On 19.02.2024 15:21, Malcolm McLean wrote:

On 19/02/2024 07:58, David Brown wrote:

On 19/02/2024 03:26, Malcolm McLean wrote:

Our maths teacher was Selby Brock (a brilliant man who taught that
the earth was flat),

Confirmed by Terry Pratchett. (How else could earth reside stable
on top of these elephants.) Or was that yet another [flat] planet?

Isn't that somewhat of an oxymoron? I had one primary teacher that
taught that there were 10 planets (Pluto was still called a planet at
the time,

Well, at that time there was at least a hypothesis about a tenth
planet, they named it "Transpluto".

and he counted the sun as a planet) and another that did not
consider the sun to be a star or the earth to be a planet. But these
were /not/ good teachers.

It's conceivable that this person was a good teacher of maths, but
that's different from being a good teacher in general.

Come on. Some respect and some intelligence. If I say he was a brilliant
man I wouldn't say that lightly, and he had very good reasons for
teaching that the Earth was flat.

What were these "very good reasons"? (Maybe personal, maybe religious
reasons, but obviously not good reasons as far as science and reality
is concerned.)

It was nonsense, everyone knew it was
nonsense, but no boy ever actually managed to prove that it was
nonsense, and that was the point.

Frankly, I'm reluctant to stir this up. But I assume by "no boy"
you mean the boys that were in this school?

Long ago, since more than 2200(!) years it had already been proven
that the earth is spherical, and even its diameter was calculated
with good accuracy. This was long before mankind went into earth
orbit to even directly observe the earth as sphere.

But it perfectly justified "Selby always teaches crap".

Janis

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 19/02/2024 16:20, Janis Papanagnou wrote:

Long ago, since more than 2200(!) years it had already been proven
that the earth is spherical, and even its diameter was calculated
with good accuracy. This was long before mankind went into earth
orbit to even directly observe the earth as sphere.

That early calculation of the earth's diameter was accurate by sheer
luck - Eratosthenes made a variety of errors that each lead to huge inaccuracies in the calculations, but by chance happened to cancel each
other out. He gets full credit for his methods, however. Later
astronomers, especially in the Islamic world, measured it more
accurately. And the main point - that the earth is basically round, not
flat - was proven and known to the classical Greeks and others.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 19.02.2024 16:52, David Brown wrote:

On 19/02/2024 16:20, Janis Papanagnou wrote:

Long ago, since more than 2200(!) years it had already been proven
that the earth is spherical, and even its diameter was calculated
with good accuracy. This was long before mankind went into earth
orbit to even directly observe the earth as sphere.

That early calculation of the earth's diameter was accurate by sheer
luck - Eratosthenes made a variety of errors that each lead to huge inaccuracies in the calculations, but by chance happened to cancel each
other out. He gets full credit for his methods, however. Later
astronomers, especially in the Islamic world, measured it more
accurately. And the main point - that the earth is basically round, not
flat - was proven and known to the classical Greeks and others.

(By "sheer luck" - well, okay. Not worth a dispute for me.)

Note also that there was another one (before Eratosthenes with
his experiment) who concluded a spherical earth by other means.
(I just recently heard/read about it but unfortunately forgot
the details.) So it was, anyway, long well known knowledge, and
it's amazing if some folks still deny that.

Janis

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 19.02.2024 18:04, Janis Papanagnou wrote:

Note also that there was another one (before Eratosthenes with
his experiment) who concluded a spherical earth by other means.
(I just recently heard/read about it but unfortunately forgot
the details.) [...]

Just searched for it and found: Aristoteles deducing it from
the shape of the earth's shade on the surface of the moon.

(And some more people, like Pythagoras and Platon, assumed a
spherical earth; but I cannot find any experimentally backed
up reason for that given in the text I found.)

Janis

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical
earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence that the Earth is round?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2024 06:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical
earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence that the Earth is round?

No, you'd still have a horizon if you were standing on an infinite plane
(or a round disc of large but finite size, or many other shapes). The
fact that things get lower over the horizon is proof of a round earth
(or at least a cylindrical one). But in ancient times that was
difficult to see - ships were small and rarely ventured out of sight of
the coast, so the effect was hard to distinguish from the movement of
the waves, and could just be "things get harder to see at a distance".

