Re: Google Earth Plus

Laurence Doering <ljd@xxxxxxxx> wrote:

>On Fri, 26 Aug 2005 15:34:55 +0100, Terry Pinnell <terrypinDELETE@xxxxxxxxxxxxxxxxxxx> wrote:
>> Chris Blunt <chris_blunt@xxxxxxxxxxxxx> wrote:
>>
>>>On Fri, 26 Aug 2005 07:30:00 +0100, Dave Fawthrop
>>><invalid@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>>><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

You're right, my hasty reply to Chris was clearly wrong. Thanks for
the lucid explanation <g>.

--
Terry, West Sussex, UK
.

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