# Is 'straightness' relative to earths curviture?

1. Feb 8, 2012

### a1993h

I'm new to the forum and physics in general so forgive the stupidity. I was wonderings if, hypothetically, if one had a 'straight' rod 1000 miles long and balances it horizontal to the earth, would the rod be 'straight [i.e. I shape] or curved [i.e. ( shape]. What I mean is: Is what I consider straight relative to the earths curviture or not?

Sorry if this is ridiculously explained..

2. Feb 8, 2012

### James Leighe

"Is what I consider straight relative to the earths curviture or not?"

That's up to you!

But if the rod bends for any reason, it's not defined as straight.

3. Feb 8, 2012

### a1993h

That's true, I guess.

But is what we see as straight actually 0 degrees or is it our distorted perception of reality?
e.g. Hypothetically speaking, if a straight ruler was stretched out for thousands and thousands of miles would it eventually travel around earth and meet itself on the other side or would it continue through the atmosphere and into space.

4. Feb 8, 2012

### Born2bwire

I guess it's how you decide to measure "straight." Less technologically advanced methods like bubble levels and the surface of a liquid rely on the Earth's gravity. So in this way, if you were to determine the straightness of an object over a large distance you would inevitably have the object follow the Earth's curvature since gravity points toward's the Earth's center. However, we now have more sophisticated means of measuring straightness, the least of which would be a laser. While the Earth's gravitational field would still bend the laser, it does so to a very minor degree and so objects built to these kinds of standards would project off the Earth's surface given sufficient size. One could also use a plumb level which would work too but then it would be troublesome to use it for a large object.

5. Feb 8, 2012

### DaveC426913

If you put a laser on the ground and shone it perfectly horizontally, then walked along beside it for a few tens of kilometers, you would indeed come to a point where you could comfortably walk under the beam. Walk far enough and the beam would disappear into space over your head.

6. Feb 8, 2012

### DaveC426913

The plumb level would suffer the same problems as your bubble level or water-level.

7. Feb 8, 2012

### Born2bwire

What I mean is, the plumb works vertically, so one could place the object vertically and you could measure along it that way.

8. Feb 8, 2012

### houseii

it would be troublesome to use it for a large object. http://www.bosin.info/g.gif [Broken]

Last edited by a moderator: May 5, 2017
9. Feb 8, 2012

### DaveC426913

Placing the object vertically has nothing to do with the OP's scenario.

10. Feb 8, 2012

### DaveC426913

Measuring an object so large as to make Earth's curvature visible would make any device troublesome to use. :grumpy:

Last edited by a moderator: May 5, 2017
11. Feb 9, 2012

### Lsos

You said it yourself that the earth is curved. Therefore, if an object would wrap perfectly around it, it must be curved as well.

Indeed, some large structures such as bridges do actually have a curve engineered into them, in order to follow the earth's curvature.

I suggest simply imagining going out far into space and observing the earth and the long rod. You would then clearly see which one is curved and which one is straight.

12. Feb 9, 2012

### Born2bwire

It has to do with first determining whether or not the object is straight. How we determine straightness is how the beam is going to follow the Earth's curvature. Measuring the straightness of the beam in a horizontal position with a device that works off of gravity would give a bend in the beam. However, we can use these devices to true the beam if the beam is measured while it is vertically placed despite how infeasible that might be since then the gravitational force will be pointed along the same direction for the length of the beam. Just like in the same way we can measure the straightness of a building regardless of its height using the plumb bob.

13. Feb 9, 2012

### Studiot

Surveyors make a correction to level lines and theodolite angles, for the curvature of the earth, when sighting betwen stations.
This correction usually incorporated an element for the refraction that also occurs.

14. Feb 9, 2012

### HowlerMonkey

I test cars on nasa's shuttle landing facility and it's runway is said to be compensated for the curvature of the earth meaning it is dead straight and not following the curvature.

15. Feb 9, 2012

### Studiot

Funny you should say that, because I did come across a situation once in a large flowline manufacturing plant where the flowlines has to be dead level not 'dead straight' as you put it.
Unfortunately the constructing contractor set out the bases using a laser.
From one end of the factory to the other there was a difference of about 15mm between my old fashioned spirit levelling and the contractor's laser. I was shown to be correct.
This resulted in much of their work having to be ripped out and recast.

I later worked out the curvature correction for their laser to be at about 12 to 13 mm.

go well

16. Feb 9, 2012

### DaveC426913

None of which has anything to do with the OP's question.

17. Feb 9, 2012

### DaveC426913

I call foul. This makes no sense. Not only is the runway not significantly longer than standard runways but what difference would it make if the runway were dead straight or not? The shuttle is subject to Earth's atmo and gravity. A few cm is silly. What would it accomplish?

How do you know this to be so? 'it's said' implies that it is via rumour.

18. Feb 9, 2012

### Lsos

Just for curiosity, the towers of the Verrazano Narrows Bridge in NY are 1 5⁄8 inches further apart at the top than the bottom, due to the curvature of the earth.

Not sure about the actual span though, which would probably be of more interest...

19. Feb 9, 2012

### davenn

yes exactly.. the straight laser light beam is going to go off tangentally to the curved surface of the earth

Dave

Last edited: Feb 9, 2012
20. Feb 15, 2012

### HowlerMonkey

That runway is over 15,000 feet long which makes it significantly longer than standard runways.

It's actually significantly longer as it has a significant displaced threshold.

I know of at least 1 other runway in florida that was built this way and it has to do with the state of instrument landing systems at the time they were built......some of which were line of sight radio.

I'll ask them next month during a F104 ride.