As Janis said, some of the Greek philosophers realised that the curved
shadow crossing the moon on a lunar eclipse was the earth's shadow, and
that is far more solid evidence of a round earth.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2/20/24 00:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical
earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence that the Earth is round?

No, you would still have a horizon if the Earth were an seriously oblate spheroid, such as a US football. Or even if it was a gently rounded
cube. The existence of the horizon only tells you something about the
shape of the Earth within the distance to the horizon, and tells you
little about the shape of the part beyond the horizon.
The key insight was that lunar eclipses were created by the shadow of
the Earth hitting the Moon's surface. The key geometrical argument is
that a sphere is the only shape that would always cast a shadow with a
circular edge, regardless of the orientation of the Earth. Of course,
they had observations from only a relatively small number of different orientations, all restricted by the Moon's orbital plane, but it seemed
a safe assumption to the Greeks, given their obsession with "perfect"
shapes.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20.02.2024 06:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical
earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence that the Earth is round?

Not per se (as others have already explained), and certainly not from
a static perspective.

But there's a chance when travelling per ship and approach Alexandria
from sea. The light from the lighthouse of Pharos might first be seen,
and when approaching further some lower details of the building or of
the surface. Not sure, though, whether one's eye-sight is sufficient
to recognize the effect, I don't think they had spyglasses back then.

Janis

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 19.02.2024 19:14, Malcolm McLean wrote:

The argument was that the Earth is indeed flat and only appears to be
round because light does not travel in straight lines. It was extremely carefully constructed to be very difficult to disprove, and Selby
claimed that you could only disprove it if you knew the principle of the constancy of the velocity of light. Now it wasn't Selby's invention. It
was his brilliance to realise that he could use that pedogogically to
teach boys how to think.

Assuming you and your teacher respectively refer to the Theory of
Relativity. - I don't think that was "brilliant". - If you'd have
bending effects of the light in presence of a huge gravity field
then you'd notice speed differences of the sunlight while the sun
is wandering across the ecliptic.

Muddying simple geometrical things (the round earth) by introducing unnecessarily and only partly(!) explained other theories or facts
(like the velocity of light) is, "pedagogically", not very clever.
It would certainly had been better if teachers teach facts and not
try (and very likely to fail) to be "brilliant" and make some kids
doubt in centuries' old knowledge that's proved by evidence. (YMMV)

Given that the spherical earth was not only proven but also the
accepted knowledge since long; it's more "amazing" that some folks
without necessity make up absurd theories or thought experiments,
and yet more are willing to accept such nonsense. (Generally said,
not related to your teacher whose ideas I just got by hearsay here.)

Janis

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical
earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence >> that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the
Earth must indeed be round. (The geography teacher taught that). But, as Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2024 18:26, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence
that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the
Earth must indeed be round. (The geography teacher taught that). But, as >>> Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force). That's the stuff lenses (physical and gravitational) are made of :-)

The Earth is not a black-hole, and the atmostphere is not that thick.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence >>> that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the
Earth must indeed be round. (The geography teacher taught that). But, as
Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force). That's the stuff lenses (physical and gravitational) are made of :-)


--
Lew Pitcher
"In Skills We Trust"

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2024-02-20, Lew Pitcher <[email protected]> wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence
that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the
Earth must indeed be round. (The geography teacher taught that). But, as >>> Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force).

Pockets or layers of air at different temperature? That would never
affect a view of the horizon, right?

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @[email protected]

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2024 18:10, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>> earth ...

Isn�t the simple fact of the existence of the horizon sufficient evidence >>> that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the Earth must indeed be round.
(The geography teacher taught that). But, as Selby pointed out, that only holds if we assume that
the light is travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Light can deviate from straight lines when passing through the atmosphere due to passing through
areas of different density. This can be observed when looking over a plain heated by the sun, where
it results in a kind of shimmering of distant objects. (It can also lead to the optical illusion of
water in the distance when there is no water.)

