Expert: Philip Stahl - 11/11/2004

Question
Dear Mr. Stahl,
Without referring to any scientific text, how do I know that the
earth revolves around the sun?  Can you prove this to me?

Thanks,
Yasir

Answer
Hello.

First a word on scientific process, "knowing" and "proof". In scientific inquiry, there is actually very little attention to what is called "proof" - we tend to leave that to the mathematicians!

Solutions of scientific problems are instead assessed for adequacy, that is, in respect to the extent to which the observations - evidence meet reasonable demands.

To fix ideas, I may present you with something that constitutes adequate evidence of what you seek - but from your frame of reference (for whatever reason) it may be declared "inadequate". Fine, then I can dig deeper and provide more detailed - beyond adequate- material.

"Proof" is another thing entirely. Nothing in science, really is proven for all times and conditions. And this is what is assumed when one mentions or demands "proof", e.g. an ironclad, irrefutable, implacable and seamless web of observations or evidence, not subject to exceptions or variations.

But in twenty -five years of research experience I've seen nothing like that.

So, it is doubtful that I can "prove" to you that the Earth revolves around the Sun, irrespective of what I say. (Since you may always find or think of some loophole wherein the tended "proof" falls or fails in one way or other).

Generally, demonstration is limited by a set of assumptions, and also by acceptance of what given observations mean. If you therefore reject either or both of these, it's doubtful that whatever I provide will be found satisfactory.

First, one must assume that: Earth is a planet and not some confection of the mind, or "virtual entity" within we all find ourselves. In other words, there exists an objective and independent reality.

Also, one must assume that whatever I can tell you that I can see, or observe, you can also. If you are blind, for example, even partially, this assumption fails and what I provide is useless.

Third, we must assume that the language I am referencing is also understood by you to mean the same thing. Else, all bets are off.

In terms of the observations, it must be clear that what I describe is reasonable to you, and moreoever can be confirmed and duplicated where you are. It must also be at least approximately true, that the meaning of the observations as I interpret them, is also shared by you.

If any of these breaks down, then what I tender will be dismissed.

That out of the way, let's get to specifics. If the Earth revolves around the Sun - and is not static in space - then it must be true that over the course of a year say, we observe differences - for example in:

i) the stars that appear at the same time in the night sky

ii) the altitude and azimuth (position with respect to the horizon's N, S points) of the Sun.

The first is easily verified, say over the course of obseving the night sky at the same time (say, 8 p.m. local time) each night. You will therefore see a procession of different stars, objects as time goes by.

This is the first indicator that Earth must be moving through space and not stationary.

A further observation to reinforce this is *revolution* and not merely linear displacement is obtained by repeating said observations *year after year* and making notes of the objects seen.

In the same 6-month period, therefore, you ought to see the same objects in the night sky at the same time.

This implies repetitive motion, and hence that the Earth is not merely linearly moving in space, but returning time and again to the same relative position in space (e.g. in it s orbit)

Second, the position, altitude of the Sun. If you do the same thing for the Sun, you will note its changing positions both in altitude - at specific calendar dates - and its rising (and setting) positions along the horizon.

Thus, it ought to be obvious - again, say over a 6-month period of observation - that these are changing.

Again, if you repeat them *year after year* you will see the exact same positions duplicated, suggesting that the Earth is returning to its same position in space relative to the Sun. (The seasons, of course, are also indicators of this)

Of course, you can refine measurements - say of the Sun's changing altitude - through the use of an instrument like an astrolabe (which can easily be made using a lead bob attached to a string, and affixed to a wooden or cardboard protractor).

Hopefully, this will help you to see how we know the Earth revolves around the Sun - even though I do not claim it is "proof".