Science Questions

If heat cannot travel through a vacuum why does the sun feel hot?

Tue, 6th Sep 2016

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Question

Peter Davies asked:

Which way is North when you leave the earth?

Which way is up when you are in space?

If heat cannot travel through a vacuum why does the sun feel hot?

Answer

Kat Arney put this to Caroline Steel...

Caroline - So we kind of mentioned this earlier when we were talking about ways to die in space. So, heat does travel through a vacuum. It just doesnít travel in a normal kind of conduction way that we quite often automatically think of. So for something to conduct like when you put a metal spoon in a saucepan and it gets really hot, you acquire particles as the heat energy is transferred through the particles. But heat can also travel through radiation which is part of the electromagnetic spectrum so waves travel from the Sun to the Earth and heat up the atmosphere. So yeah, heat does travel through space and it probably would give you really bad sunburn if you were stuck out there with no suit or anything.

Kat - Is this why things like you get infrared heating lamps and stuff like that?

Caroline - Thatís also radiation, yeah. So itís a different way of transferring heat. Itís using heat waves rather than sort of transferring heat through collisions of particles.

 

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North is only defined with respect to this planet. Other planets spin and can therefore be said to have north and south poles according to how closely their spin axes are aligned with ours. Interestingly (for those of a curious mind) Southend Airport recently renumbered its runways because the drift of the magnetic north pole had rendered them out of specification. Runway names have two digits that corrrespond to the nearest 10 degree point on the magnetic compass, so "two six" lies between 255 and 265 degrees magnetic. Instrument landing systems and approach charts are maintained to bring you within 5 degrees of the runway centerline at the published final fix point, and it's disconcerting to look up from the compass and see nothing but trees.

Up, in the absence of gravity, is whatever you want it to be. In a rotating spacecraft it is most conveniently the direction towards the spin axis. If you are travelling between planets, it's initially the direction towards your destination.

Obviously, heat can travel through a vacuum. Always best to start with the observation rather than the hypothesis. alancalverd, Thu, 18th Aug 2016

There are multiple ways that heat can travel. Conduction, convection, and radiation are the three most commonly discussed. Conduction and convection are both impossible in a vacuum because they require a medium to conduct or convect. Radiation, however is most efficient in vacuum (nothing to get in the way and absorb, reflect or scatter the radiation).

In most cases that we are intimately familiar with from day-to-day life, radiation is a minor player in heat transfer, because it is really only significant for very hot objects. However, for things that are really hot (the filament of a light bulb, a flame, a glowing coal, the sun etc.) they  glow (both in the visible and infra-red). Ever notice how you can feel the heat of a lamp on your face? That's most likely due to heat transfer by radiation. chiralSPO, Thu, 18th Aug 2016

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