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Do you really know what you know?

Here is a suggestion for an interview question.  It is not a trick or a problem to solve, it is simply asking the interviewee to demonstrate the way that they can form arguments and evaluate the strength of those arguments.

How do you personally know that the Earth is (roughly) a sphere? Prove it based on your own observations, not what you have read in books, seen on TV, or been told by teachers or others.

Is your "proof" really a proof or just an argument?  Are you really proving that it is a sphere or just not flat?  What experiments could be done to really prove it?  What are the most practical experiments?

Jason Robbins
Thursday, March 24, 2005

I like that.  It's open enough to encourage logical thinking and discussion to see if the person is smart.  Then at the end you could tell them to prove it, to make sure that they get things done :)

Michael H. Pryor
Fog Creek Software
Friday, March 25, 2005

I tried figuring out the shape of our planet, and started walking, and sailing on the oceans. I never saw the end of my journey. But at last one day I reached back to the point where I started. So the possible shapes of earth are sphere, ellipsoid, or any such curvy shapes. So take this as initial step of my experiment, and start working on step-2. Is there anyone that could work on step-2 and pour in your thoughts?

Friday, April 1, 2005

balaji, you are wrong,

it is a torus (a donut)

Peter Miehle
Saturday, April 2, 2005

Donut..possible...It;s another possibility..That does not mean I'm wrong..

Saturday, April 2, 2005

What a dump question. What is to argue about ? Take any book of elementary physics, and you will find several simple experiments that will prove (not argue, prove !) that our planet is sphere (not just "not flat").

Monday, April 4, 2005

I doubt there`s a physical experiment that will prove that the planet is a sphere since it is in fact not a sphere but a rather an egg-shaped ellipsoid

Sunday, April 10, 2005

An experiment that you could conduct by your own, without conplicated equipment will prove that it is sphere. Using triangulation with presize measurment can show that it's not perfect sphere. Only measurements performed on very long distances with presize equipment will show egg-formed ellipsoid

Wednesday, April 13, 2005

would you entertain people who talk of surface tension with analogy of drops of water.

what about: i see the sun is a circle, the moon is a circle, so the earth must be a circle, and if it is three dimensional, it must be a sphere (rough or perfect is moot).

what about: when i call people far away they report a different time from my time.  when i thought about it, i figured out that the earth must be a sphere.

what about: for the gravity pull to be equal all over the planet, it must be a sphere. if it was any other shape, we would be able to jump off the planet from the edges.

what about: it is not possible to casually prove that the earth is any particular shape.  we will need to study the stars over a number of years to conclusively prove anything.

i guess the last answer the most correct

Wednesday, April 13, 2005


Walking until you reach where you started from is possible on pretty much any shape except a line.  Just walk in a circle, or a triangel, or a square.  Is a line a shape?


Ed Tignor
Saturday, April 23, 2005

I dont think a lot of these answers meet the criteria of the question :

  "How do you personally know that the Earth is (roughly) a sphere? Prove it based on your *own observations*.."

you guys been walking circles around the earth lately?  you might as well just say "well, when i get into a spaceship and leave earth's orbit i can clearly see it's a sphere".

This is a really hard question.  The closest I can say is that on a very clear day, I can only see the tops of tall mountains beyond the distant horizon and I know those mountains aren't in a valley but topographical elevation differences could still influence this view but from personal observations this is the only indication I've experienced.

johnny bucyk
Friday, May 13, 2005

I agree.. I cannot think of any personal observations either...

Something on similar lines is this - if you have ever been to the seaside and observed ships or boats near the horizon, initially you can only see the fag end of the boat's mast or sail.. As the boat moves closer to land, you can see more and more of it..

While this clearly proves that the earth is not flat (i.e. otherwise, the boat would have been visible throughout and just grown in size as it got closer), it still is not conclusive evidence for the fact that the earth is a sphere (or ellipsoidal)...

Thursday, May 26, 2005

How about using Occam's Razor?

If we measure the angle between Polaris north star and the horizon (which we shall call our latitude) and the period precession of a Foucault pendulum (driven by the coriolis force), we will find a curious correlation between our latitude and and the coriolis force. Namely a faster procession as the north star is higher above the horizon. The precise numerical relationship between coriolis force and latitude is most simply explained by the earth as an approximate sphere of radius 6400Km rotating about its axis once every sidereal day (23 hours 55 minutes for a star to make a circuit around polaris).

We can also use Eratosthenes calculation of the circumference of the earth by measuring the angle of a shadow at high noon on two separate points one of which is a fair distance (say 500 miles) north of the other (you could get a friend to help with this).

The 6400km radius sphere model then becomes the simplest way of explaining both experiments without recourse to unexplained coincidence.

Friday, May 27, 2005

its not sphere or global or flat its form of just lik that means that some bushy some cloudy some some thing just lik that that that

Tuesday, May 31, 2005

we can say wat ever we think so lik lik sphere means sphere flat means flat wat ever we think just lik we thinlk gud its gud only bad means bad only lkay

Tuesday, May 31, 2005

Not quite sure what sivan's saying - nor do i know a good answer to this question, but can I just make a quick point about Occam's Razor?

Whilst it's an excellent ontological tool (that is, it helps you decide whether or not to posit *types* of entities, e.g if we need to speak of numbers as entities in order to talk about stuff then we can (and should) think of numbers as existing in some philosophical sense), it is clearly no use in a practical proof. e.g. if I come into my house and see my stereo missing then a practical application of Occam's Razor would force me to deduce that someone i already know has nicked it. This is obviously not always going to be the case.

Equally if you find a model of the universe which fits with all the data you have, it does not mean you have proved that this model is correct (unless you are a philosophical idealist who beleives there is not external reality in which case i guess occam's razor can be used freely).

William of Occam clearly didn't mean his tool to be used like this, but it is often mis-applied - c.f. "The Curious Incident of The Dog In The Nighttime", which is an otherwise excellent book.

In short, Occam's Razor proscribes "economy of ontology", not closed-mindedness.

Chris Harris
Tuesday, May 31, 2005

if i were asked this question in an interview the following  would have been my answer ?

How do you personally know that the Earth is (roughly) a sphere? Prove it based on your own observations, not what you have read in books, seen on TV, or been told by teachers or others.

if we travel around the world we see the gradual change of day into night or the vice versa. In otherwords the change from day to night is gradual...this we know from high school science is because the earth rotates.  if the earths shape were anything other than a sphere say a  cube or cuboid (flat oobjects) then the change would not be gradual, infact at the edges  it would be sudden. this day-night change or a gradual change in light sensivity (increase or decrease - depending on direction ) which can be observed even when we travel in air (flight journey) gives a sense of feeling that earth is spherical .

Is your "proof" really a proof or just an argument?

it is not really a proof, may be i can call it a argument

Are you really proving that it is a sphere or just not flat? 

I am not really trying to prove here but may be trying to analyse why earth should be a sphere

What experiments could be done to really prove it?  What are the most practical experiments?

i do not know of practical experiments but read this in one of the web pages

The sphericity of the earth is proved by the evidence of ... lunar eclipses," Aristotle says. "For whereas in the monthly phases of the moon the segments are of all sorts--straight, gibbous [convex], crescent--in eclipses the dividing line is always rounded. Consequently, if the eclipse is due to the interposition of the earth, the rounded line results from its spherical shape" Of course a frisbee, properly angled, would make a round shadow too. But if the frisbee rotated while the eclipse was in progress, the curvature of its shadow would change. The earth's does not.

Wednesday, June 15, 2005

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