I recall watching a "flat earth" video on YouTube which discussed viewing Chicago(?) across Lake
Michegan, where this was a real factor that needed to be considered in the picture - i.e. either
quite a bit more or less of the Chicago skyscrapers was visible than would be the case if the light
was travelling in straight lines.

Clearly light travels in "mostly straight" lines, but really if someone wants to argue this as
/proof/ that the world is not flat, the onus is on the arguers to /prove/ that the light path is
perfectly straight...


Regards,
Mike.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2024-02-20, Richard Harnden <[email protected]d> wrote:

On 20/02/2024 18:26, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force). >> That's the stuff lenses (physical and gravitational) are made of :-)

The Earth is not a black-hole, and the atmostphere is not that thick.

The atmopshere can make reflective pools of water appear where there are none, rigid objects appear to flutter and steady lights to flicker.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @[email protected]

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 18:30:55 +0000, Richard Harnden wrote:

On 20/02/2024 18:26, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence
that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the >>>> Earth must indeed be round. (The geography teacher taught that). But, as >>>> Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force). >> That's the stuff lenses (physical and gravitational) are made of :-)

The Earth is not a black-hole, and the atmostphere is not that thick.

Neither of those negate my answer to the question "Why wouldn't light
be travelling in a straight line?"

Anyway, have you ever seen a mirage (https://en.wikipedia.org/wiki/Mirage)?


--
Lew Pitcher
"In Skills We Trust"

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2024 18:59, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:30:55 +0000, Richard Harnden wrote:

On 20/02/2024 18:26, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>>>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence
that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the >>>>> Earth must indeed be round. (The geography teacher taught that). But, as >>>>> Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force). >>> That's the stuff lenses (physical and gravitational) are made of :-)

The Earth is not a black-hole, and the atmostphere is not that thick.

Neither of those negate my answer to the question "Why wouldn't light
be travelling in a straight line?"

Anyway, have you ever seen a mirage (https://en.wikipedia.org/wiki/Mirage)?

On a hot road, yeah.

Wouldn't that lead to the conclusion that the earth is surrouned by
water, though? Not that its flat.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2024-02-20, Richard Harnden <[email protected]d> wrote:

On 20/02/2024 18:59, Lew Pitcher wrote:

Anyway, have you ever seen a mirage (https://en.wikipedia.org/wiki/Mirage)? >>

On a hot road, yeah.

Wouldn't that lead to the conclusion that the earth is surrouned by
water, though? Not that its flat.

If you're a conservative, any scientific findings that create any sort
of doubt in anything are used to more deeply dig your heels into
your a belief belief such as that the Earth is flat.

In this situation, mirages are proof that light doesn't travel in
straight lines, therefore observations of a curved horizon being used to suggest that the Earth might be round ridiculed as lunacy, even if the curvature can be confirmed at all sorts of atmospheric conditions,
whereas mirages vary wildly.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @[email protected]

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 19:27:06 +0000, Richard Harnden wrote:

On 20/02/2024 18:59, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:30:55 +0000, Richard Harnden wrote:

On 20/02/2024 18:26, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>>>>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence
that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the >>>>>> Earth must indeed be round. (The geography teacher taught that). But, as >>>>>> Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force). >>>> That's the stuff lenses (physical and gravitational) are made of :-)

The Earth is not a black-hole, and the atmostphere is not that thick.

Neither of those negate my answer to the question "Why wouldn't light
be travelling in a straight line?"

Anyway, have you ever seen a mirage (https://en.wikipedia.org/wiki/Mirage)? >>

On a hot road, yeah.

Wouldn't that lead to the conclusion that the earth is surrouned by
water, though? Not that its flat.

To follow the gist of this thread,

The phenomenon of a mirage shows that, sometimes, light does not travel
in a straight line.

If light does not travel in a straight line, then the curvature of the
horizon may be explained as the effect of light not travelling in a
straight line.

Ergo, the fact that the horizon appears curved is /not/ a proof that
the world is round.


