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Unformatted text preview: distance
 d
 is
 I
 =
 Ie(de/d)2,
 because
 energy
 flux
 decreases
 as
 the
 inverse
 square
 of
 distance.
 Call
 the
 plant’s
 radius
 R,
 and
 suppose
 that
it
absorbs
a
fraction
α
of
the
incident
sunlight,
reflecting
the
rest
back
into
space.
 The
 planet
 intercepts
 a
 disk
 of
 sunlight
 of
 area
 πR2,
 so
 it
 absorbs
 a
 total
 power
 of
 πR2α
I.

Earth’s
radius
is
about
6400
km.

 
 The
Sun
has
been
shining
for
a
long
time,
but
Earth’s
temperature
 is
roughly
 stable:

The
planet
is
in
a
steady
state.

For
this
to
happen,
 the
absorbed
solar
energy
 must
 get
 reradiated
 back
 to
 space
 as
 fast
 as
 it
 arrives
 (see
 Figure
 1.2). 
 Because
 the
 rate
 at
 which
 a
 body
 radiates
 heat
 depends
 on
 its
 temperature,
 we
 can
 find
 the
 expecte...
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