Seeing the curvature of the earth.

[Humblebee]

When I stand on the cliff tops near my house the horizon appears to bend downwards at either visible end. I've also read that ancient mariners knew the earth wasn't flat because they could see it's curvature.
But how can this be? If one was on a small island with the horizon visible through 360 degrees, or on a boat in mid ocean, the horizon would appear flat in all directions - horizontal indeed. So is the apparent curvature when seen from land an illusion? Please, someone, enlighten me.

 

[Woodlouse]

Go and hold a ruler up to the horizon. It's not an illusion.

 

[sarabande]

the phenomenon is related to the height of eye above your nadir.

 

[C08]

I have only noticed marked curvature of the horizon from a boat in the Irish Sea-not quite sure why that would be?

 

[Robin]

I have only noticed marked curvature of the horizon from a boat in the Irish Sea-not quite sure why that would be?

Leprechauns and Paddy's?

 

[pvb]

Surely it's obvious every time you go to sea?

 

[DJE]

Horizon's flat - your eyes are round!

 

[JumbleDuck]

But how can this be? If one was on a small island with the horizon visible through 360 degrees, or on a boat in mid ocean, the horizon would appear flat in all directions - horizontal indeed. So is the apparent curvature when seen from land an illusion? Please, someone, enlighten me.

The horizon is a circle, so it's both flat and curved.

HTH, HAND.

 

[johnalison]

The horizon is only a straight line when you are at water level. Even verticals are curved except when in the centre of your vision.

Edgar Allan Poe wrote a short story about going to the moon in a balloon, during which he conjectured that the occupants of the balloon would experience an optical illusion that the surface of the Earth below them was concave. it is very gratifying that the experience of astronauts has shown him to be correct.

 

[arto]

It's clearly an illusion. The earth is well known to the flat.

If you need convincing, just do a search for "flat earth" on Youtube. The "authorities" or "satanists" or the UN or other conspirators are responsible.

https://www.youtube.com/watch?v=sfMZ9sDXAp0

Amazing.

 

[Humblebee]

Go and hold a ruler up to the horizon. It's not an illusion.

Fair point, Woodlouse, but it doesn't tell me how this happens, or why. I'm still mystified.

 

[Humblebee]

Surely it's obvious every time you go to sea?

Apparent, yes, but not obvious why. Still no wiser.

 

[Humblebee]

The horizon is a circle, so it's both flat and curved.

HTH, HAND.

OK, so it will look to be a straight line when viewed from water level, I see (!) that, and a disc, like the moon, when seen from a distance. But I still can't quite grasp why it looks to be a level line when seen from, say, the mast of a tall ship but a curve when seen from the shore - both beach or cliff height.

 

[JumbleDuck]

But I still can't quite grasp why it looks to be a level line when seen from, say, the mast of a tall ship but a curve when seen from the shore - both beach or cliff height.

You probably just don't have time to contemplate its shape fully when at the masthead.

 

[oldharry]

Visual perception is a complex set of interactions by the brain. Ever noticed how much bigger the full moon is when its rising? Because the brain has the landscape to match it against, while once it is high in the sky its no further away, but the brain perceives it as such so 'sees' it as smaller. I often pondered the curvature phenomenon, and noticed that it is more pronounced if you have reference points each side near the edge of vision, like standing on the shore of a bay, though take a photo and the illusion disappears in the pic. So which is seeing the reality? The brain, which knows the horizon is curved as it must be, or the camera..... which never lies, of course.

Of course, if you are wearing glassess too, then they can bend a straight line too - mine drive me round the bend when I'm doing carpentry, because I cant see a straight line any more. So, is the curvature of the horizon a function of the lens in the eye? Photgraphs would tend to suggest it is. Or is it?

 

[bikedaft]

so what happens when you hold a ruler up to the horizon?

pretty sure i did this in the past and it looked straight

i've always wondered at what point on the way to the moon it looks like a round earth as opposed to a flat disc under you?

or am i daft(!)

 

[mjcoon]

Fair point, Woodlouse, but it doesn't tell me how this happens, or why. I'm still mystified.

Is this because you cannot imagine floating around a circle such as a hoola-hoop. If you view from the centre of the circle (or anywhere within the plane) then you cannot see the curvature. As soon as you leave the plane it is obvious.

Now replace the hoop with a huge sphere (Earth) with a large hole cut out of it (where your boat is) and you will see exactly the same thing.

Mike.

 

[capnsensible]

With a height of eye of 2m, the horizon is 2.9 miles away. Not a lot of people know that. Thanks Reeds.

 

[GHA]

With a height of eye of 2m, the horizon is 2.9 miles away. Not a lot of people know that. Thanks Reeds.

Yeah, right . It must be a lot more than that, spent ages sailing towards the horizon and never reached it yet...

 

[johnalison]

A I mentioned, it's not nly horizons that cause confusion. Artists invariably draw buildings with walls straight within the picture, whereas the only accurate camera, a pinhole camera, will show verticals converging. If you think about it, you will see that only an infinitely short section of a wall level with the eye is seen as straight while the rest gently curves into converging lines. If you are halfway up a traditional skyscraper, this will be observed both upwards and downwards.