What is it that actually heats the Earth?

Most of us would answer: the heat of the sun.

• And yet the temperature at its surface does not exceed 6000 °C.
• It also sits 150 million kilometres away, across a vacuum, and a vacuum is an excellent insulator.

So the explanation has to lie somewhere else.

You will have noticed it in summer: with the sun overhead on a cloudless day, the sand is twice as hot as the air, even though the sun delivers the same energy to both.

How is that possible?

The sun's rays carry a form of light we call infrared. Light passes through the air around us, but it is stopped by matter, by sand or by skin, which absorbs it entirely.

These packets of energy interact very little with air. They excite the molecules and atoms that make up matter, and that agitation is what we feel as heat.

For most people, thermal comfort is a question of air temperature and of how that air is circulated.

Taking the sun as the model, two things follow:

1. heating matter uses half the energy of heating air,
2. and emitting infrared light from the ceiling warms the skin directly, with nothing in the way.

A radiant ceiling is the form of direct heating that comes closest to reproducing what the sun does.