the FoV. That should make things look bigger and smaller.

Same problem gets complained about a lot in Orbiter. Decrease your FOV to around 25-30 degrees, and the Moon should look a bit more normal.

Also, part of what makes the Moon look larger from Earth is optical illusion. Visual cues around you make you think the Moon is a lot larger than it really is. This is especially apparent when the Moon rises or sets. The moon at Moonrise and Moonset is exactly the same size as it is overhead, but you'd swear it was twice the size. In space, with no visual cues to throw you off, the Moon looks much smaller.

[pint]

everyone says that the moon being so big in real life is an illusion. pals! everything you see is an illusion!!! what is the difference between looking to a screen and looking to the sky? same photons entering your eye, same receptors sensing them, same neurons processing ... so if you see any difference, the output on screen is wrong.

in my case, the real geometry is something like 2:3.5 screen width:distance, that means about 30 deg FOV. it is smaller than 45, but still not enough.

i found that, for me, 15-17 deg FOV is makes the most realistic results. i'm curious to know why.

[Guest]

I thought that what I was looking at with Celestia was exactly what I would see I were in space at that very spot looking in that particular direction.

The reason I thought that was that on the celestia home page, there is a link to an HTML document about how to use Celestia. Chapter 6 of that document has that quote:

"The beauty of Celestia is its accuracy. The image in front of you is exactly what you would see if you were really positioned in space at that very spot right now."

So are you saying that what that Celestia document is saying not true? That what we see with Celestia is NOT what we would see if we were really in Space at that place?

[Guest]

Hi,

I have measured my 15" monitor and how far my eyes are from it.

It's 60cm from my eyes and its width is 30cm.

From this I can calculate what is my viewing angle for the monitor:

tan alpha=15/60 --> alpha = 14 degrees

This means, that if I set FOV in celestia to 14 degrees than I should see objects the same size like in real life.

You can measure and calculate your configuration easily.

Hope this helps.

Docens[/img][/b]

[julesstoop]

Part of the explanation, besides the optical illusion, might be the fact that the real moon is infinately more 'sharp' than a moon on a pixellated computerscreen. The real moon's edge never looks jagged; celestia's rendering on standard hardware does noticably, so even with a 20deg fov on a 17 inch monitor @ 1024*768 it's clearly just a 'fake'.

If we were to have a 300 dpi screen, objects would look a lot more real imo.

[Evil Dr Ganymede]

"The beauty of Celestia is its accuracy. The image in front of you is exactly what you would see if you were really positioned in space at that very spot right now."

So are you saying that what that Celestia document is saying not true? That what we see with Celestia is NOT what we would see if we were really in Space at that place?

No.... it should be what you'd see in space. It's just that the field of view might be different to what you expect. That's nothing to do with the accuracy of what's there - the FoV is just a matter of personal aesthetics. I mean, isn't the natural FoV of the human eyes actually more like 120 degrees? If you put that into Celestia you'd get a very distorted view. So it has to be something smaller in the program. If it miffs you that much just change it, it's not like it's a major thing or impossible to do - but it's somewhat hasty to leap to the conclusion that you're being deceived by the documentation.

[fsgregs]

The quote in the document is correct. All celestial objects are positioned and oriented exactly as you would see them. The other forum posts do properly explain that the magnification of the image (the field of view) is an arbitrary one, since Celestia has to go from your 3 dimensional view of the world with binocular vision, and recreate it on a flat screen. As already mentioned, your FOV in your vision is over 120 degrees but your screen is only 15 - 20 inches wide. Something has to give. A 45 degree FOV is a reasonable compromise. To change it, simply hold down the shift key and left-click-drag. A 30 degree field of view will magnify the moon to a reasonable degree, but forces the screen to narrow its view.

Frank

[Guest]

Using guest's formula, I calculated 31 degrees and put that as my FOV and put my screen full screen and I got pretty much the same result. A small dot a little bigger than all the other dots.

If that is what the astronauts in space see when they look at the moon, no problem but is that what the astronauts see or not?

The quote from the documentation says that what we see is what we would see from space, so I expected to see the moon like I would see it if I were really in space.

if that's impossible, then so be it, but no one has said that yet.

