Naked Science Forum
General Science => General Science => Topic started by: neilep on 11/11/2007 21:55:29
-
Dearest All Frequent Flyers,
See this aeromobile ?
[ Invalid Attachment ]
Nice isn't it ?...I'm in First Class of course !..(as a stowaway ! [;)])
This is the perfect picture actually because my question is......does an Aeromobile at 35000 feet cast a shadow on a clear day ?...how big would that shadow be ?...I imagine it would be ...'well big '...
How come we don't see airplane shaped great big shadows wooshing all over the place ?
Phew !!...it's hot in here...think I'll open the windowwwwwwwwwwwwwwwwwwwww !!!!!
-
How come we don't see airplane shaped great big shadows wooshing all over the place ?
If you ever see it going over the sun then its shadow is on you -fleetingly.
-
Aircraft that are flying relatively low do cast noticeable shadows but up at 35000 ft the shadow is barely noticeable. If the sun was a point source the shadow would be essentially the same size as the plane and equally visible at all altitudes. The sun's angular diameter is about half a degree, so as the plane gets higher the penumbra (the bit where only part of the sun is obscured expands from the size of the plane at an angle of about half a degree while the umbra the part where the whole of the sun is obscured contracts at an angle of about half a degree. Normally one would only count the umbra as a true shadow so that eventually contracts to zero when the looking up at the plane there is no part of the whole body that is capable of blocking out the sun
half a degree is approximately .01 of a radian so at 35000 feet the extra radius of the Shadow is 35,000 x .01 = 350 feet so the penumbra of the shadow if the plane is the outline of the plane extended by 350 feet all round but as the smallest crosssection of even the largest plane is less than 350ft there is no umbra so the shadow is not really noticeable across the land although sometimes it is possible to notice a brief dimming of the sun when an aircraft at altitude crosses in front of it and you are briefly in the penumbra.
Another feature of this is that even with aircraft zooming all over the place this is a relatively improbable event an aircraft travelling from horizon to horizon draws a line across the sky and this line must cross the half degree area of the sun for a shadow to pass over you just assuming that one considers the number of possible half degree sections in all parallel lines across the sky. there are 360 of them to for every aircraft that passes across the sky near you there is a 1 in 360 chance of the line of its passage passing in front of the sun so most of the planes you see will not pass over the sun and cast a shadow over you.
However most commercial aircraft at altitude are travelling in air lanes and so their tracks are precisely defined and at certain times of the day the shadows will track at predictable locations so it is possible to predict area where it is more probable to experience an aircraft eclipse. conversely there are many areas where the probability is very low.
Mind yo if you live where I do we have military aircraft flying by at speed so close we can wave at the pilots from time to time and they cast spectacular shadows, particularly if we get a convoy of Hercules aircraft practicing hedgehopping manoeuvres!
-
LOL LOL.. That is a good question... I never thought about why it would not.. never paid attention..
Nice answers you guys!
-
SOPHIECENTAUR...THANK YOU !!
IAN !!...well !..phew !!...THANKFULLY dilettantes like myself do not keep people like you away.......