# Horizon

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 639335.  Sun Nov 22, 2009 3:39 pm ....

639388.  Sun Nov 22, 2009 7:16 pm

 Davini994 wrote: Why would there still be a horizon if the Earth was flat?

Jiggered if I can remember, now - but does this help:
 Quote: In many contexts, especially perspective drawing, the curvature of the earth is typically disregarded and the horizon is considered the theoretical line to which points on any horizontal plane converge (when projected onto the picture plane) as their distance from the observer increases. For observers near the ground the difference between this geometrical horizon (which assumes a perfectly flat, infinite ground plane) and the true horizon (which assumes a spherical Earth surface) is typically imperceptibly small.

?

(s: wiki, of course)

639391.  Sun Nov 22, 2009 8:02 pm

 Davini994 wrote: Why would there still be a horizon if the Earth was flat?

Assuming there to be no optical "bending" or obscuring by the atmosphere, the horizon, even if the plane you were standing on was infinite, would be on your level eyesight line.

If the horizon is where 2 receding parallel lines appear to meet, then one of the lines can be drawn on the plane on which you're standing and the other would be your line of sight. The point at which they appear to meet cannot be higher than your line of sight. If you look up or down, the sight line is not parallel to the ground plane.

To carry the point beyond absurdity: if there were 2 Earths, both flat and both infinite, one being above the other but inverted, the horizon would be where they appeared to meet, since (4+ dimensional space apart) the 2 planes could not intersect.

Well, it made sense when I began to type...

639401.  Mon Nov 23, 2009 2:28 am

Flash wrote:
 Davini994 wrote: Why would there still be a horizon if the Earth was flat?

Jiggered if I can remember, now - but does this help:
 Quote: In many contexts, especially perspective drawing, the curvature of the earth is typically disregarded and the horizon is considered the theoretical line to which points on any horizontal plane converge (when projected onto the picture plane) as their distance from the observer increases. For observers near the ground the difference between this geometrical horizon (which assumes a perfectly flat, infinite ground plane) and the true horizon (which assumes a spherical Earth surface) is typically imperceptibly small.

?

(s: wiki, of course)

Thanks for that Flash. I'm confident then that there isn't a horizon on flat ground, as this is just saying that it's hard to see things that are a long way away with great detail.

639441.  Mon Nov 23, 2009 9:02 am

 bobwilson wrote: If you stood on the surface of a really massive body then the horizon would be more than 3 miles (if it was massive enough, you'd be able to see the back of your head).

Although, if the body was that massive, it'd be very hard to see the back of your own head seeing as how it would have been squished into a small puddle of goo.

639455.  Mon Nov 23, 2009 9:52 am

 Davini994 wrote: I'm confident then that there isn't a horizon on flat ground, as this is just saying that it's hard to see things that are a long way away with great detail.

I guess this is another semantics issue? Looking up "horizon dictionary" on Google, the highest-ranked source gave, among others, these two definitions:

1. the line or circle that forms the apparent boundary between earth and sky.

and

3. the limit or range of perception, knowledge, or the like.

I think (1) would include the infinite flat Earth concept, (3) would not. So, you can pick your preferred definition of the word.

639478.  Mon Nov 23, 2009 11:13 am

dr.bob wrote:
 bobwilson wrote: If you stood on the surface of a really massive body then the horizon would be more than 3 miles (if it was massive enough, you'd be able to see the back of your head).

Although, if the body was that massive, it'd be very hard to see the back of your own head seeing as how it would have been squished into a small puddle of goo.

That's just being pedantic ;)

641586.  Sat Nov 28, 2009 6:08 pm

 Flash wrote: Would this be an interesting Gen Ig (not in the sense of a trick question, but something that people generally won't know)? How far away is the horizon?

Would "46 billion light years" be an acceptable response for QI points. 3 miles may be the furthest you one can see along the Earth's surface from standing at sea level, but 46 billion light years is the furthest distance we can see anything!

641643.  Sun Nov 29, 2009 5:17 am

 gruff5 wrote: I guess this is another semantics issue? Looking up "horizon dictionary" on Google, the highest-ranked source gave, among others, these two definitions: [i]1. the line or circle that forms the apparent boundary between earth and sky.

Not semantics AFAIC; I'm saying that the horizon on flat ground would be infinitely far away.

641704.  Sun Nov 29, 2009 8:57 am

 Davini994 wrote: Not semantics AFAIC; I'm saying that the horizon on flat ground would be infinitely far away.

Which I don't dispute & I agree with, but say you can also describe the elevation of the horizon in our field of view. Thus, the elevation of the horizon of Earth is below the "horizontal" and elevation of the horizon on the Moon is lower still and the elevation of the horizon of flat* ground is at 0 degrees.

* idealised flat ground, not taking into account the 46 billion light year horizon of the real universe alluded to by Spamperial.

641745.  Sun Nov 29, 2009 11:18 am

 Quote: 46 billion light

4.6 surely?

641748.  Sun Nov 29, 2009 11:26 am

bobwilson wrote:
 Quote: 46 billion light

4.6 surely?

Nope, the edge of the observable universe is about 46.5 billion light years away. I'm not totally sure how this works as the universe is only 13.7 billion years old, but I'm told it's something to do with expansion.

951662.  Sun Nov 18, 2012 12:21 pm

Moosh wrote:
bobwilson wrote:
 Quote: 46 billion light

4.6 surely?

Nope, the edge of the observable universe is about 46.5 billion light years away. I'm not totally sure how this works as the universe is only 13.7 billion years old, but I'm told it's something to do with expansion.

Would the figure that it is 46.5 billion light years away be wrong then? Given that the light didn't take 46.5 billion years to travel from there to here? Perhaps we need a new yardstick?

 951680.  Sun Nov 18, 2012 1:53 pm It's because space itself has gotten bigger over time, as Moosh noted there. If we were to limit ourself to the "Minkowski space"* of special relativity, then the distance from the middle to the edge could not possibly be greater than the speed of light times the age of the universe. But if we understand general relativity (which I am certainly not claiming to do) we need not limit ourselves to that. The Universe started as a point, and has steadily blown up like a balloon - the edges get further from the middle all the time. There are hard equations which you can consult if you want to know how much further how quickly. Now, at this point, it becomes very clear that I know one seventh of fuck all about Physics. I start asking questions like "But if the Universe is getting bigger all the time, what is outside it that is being pushed out the way". Apparently you're not allowed to ask that. Accordingly, I'll get out the way and leave the floor free for someone who actually knows what they're talking about. * Minkowski space is a theoretical four dimensional concept of space. Named after a Polish guy who was briefly Einstein's Physics teacher.

 951728.  Mon Nov 19, 2012 1:17 am yes, quite right. The furthest galaxies that Hubble has imaged "look" as though they are about 13.7 billion l.y. away (ie size and brightness-wise), but the objects from which that light has come are now 46 billion l.y. away.

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