Great Circle routes

RunAgroundHard said:
How do you define a straight line, plane or arc?
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I don't but I believe that Euclid did, starting with a straight line being the shortest distance between two points. The trouble started when some idiot invented non-Euclidian geometry, which put a kybosh on the whole subject and resulted in light getting itself bent.
 
Daydream believer said:
Well it is not entirely flat is it. Because there are mountains on the land & obviously waves on the sea.
We are not stupid. :rolleyes: :(:love:
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An amusing stat - if you take the earth and maintain its relative roughness; ie highest peaks to lowest depths of the ocean and shrink the earth down to the size of a “perfect” snooker ball, then the earth will be far far far smoother than a snooker ball could be made.
 
Minerva said:
An amusing stat - if you take the earth and maintain its relative roughness; ie highest peaks to lowest depths of the ocean and shrink the earth down to the size of a “perfect” snooker ball, then the earth will be far far far smoother than a snooker ball could be made.
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Why would you use a snooker ball when a Pizza is more relevant
If you wanted to go 3D you may as well use a banana :rolleyes: :p
 
Minerva said:
An amusing stat - if you take the earth and maintain its relative roughness; ie highest peaks to lowest depths of the ocean and shrink the earth down to the size of a “perfect” snooker ball, then the earth will be far far far smoother than a snooker ball could be made.
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The ellipsoid that is the best fit to the form of the earth differs from a perfect sphere by up to ±11 km - approximately a 600th of the diameter. The geoid (surface of gravitational equipotential; approximately sea level) deviates from that by a maximum of ±100m - mostly a lot less. The maximum relief goes from +8849 m (Mt Everest) to -11034m (Challenger Deep); an amount similar to the deviation because of the ellipsoidal form of the earth. So the deviation from a sphere is up to ABOUT 1/300th of it's diameter. I have no statistics for snooker balls, but I'd guess the earth is less spherical and slightly rougher than a good snooker ball. A snooker ball with a few scratches perhaps comes closer!
 
wonkywinch said:
I don't think you can. It's an illusion where you see a radius of only 3-4 miles so the impression is that it's curved.

I spent many hours staring out the front at 40,000 feet and you can't see curvature at that height. Concorde was different and I was fortunate enough to witness that a couple of times.

To put a plane's altitude in perspective, 40,000 feet is 7.5 miles. Here's a pic of a globe covering the route mentioned above. Picture how far 7.5 miles is on that globe. It's tiny, so a flight is really very close to the surface. The atmosphere is not much thicker so it's like a very thin membrane over the surface of the earth. A scary thought.View attachment 171152
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Thanks for that explanation. It must be a small boat illusion!
 
AntarcticPilot said:
The ellipsoid that is the best fit to the form of the earth differs from a perfect sphere by up to ±11 km - approximately a 600th of the diameter. The geoid (surface of gravitational equipotential; approximately sea level) deviates from that by a maximum of ±100m - mostly a lot less. The maximum relief goes from +8849 m (Mt Everest) to -11034m (Challenger Deep); an amount similar to the deviation because of the ellipsoidal form of the earth. So the deviation from a sphere is up to ABOUT 1/300th of it's diameter. I have no statistics for snooker balls, but I'd guess the earth is less spherical and slightly rougher than a good snooker ball. A snooker ball with a few scratches perhaps comes closer!
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at about 1/300th would mean that a snooker ball could have indentations of about 7 thou. That seems like a rather battered old snooker ball to me!
 
wonkywinch said:
To put a plane's altitude in perspective, 40,000 feet is 7.5 miles. Here's a pic of a globe covering the route mentioned above. Picture how far 7.5 miles is on that globe. It's tiny, so a flight is really very close to the surface. The atmosphere is not much thicker so it's like a very thin membrane over the surface of the earth. A scary thought.
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But surely the atmosphere is vastly higher than 7.5 miles. For a start there would be nothing for the plane's wings to displace to fly on. :unsure: ( & that applies whether you think that it is a snooker ball or a pizza )
 
Daydream believer said:
But surely the atmosphere is vastly higher than 7.5 miles. For a start there would be nothing for the plane's wings to displace to fly on. :unsure: ( & that applies whether you think that it is a snooker ball or a pizza )
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There isn't really a "top" to the atmosphere - it merges with the solar wind at some height.

The definition used for the award of the astronaut label is 100km.

Oxygen is required above about 3000 m. There's an altitude above which we need a pressure suit; not sure how high, but perhaps 10,000m??

Oddly, the atmosphere of Mars extends further from the surface than that of Earth - it's to do with the gravity gradient, so although the Earth's atmosphere is thicker at ground level, it thins more quickly with height than Mars' does.
 
AntarcticPilot said:
Oxygen is required above about 3000 m. There's an altitude above which we need a pressure suit; not sure how high, but perhaps 10,000m??
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There are records ( But please do not ask me to quote them) of early climbers going above 3000M & not using oxygen masks. I am also sure that I read somewhere of a town much higher. Did Top Gear go to it? 4000m high or something? They had to be treated for oxygen starvation & bought down a bit sharppish. Is that correct?
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Or am I dreaming that the earth is the shape of a sphere again?
 
Daydream believer said:
There are records ( But please do not ask me to quote them) of early climbers going above 3000M & not using oxygen masks. I am also sure that I read somewhere of a town much higher. Did Top Gear go to it? 4000m high or something? They had to be treated for oxygen starvation & bought down a bit sharppish. Is that correct?
.
Or am I dreaming that the earth is the shape of a sphere again?
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El Alto airport near La Paz, Bolivia, tops 4k m. A bit frightening the time it takes to a landing plane before coming to a stop.
One has to change plane at some other airport (ex Lima) to take a smaller plane, IIRC a 747 would need a runway many many kilometers long.
Similarly, when the plane is running after having the engine into full power, it takes ages to lift, one keeps thinking "will we ever take off -will we ever take off - will we ever take off
 
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There may be atmosphere above 40,000 feet but there isn't much ability to support life. As said, anything above 10k starts to get difficult so it's still a mere gossamer above the surface, like laying tissue paper on the globe picture above.
 
Jonny A said:
Going Canaries - Caribbean I was forced by southerly (!) winds to start heading almost due west several days earlier than I wanted... at that point the great circle route did still matter.
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Yes but you went north because of the wind, not because of the reduction in distance. You wouldn't go north with regular trades, would you?
I suppose a consequence of the ARC seasonally early start?
 
Daydream believer said:
Did you find breathing difficult after any exercise. ie short walk etc?
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Just a little, indeed there were airport personnel ready with oxygen masks around. Highest I have been was on top of Venezuela highest mountain, well over 5k meters, that was with a cableway of course, I went for a very short walk to a nearby small lake and I felt like having ran a marathon (not that I know about it).
 
Used to fly the 747 into Bogota, 8,700 feet airfield elevation where the walkround was tricky as I sprinted up the jetty steps and up to the flight deck, needed to pop out the crew oxygen masks for a quick whiff to stop the light headedness.
 
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