So how much energy does fall on the surface of the Earth from the Sun?
You can work it out with a beautiful simple experiment
using only thermometer, tin full of water, and an umbrella.
Basically, you let the water heat off in a tin to an ambient temperature,
which here in Death Valley today is about 46°C.
And then you put thermometer in the water
you take the shade away and let the Sun shine on the water.
In direct sunlight the water temperature begins to rise.
By timing how long it takes the Sun to raise the water temperature by one degree Celsius,
you can figure out exactly how much energy the Sun has delivered into the can of water.
And from that, how much energy is delivered to a square metre of the surface.
It turns out on a clear day, when the Sun is vertically overhead,
that number is about a kilowatt.
That's ten a hundred watt bulbs can be powered by the Sun's energy
for every meter squared of the Earth's surface.
In an audacious leap of imagination, Herschel used this figure to calculate
the entire energy given off by the Sun.
So imagine adding up those kilowatts over this entire landscape
and then imagine, following the Sun's rays, as they cover the entire surface of the Earth.
But then, imagine this:
The Earth is 150 milion kilometres away from the Sun.
So actually, the Sun is radiating the energy out across a giant sphere
with the radius of 150 milion kilometres surrounding our star.
And how much energy does that make?
It's four times π times the distance to the Sun squared
which is about three times...
It's four hundred million million million million Watts.
That is a million times the power compsumption of the United States
every year radiated in one second.
And we worked that out by using some water, a thermometer, a tin, and an umbrella.
That's why I love physics.