[HankR]

The moon, as seen from earth, actually appears pretty small. Its apparent diameter is about 1/2 degree, so if your field of view is set to 31 degrees, then the moon should be about 1/62 the height of your view. You'll also need the correct magnification factor, of course. I believe the default assumes your screen is set to 96 dpi and your viewing distance is 400mm.

- Hank

[jamarsa]

I mean, isn't the natural FoV of the human eyes actually more like 120 degrees?

Yes it is, but when you are focusing an object it becomes really smaller (you discard the rest of your FoV). I think that's part of the reason why you see the moon bigger near the horizon.

And too (as it's said), your monitor is around 15 % of your total FoV...

[Guest]

so what I understand from all this is that it is imposible for a solar system simulator such as Celestia to show us exactly what we would see if we were at that spot in space. that's what the documentation said it did, but it doesn't do it because it is not possible to do it.

I suppose it's the same thing when I look at all the other objects in the program, i don't see them like I would see them if I were really in that position in space looking in that direction

It's ok really, if it's not possible, then it can't be done.

thank you.

[jamarsa]

The 'reality' is always different of how do you 'perceive' it. It's a issue of interpretation. Not only your eyes intervene, your brain too. Celestia tries to model the 'reality'. Didn't happen to you to see yourself in a photo different as how you 'perceive' yourself in a mirror?

[granthutchison]

everyone says that the moon being so big in real life is an illusion. pals! everything you see is an illusion!!! what is the difference between looking to a screen and looking to the sky? same photons entering your eye, same receptors sensing them, same neurons processing ... so if you see any difference, the output on screen is wrong.

No, it turns out that you're wrong - there are a lot more data entering your brain than just those photons. Even though you don't seem to have noticed it, your brain knows very well that you are focussing close up when you look at your screen, and focussing far away when you look at the Moon. It therefore interprets the size differently. (This is a handy software feature, evolutionarily speaking, which allows you tell the difference between a nearby cat and a distant lion.)
Because you are focussing far off when you look at the Moon, and because you see a clutter of landscape features between you and the Moon when it is low on the horizon, your brain says: "Hellfire, that thing's huge" - but when you look at your screen, your brain just sniffs dismissively, because it knows very well that the dot on the screen is only a few decimetres away. This effect is worsened by the relatively low resolution of a computer screen compared to what your eye is capable of resolving. A computer screen just can't fit enough detail into the limited angular diameter of the Moon when it's displayed at an appropriate size, so it only starts to look "right" when you enlarge it.

So Celestia does show you what you would see - but (surprise) only within the limits of the technology.

Grant

[Evil Dr Ganymede]

Speaking of FoV, is there a way in Celestia to just type one in, rather than faff around with drag+shift+left mouse button? I can't get exact, round-numbered FoVs (eg 30 degrees exactly) using the mouse, it's always a few minutes off.

[chris]

Speaking of FoV, is there a way in Celestia to just type one in, rather than faff around with drag+shift+left mouse button? I can't get exact, round-numbered FoVs (eg 30 degrees exactly) using the mouse, it's always a few minutes off.

There's no way to do that right now . . . I'll add it to my list

--Chris

[Evil Dr Ganymede]

Cool! Thanks!

[jamarsa]

Even though you don't seem to have noticed it, your brain knows very well that you are focussing close up when you look at your screen, and focussing far away when you look at the Moon. It therefore interprets the size differently. (This is a handy software feature, evolutionarily speaking, which allows you tell the difference between a nearby cat and a distant lion.)
Because you are focussing far off when you look at the Moon, and because you see a clutter of landscape features between you and the Moon when it is low on the horizon, your brain says: "Hellfire, that thing's huge" - but when you look at your screen, your brain just sniffs dismissively, because it knows very well that the dot on the screen is only a few decimetres away. This effect is worsened by the relatively low resolution of a computer screen compared to what your eye is capable of resolving. A computer screen just can't fit enough detail into the limited angular diameter of the Moon when it's displayed at an appropriate size, so it only starts to look "right" when you enlarge it.

That's really an excellent explanation!! specially the lion part ... and the brain dialogues too!!

[Guest]

i think the trick here when calculating FOV is to assume that your monitor is like a window in a spacecraft. Your FOV may be 120 deg. but only part of that includes the window. If you were to push your face up against the glass as you could in a spaceship then you would see the 120 but you can't do that with your monitor.