# Do photons lose energy while travelling through space? Or why are planets closer to the sun warmer?

My train of thought was the following:

The Earth orbiting the Sun is at times 5 million kilometers closer to it than others, but this is almost irrelevant to the seasons.

Instead, the temperature difference between seasons is due to the attack angle of the rays, so basically the amount of atmosphere they have to pass through.

Actually, it makes sense, heat comes from the photons that collide with the surface of the earth (and a bit with the atmosphere) and gets reflected, and there's nothing between the earth and the sun that would make a photon lose energy over a 5 million km travel on vacuum. Or is it? (Note I'm not wondering about the possible lose of energy related to the redshift of the expanding universe.)

So why then are the planets closer to the sun warmer? It seems silly, the closer you are to a heat source, the warmer it feels, but that's because of the dispersion of the heat in the medium, right? If there's no medium, what dissipates the energy?

• No, no, no! The amount of atmosphere has little to do with the temperature difference between seasons. See Michael Brown's answer. Jan 3, 2013 at 9:11

The reason being closer to a heat source makes you warmer is the inverse square law. Think of it this way: If you have a $1~\mathrm{m}^2$ piece of material facing the Sun and located at Mercury's orbit, it will be quite hot. What does the shadow of this square look like at Earth's orbit (about $2.5$ times further away than Mercury)? Well, it will be $2.5$ times bigger in both directions, covering about $6~\mathrm{m}^2$. So the same amount of power can be delivered either to $1~\mathrm{m}^2$ on Mercury or to $6~\mathrm{m}^2$ on Earth. Every square meter of Earth gets about $6$ times less Solar power than every square meter on Mercury. The light is not losing energy to the surrounding medium, even if the medium exists.

• This is off-topic, but doesn't light lose in intensity (photons get absorbed and scattered) more or less inside mediums like air, water, glass, exponentially with distance travelled (Beer-Lambert law)? In that case you'd have to take into account that, along with the inverse square law, right? Jan 3, 2013 at 10:34
• @Thomas Well yes, certain wavelengths will diminish in any given medium. At the same time, others will go through rather unimpeded.
– user10851
Jan 3, 2013 at 11:40