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So how much energy does fall 
on the surface of the Earth from the Sun?

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You can work it out with a beautiful simple experiment

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using only thermometer, 
tin full of water, and an umbrella.

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Basically, you let the water heat off in a tin 
to an ambient temperature,

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which here in Death Valley today 
is about 46°C.

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And then you put thermometer in the water

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you take the shade away
and let the Sun shine on the water.

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In direct sunlight 
the water temperature begins to rise.

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By timing how long it takes the Sun to raise 
the water temperature by one degree Celsius,

10
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you can figure out exactly 
how much energy the Sun has delivered 
into the can of water.

11
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And from that, how much energy is delivered 
to a square metre of the surface.

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It turns out on a clear day,
when the Sun is vertically overhead,

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that number is about a kilowatt.

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That's ten a hundred watt bulbs 
can be powered by the Sun's energy

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for every meter squared of the Earth's surface.

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In an audacious leap of imagination,
Herschel used this figure to calculate

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the entire energy given off by the Sun.

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So imagine adding up those kilowatts 
over this entire landscape

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and then imagine, following the Sun's rays,
as they cover the entire surface of the Earth.

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But then, imagine this:

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The Earth is 150 milion kilometres away 
from the Sun.

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So actually, the Sun is radiating the energy out
across a giant sphere

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with the radius of 150 milion kilometres
surrounding our star.

24
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And how much energy does that make?

25
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It's four times π
times the distance to the Sun squared

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which is about three times...

27
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It's four hundred million million million million Watts.

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That is a million times the power compsumption
of the United States

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every year radiated in one second.

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And we worked that out by using some water,
a thermometer, a tin, and an umbrella.

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That's why I love physics.