Re: Google Earth Plus

On Fri, 26 Aug 2005 13:27:04 +0800, Chris Blunt <[email hidden]>
wrote:

| This is an old trivia question. The shadow on the ground cast by an
| aircraft is the same size regardless of what height the aircraft is
| flying at.

<pedant mode>
Only if the sun is an *infinite* distance away.

--
Dave Fawthrop <dave hyphenologist co uk>
"Intelligent Design?" my knees say *not*.
"Intelligent Design?" my back says *not*.

Dave Fawthrop wrote...

Quoted message said:

| This is an old trivia question. The shadow on the ground cast by an
| aircraft is the same size regardless of what height the aircraft is
| flying at.

<pedant mode>
Only if the sun is an *infinite* distance away.

<Physicist mode>
.... which of course it IS - to any practical degree of approximation.

David

Dave Fawthrop said:

On Fri, 26 Aug 2005 13:27:04 +0800, Chris Blunt <[email hidden]>
wrote:

| This is an old trivia question. The shadow on the ground cast by an
| aircraft is the same size regardless of what height the aircraft is
| flying at.

<pedant mode>
Only if the sun is an *infinite* distance away.

That would only be true if the source was a single point of light, or
small in relation to the size of object it was illuminating. The sun
is not a single point source of light, its 1.4 million kms in
diameter.

Chris

David Lee said:

Dave Fawthrop wrote...

Quoted message said:

| This is an old trivia question. The shadow on the ground cast by an
| aircraft is the same size regardless of what height the aircraft is
| flying at.

<pedant mode>
Only if the sun is an *infinite* distance away.

<Physicist mode>
... which of course it IS - to any practical degree of approximation.

No it isn't, because it it was it would be an approximate point
source, rather than being a quarter degree across as it is in the sky.

--
Nobby

Nobody Here said:

...rather than being a quarter degree across as it is in the sky.

Half a degree across :-)

--
Alan White
Twenty-eight miles NW of Glasgow, overlooking Loch Goil and Loch Long in Argyll, Scotland.
Web cam and weather:- http://www.windycroft.gt-britain.co.uk/weather/kabcam.htm
Some walks and treks:- http://www.windycroft.gt-britain.co.uk/walks/

Alan White said:

Nobody Here said:

...rather than being a quarter degree across as it is in the sky.

Half a degree across :-)

Ah yes, I obviously meant "across from the middle to the edge" ;-)

--
Nobby

Chris Blunt said:

Dave Fawthrop said:

On Fri, 26 Aug 2005 13:27:04 +0800, Chris Blunt <[email hidden]>
wrote:

| This is an old trivia question. The shadow on the ground cast by an
| aircraft is the same size regardless of what height the aircraft is
| flying at.

<pedant mode>
Only if the sun is an *infinite* distance away.

That would only be true if the source was a single point of light, or
small in relation to the size of object it was illuminating. The sun
is not a single point source of light, its 1.4 million kms in
diameter.

Chris

But it's about 150 million km away! So to all intents and purposes, it
is a single point.

--
Terry Pinnell
Hobbyist, West Sussex, UK

Andy Champ said:

However I still
don't see how you can tell from the double image that the photo was
taken from a satellite,

Hi Andy,

I have the URLs not at hand . . .
.. . . so I say it with my words:

the camera in the satellite doesn't make normal pictures. It uses two
different light frequencies to scan the ground. This scanning runs one
after the other (not at the same part of second). Due to the great
yelocity of the satellite both pictures have to be shifted to match each
other. If an object with great velocity is scanned and shifted, you can
see this "ghost twin", mostly kind of blue. In my GoogleMaps-URL you can
see 4 planes, one plane, one shadow and two "ghost twins".

hth
Anton

Terry Pinnell said:

Chris Blunt said:

Dave Fawthrop said:

<pedant mode>
Only if the sun is an *infinite* distance away.

That would only be true if the source was a single point of light, or
small in relation to the size of object it was illuminating. The sun
is not a single point source of light, its 1.4 million kms in
diameter.

Chris

But it's about 150 million km away! So to all intents and purposes, it
is a single point.

Um, no, not exactly. You can demonstrate this for yourself by going outside
and looking at the sun, and noticing that it appears to be a disk 30 arcminutes
(half a degree) in diameter, about the same size as the nail of your little
finger held at arm's length. Go out again later, when it's dark, and notice
that the stars (which are effectively point sources of light) look much
much smaller than the sun does.

If an object between you and the sun subtends an angle much larger than the
sun it will cast a sharp shadow. If it's close to the same angular size as
the sun (like an airplane flying at an altitude of several thousand feet)
it will cast a diffuse shadow, since it will block only a portion of the
sun's light. If it's an airliner at cruising altitude, it will appear to
be much smaller than the sun, and will not cast a visible shadow on the
ground.

If you can see a sharp shadow of an airplane in a Google Earth image, that
airplane is close to the ground. If you know the sun angle when the image
was taken, you can measure the distance between the image of the airplane
and its shadow and determine its altitude.

ljd

Anton Betzler said:

Hi Andy,

I have the URLs not at hand . . .
. . . so I say it with my words:

the camera in the satellite doesn't make normal pictures. It uses two
different light frequencies to scan the ground. This scanning runs one
after the other (not at the same part of second). Due to the great
yelocity of the satellite both pictures have to be shifted to match each
other. If an object with great velocity is scanned and shifted, you can
see this "ghost twin", mostly kind of blue. In my GoogleMaps-URL you can
see 4 planes, one plane, one shadow and two "ghost twins".

hth
Anton

It does, thanks. Next question: How do you convert 2 images into the
three colours our eyes need?

(I notice BTW on the "application" version of google earth there's a
different image of the airport with no plane, and all the other ones
have moved)

Andy