0:00:00.264,0:00:04.463
So how much energy does fall [br]on the surface of the Earth from the Sun?

0:00:04.843,0:00:07.458
You can work it out with a beautiful simple experiment

0:00:07.498,0:00:11.877
using only thermometer, [br]tin full of water, and an umbrella.

0:00:12.143,0:00:15.909
Basically, you let the water heat off in a tin [br]to an ambient temperature,

0:00:16.009,0:00:20.943
which here in Death Valley today [br]is about 46°C.

0:00:21.020,0:00:25.138
And then you put thermometer in the water

0:00:25.138,0:00:30.722
you take the shade away[br]and let the Sun shine on the water.

0:00:32.664,0:00:36.693
In direct sunlight [br]the water temperature begins to rise.

0:00:36.705,0:00:42.608
By timing how long it takes the Sun to raise [br]the water temperature by one degree Celsius,

0:00:42.608,0:00:48.409
you can figure out exactly [br]how much energy the Sun has delivered [br]into the can of water.

0:00:48.424,0:00:53.207
And from that, how much energy is delivered [br]to a square metre of the surface.

0:00:53.843,0:00:57.049
It turns out on a clear day,[br]when the Sun is vertically overhead,

0:00:57.049,0:00:59.120
that number is about a kilowatt.

0:00:59.120,0:01:03.971
That's ten a hundred watt bulbs [br]can be powered by the Sun's energy

0:01:03.971,0:01:06.470
for every meter squared of the Earth's surface.

0:01:11.826,0:01:16.017
In an audacious leap of imagination,[br]Herschel used this figure to calculate

0:01:16.063,0:01:19.681
the entire energy given off by the Sun.

0:01:23.376,0:01:28.129
So imagine adding up those kilowatts [br]over this entire landscape

0:01:32.169,0:01:38.338
and then imagine, following the Sun's rays,[br]as they cover the entire surface of the Earth.

0:01:43.265,0:01:45.089
But then, imagine this:

0:01:45.189,0:01:48.632
The Earth is 150 milion kilometres away [br]from the Sun.

0:01:48.648,0:01:55.322
So actually, the Sun is radiating the energy out[br]across a giant sphere

0:01:55.322,0:02:00.567
with the radius of 150 milion kilometres[br]surrounding our star.

0:02:01.117,0:02:02.788
And how much energy does that make?

0:02:02.926,0:02:07.796
It's four times π[br]times the distance to the Sun squared

0:02:07.906,0:02:11.142
which is about three times...

0:02:14.727,0:02:20.447
It's four hundred million million million million Watts.

0:02:26.177,0:02:31.017
That is a million times the power compsumption[br]of the United States

0:02:31.090,0:02:35.010
every year radiated in one second.

0:02:35.094,0:02:43.334
And we worked that out by using some water,[br]a thermometer, a tin, and an umbrella.

0:02:44.488,0:02:46.588
That's why I love physics.