--
Lew Pitcher
"In Skills We Trust"

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 09:00:39 +0100, David Brown wrote:

No, you'd still have a horizon if you were standing on an infinite plane

On an infinite plane, objects moving away from you on the plane would
never disappear over any horizon.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 16:16:29 +0100, Janis Papanagnou wrote:

... If you'd have
bending effects of the light in presence of a huge gravity field then
you'd notice speed differences of the sunlight while the sun is
wandering across the ecliptic.

Velocity differences, yes, speed differences, no. Because the speed of
light is a constant.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 18:30:55 +0000, Richard Harnden wrote:

The Earth is not a black-hole, and the atmostphere is not that thick.

The atmosphere is thick enough that the timings of sunrise and sunset are affected by refraction, by up to several minutes.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 03:27:37 -0500, James Kuyper wrote:

On 2/20/24 00:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical
earth ...

Isn’t the simple fact of the existence of the horizon sufficient
evidence that the Earth is round?

No, you would still have a horizon if the Earth were an seriously oblate spheroid, such as a US football.

All of which come under the definition of “round”, I would have thought.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Tue, 20 Feb 2024 19:39:37 -0000 (UTC), Lew Pitcher wrote:

If light does not travel in a straight line, then the curvature of the horizon may be explained as the effect of light not travelling in a
straight line.

Ergo, the fact that the horizon appears curved is /not/ a proof that the world is round.

But the fact that tall, straight buildings partially over the horizon
still appear straight is, I think proof. Because if the light were curved,
then those buildings would appear curved.

I think.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2/20/24 13:10, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient evidence >>> that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the
Earth must indeed be round. (The geography teacher taught that). But, as
Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

In the real world, two reasons: refraction, and gravity. However, if
Selby were trying to teach skepticism, the important thing is not
whether there are known reasons why light would not travel in straight
lines. The important point is that it must be established that it does
travel in straight lines, it cannot simply be assumed.

You can't prove that light always travels in straight lines, for the
same reason why you can't prove the truth of any other general statement
about the real world. The most you can do is collect enough evidence to
justify a high level of confidence in the truth of such a statement.
In particular, it is possible to collect evidence sufficient to support
an accurate theory about how light travels through space, including the
effects of both refraction and gravity. Such a theory can then be used
to show that a horizon remains inconsistent with a flat earth, even
taking those effects into consideration.

However, the end result of teaching skepticism properly should have been
boys who were competent to explain the evidence that rules out a flat
earth. From Malcolm's own description, none of them were able to do so,
and that makes Selby a bad teacher.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2/20/24 17:38, Lawrence D'Oliveiro wrote:

On Tue, 20 Feb 2024 03:27:37 -0500, James Kuyper wrote:

On 2/20/24 00:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a spherical >>>> earth ...

Isn’t the simple fact of the existence of the horizon sufficient
evidence that the Earth is round?

No, you would still have a horizon if the Earth were an seriously oblate
spheroid, such as a US football.

All of which come under the definition of “round”, I would have thought.

Depends upon the definition. The second definition provided by
Wiktionary.org is "Spherical", with the specific example "The ancient
Egyptian demonstrated that the Earth is round, not flat.". Given the
context, that's the one I assumed you were using.

However, even if you're using the third definition "lacking sharp
angles, having gentle curves", the existence of a horizon does not prove
that. If the Earth did have sharp angles, you would still see a horizon
- wherever there was a sharp angle, there would be many locations from
which that angle was on the horizon.

Even more to the point, as I mentioned before, the horizon you can see
tells you directly only about the shape of the Earth up to the horizon.
It gives you very little information about the shape of the Earth beyond
the horizon - only that it doesn't stick up far enough to become part of
the horizon.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2/20/24 13:51, Kaz Kylheku wrote:

On 2024-02-20, Lew Pitcher <[email protected]> wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

...

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force).

Pockets or layers of air at different temperature? That would never
affect a view of the horizon, right?

Why not? Air density drops rapidly with altitude, causing refraction as
light travels through the air. The net result is that, if the air were perfectly clear and the Earth perfectly spherical, you could see
significantly more of Earth from any given position than you would
expect if you didn't take refraction into effect. You can still see the
setting Sun for many minutes after you might otherwise expect it to
disappear below the horizon, because it's light in bent by refraction
(and, of course, see the rising Sun several minutes earlier for the same reason).

