Thread: How far away is the horizon??

Originally Posted by Saitch

Isn't it true that you can see "over" the horizon for a short distance as light rays are affected by the Earths gravity & therefore bend? Probably different on "Discworld"

Not affected by gravity, but affected by refraction through the atmosphere. If I remember right when the apparent sun is just about set the actual sun is already below the horizon, and only visible due to the refraction.
When I was surveying we had to correct observations to stars for refraction, and the amount varies with temp and pressure (these affect atmospheric density)


Martyn

Martyn
I could be horribly wrong here but I was of the opinion & training that light is affected by gravity. I'm in the surveying proffesion & in the 70's when using G8 Geodimeters, which could measure 80 + kilometres, I'm sure there was a formula to calculate curvature of the LASER caused by gravity. Surely light must be affected by gravity otherwise there'd be no black holes.

Martyn and Saitch,

You are both right. Both atmospheric refraction and gravity can influence the final path of light.

Atmospheric refraction as the light travels though different air densities can cause it to bend - hold a pencil in a glass of water and you'll see what I mean. Similarly, spear fishermen standing in the shallows never aim at the fish, they alway aim slightly short of the fish.

In Antarctica I've watched mountains hundreds of miles away that are not possible to see due to the curvature of the earth appear and disappear as I've been standing on the ground. This is due to light being refracted in layers of cold air near the surface. (I've also seen Fisher Massif (a large blocky mountain at the end of the Lambert Glacier, Antarctica) reflected upside down in the sky due to total internal reflection but I digress...)

As for gravity though, light will not not be bent by gravity per se. According to Einstein's theory, space is "bent" by gravity, thus anything travelling in a straight line (light) will appear "bend" relative to a distant observer (but not to the object moving) as it travels through the bent space.

This theory was proved early last century when during a total solar eclipse, light from a star which should have been obscured by the sun's disc (when measured by the apparent distance to other nearby stars) was visible just beside the sun. The light, travelling in a straight line through space bent by the sun appears to be moved.


More recently, Hubble took a fabulous image of the same effect on a massive scale - light from a quasar and a galazy behind a massive black hole object (black holes have gravity so strong it sucks space in on itself) bending space so that the light heading out in slightly different directions is focussed by the "bent" space producing five separate images of them are apparent. Mind blowing stuff.

NASA - Hubble Captures A "Five-Star" Rated Gravitational Lens

Yes, light is affected by gravity as in the prvious post. But the effect is too small to be observed on earth and would not be a factor in surveying, except possibly for some star shots involving light rays passing close to the sun.

John

Good point JD, I am wondering whether even with the best most accurate laser you could still couldn't measure it as both the emitter and the sender are in the same patch of "bent" space-time - i.e. in my diagram above imagine you were on the surface of the sun surveying with sub pico-mm accuracy. the laser beam would still be the same distance from the surface.

I've just read that there was to be a satellite launched to measure the effect from the earth as there's a theory called frame dragging, where as the body rotates, it pulls the curved space-time with it.

My brain hurts.

With all of our equations, theories, and navigational extracts, I defy anyone to swim to it. Or sail to it, for that matter, Bob

Originally Posted by JDNSW

Yes, light is affected by gravity as in the prvious post. But the effect is too small to be observed on earth and would not be a factor in surveying, except possibly for some star shots involving light rays passing close to the sun.

John

Not much of a problem, most stars are too hard to see during the day.


Martyn

Originally Posted by bob10

With all of our equations, theories, and navigational extracts, I defy anyone to swim to it. Or sail to it, for that matter, Bob

Quite right,
By the time i swam to the horizion i think there would not be an earth left