3 ms·

You're talking about a first-person view, aren't you? But we don't start with that, we start with a third-person view of Earth and then "pan" across the sky...

by guy2221 14y ago

You're talking about a first-person view, aren't you? But we don't start with that, we start with a third-person view of Earth and then "pan" across the sky... So wouldn't panning across the sky from whatever vantage point actually produce that movement? Same as when you point a telescope and pan, the stars move against your view...?

shardling 14y ago
Hmm, I see what you're saying, but that's not what the demo is trying to convey. At one point it says "You're now traveling at [1/5 the speed of light]" -- that would be nonsense if it was conceived as a panning motion. e: Ah, but as someone else points out, the trip must actually exceed the speed of light, so the whole thing is nonsense. The author should recast things the way you describe them, and thus solve multiple problems at once.
- guy2221 14y ago
  Let's get to the bottom of this. It is a panning motion, this much is physically, visually true. Does it still make sense to talk about 'speed of motion'? Now I'm confused. What happens when you pan from the moon to the sun (during a new moon, when they're ostensibly both visible)? If you do it quite quickly you are panning faster than the speed of light? (In the interpretatio: 'if a physical object remained at the center of your scope as you panned, and started at the moon, it would have to move faster than the speed of light, to follow your pan?) So if you pan from one thing to another and they're 1 light-minute away and you take one minute to pan, does it make sense you are 'panning at the speed of light'? For something that leaves one object and goes toward another? What do you think of this?
  - shardling 14y ago
    Any comparison to the speed of light immediately invokes other concepts that wouldn't apply to panning, so it's probably a bad idea. There might be situations where it makes sense to map an angular speed to some sort of absolute speed, but it just doesn't work in this particular example.
    - guy2221 14y ago
      The specific situation where it makes sense to map an angular speed to some sort of absolute speed is if you're told - or have some way of figuring out or knowing - the distance of the camera to the two objects (including if it is very highly zoomed, which it obviously is, from the perspective we are shown). in this sense - if there is an intuitive sense of the distance of the camera and the high level of zoom - it makes sense to speak of an object leaving earth at the velocity that lets it stay in the center of the frame as we pan. doesn't it?
finnh 14y ago
You're right, we do start with a 3rd person view of Earth .. but I still interpreted the motion as translation rather than rotation. By your interpretation, the camera lens is at a fixed point and then simply "swings" from pointing at Earth to point at Mars. But, from such a supposed point, both the Earth and Mars would be fixed points rather than objects with "multi-pixel" width. So the fact that both the Earth & Mars are viewable as non-point objects implies translation rather than rotation... and so GP's gripe stands =) [edit: oh, and what shardling says too]
- guy2221 14y ago
  Okay, it is problematic. If the camera lens is at a fixed point and then swings from pointing at Earth to pointing at Mars, and we imagine how fast something would have to travel leaving Earth to remain at the center of the camera sensor as it pans - isn't the obvious question "how far away are we??" So it doesn't really work. It also doesn't work because at different camera locations the Earth and the Mars would have different relative sizes... I suppose we should state that this will be an equilateral triangle formed between the Earth, Mars, and the Camera, the "height" of the equilateral triangle is x, and that Earth will be so many pixels wide on that camera when zoomed 2000x (or whatever). This interpretation might be specific enough and also match the experience.
- Gravityloss 14y ago
  It is simple. 1. Choose a position where the proportional sizes of the Earth, Moon and Mars are what they are on the page. This is likely far away above the ecliptic (the plane the planets are in). 2. Choose a telescope focal length to set the right scale for the planets. Ie magnification. 3. Pan and imagine there is an object in the ecliptic plane at the center of your field of view. Mention the calculated speed of the object. This all results in a moving star field.

by guy2221 14y ago

shardling 14y ago

Hmm, I see what you're saying, but that's not what the demo is trying to convey. At one point it says "You're now traveling at [1/5 the speed of light]" -- that would be nonsense if it was conceived as a panning motion. e: Ah, but as someone else points out, the trip must actually exceed the speed of light, so the whole thing is nonsense. The author should recast things the way you describe them, and thus solve multiple problems at once.

guy2221 14y ago
Let's get to the bottom of this. It is a panning motion, this much is physically, visually true. Does it still make sense to talk about 'speed of motion'? Now I'm confused. What happens when you pan from the moon to the sun (during a new moon, when they're ostensibly both visible)? If you do it quite quickly you are panning faster than the speed of light? (In the interpretatio: 'if a physical object remained at the center of your scope as you panned, and started at the moon, it would have to move faster than the speed of light, to follow your pan?) So if you pan from one thing to another and they're 1 light-minute away and you take one minute to pan, does it make sense you are 'panning at the speed of light'? For something that leaves one object and goes toward another? What do you think of this?
- shardling 14y ago
  Any comparison to the speed of light immediately invokes other concepts that wouldn't apply to panning, so it's probably a bad idea. There might be situations where it makes sense to map an angular speed to some sort of absolute speed, but it just doesn't work in this particular example.
  - guy2221 14y ago
    The specific situation where it makes sense to map an angular speed to some sort of absolute speed is if you're told - or have some way of figuring out or knowing - the distance of the camera to the two objects (including if it is very highly zoomed, which it obviously is, from the perspective we are shown). in this sense - if there is an intuitive sense of the distance of the camera and the high level of zoom - it makes sense to speak of an object leaving earth at the velocity that lets it stay in the center of the frame as we pan. doesn't it?

finnh 14y ago

You're right, we do start with a 3rd person view of Earth .. but I still interpreted the motion as translation rather than rotation. By your interpretation, the camera lens is at a fixed point and then simply "swings" from pointing at Earth to point at Mars. But, from such a supposed point, both the Earth and Mars would be fixed points rather than objects with "multi-pixel" width. So the fact that both the Earth & Mars are viewable as non-point objects implies translation rather than rotation... and so GP's gripe stands =) [edit: oh, and what shardling says too]

guy2221 14y ago
Okay, it is problematic. If the camera lens is at a fixed point and then swings from pointing at Earth to pointing at Mars, and we imagine how fast something would have to travel leaving Earth to remain at the center of the camera sensor as it pans - isn't the obvious question "how far away are we??" So it doesn't really work. It also doesn't work because at different camera locations the Earth and the Mars would have different relative sizes... I suppose we should state that this will be an equilateral triangle formed between the Earth, Mars, and the Camera, the "height" of the equilateral triangle is x, and that Earth will be so many pixels wide on that camera when zoomed 2000x (or whatever). This interpretation might be specific enough and also match the experience.
Gravityloss 14y ago
It is simple. 1. Choose a position where the proportional sizes of the Earth, Moon and Mars are what they are on the page. This is likely far away above the ecliptic (the plane the planets are in). 2. Choose a telescope focal length to set the right scale for the planets. Ie magnification. 3. Pan and imagine there is an object in the ecliptic plane at the center of your field of view. Mention the calculated speed of the object. This all results in a moving star field.