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2024 19:30, Richard Harnden wrote:

On 20/02/2024 18:26, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

On 20/02/2024 10:09, Malcolm McLean wrote:

On 20/02/2024 05:45, Lawrence D'Oliveiro wrote:

On Mon, 19 Feb 2024 18:30:08 +0100, Janis Papanagnou wrote:

(And some more people, like Pythagoras and Platon, assumed a
spherical
earth ...

Isn’t the simple fact of the existence of the horizon sufficient
evidence
that the Earth is round?

The horizon is curved, which boys sometimes advanced as proof that the >>>> Earth must indeed be round. (The geography teacher taught that).
But, as
Selby pointed out, that only holds if we assume that the light is
travelling in straight lines.

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other
force).
That's the stuff lenses (physical and gravitational) are made of :-)

The Earth is not a black-hole, and the atmostphere is not that thick.

Black holes don't bend light any more than any other object with the
same mass. It's just that you can get closer to a black hole without
being inside it. (And gravity does not "bend" light - mass bends
space-time, and the light goes in a straight line in the bent space-time.)

And the atmosphere /is/ that thick. The refractive index of air is
usually negligible, but the earth's atmosphere is big enough that we see
the sun half a degree out at sunset and sunrise.

But refraction needs changes in the density or composition of the air -
and when you are talking about measurements close to the earth's
surface, the refractive index does not change much so there is usually
very little refraction. (How thick the atmosphere is has nothing to do
with it - only differences matter for refraction.)

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2024 23:37, Lawrence D'Oliveiro wrote:

On Tue, 20 Feb 2024 09:00:39 +0100, David Brown wrote:

No, you'd still have a horizon if you were standing on an infinite plane

On an infinite plane, objects moving away from you on the plane would
never disappear over any horizon.

Correct, but you would still see the line of the horizon between the
"land" and the "sky". So it would look the same, but it would not act
the same as objects moved into the distance.

So I guess in your previous post you meant that the existence of a
horizon that things disappear over is proof of a round Earth? (Snipping
posts is helpful - snipping too much context is not.)

If the air were perfectly clear, and people's long-distance eyesight
were perfect, then what you say would be true. But objects "disappear"
with distance even if they don't go over the horizon - it is very
difficult with the naked eye to realise there is a fundamental
difference between a ship disappearing over the horizon, and a bird
flying off out of sight. It was not until people starting building
things like the lighthouse at Alexandria that you could really be sure
of the effect of disappearing over the horizon.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2/21/24 02:52, David Brown wrote:
...

Black holes don't bend light any more than any other object with the
same mass. It's just that you can get closer to a black hole without
being inside it. (And gravity does not "bend" light - mass bends
space-time, and the light goes in a straight line in the bent space-time.)

Not quite. Light follows null geodesics in the curved space-time. A
geodesic is an extension of the idea of straight lines to curved
spacetimes. In flat space time (which implies no gravity), geodesics are straight lines. However, in curved space times, geodesics can be much
more complicated shapes.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20.02.2024 19:26, Lew Pitcher wrote:

On Tue, 20 Feb 2024 18:10:30 +0000, Richard Harnden wrote:

Why wouldn't light be travelling in a straight line?

Refraction, either by a change in medium or gravity (or some other force). That's the stuff lenses (physical and gravitational) are made of :-)

There's an interesting atmospheric condition seen at the Alps (Europe)
that's called "Föhn". Warm winds from the south (Italy) are streaming
over the mountains. At the German side there's cold air and the warm
air streams over that cold air. The different density of the cold/warm
air "bends" the line of sight so that the mountains appear to be much
higher than they effectively are (i.e. as seen on non-Föhn condition).
This refraction effect is depicted in the picture behind the link[*];
blue is the line of sight, and red is the perceived line of sight.

Janis

[*] https://upload.wikimedia.org/wikipedia/commons/thumb/3/35/Foehn_Lichtweg.svg/640px-Foehn_Lichtweg.svg.png

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)