How/why is space between the sun and the earth so cold, when we can feel heat coming from the sun?


How/why is space between the sun and the earth so cold, when we can feel heat coming from the sun?

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You need to distinguish between heat and temperature to understand this. The sun radiates energy, but it doesn’t contribute to an increase in temperature until it hits something and warms that something up. Because there is nothing in between the sun and the earth, it is “cold”. But if you were floating there, with the sun full on you, it would heat you up. They need to put reflective foil on spacecraft for this reason. In the shade of something, your body would radiate heat, which would make you very cold.

The transfer of heat is either convection, conduction or radiation. The first two require some medium. The vaccum of space is the absence of a medium. That leaves radiation. Electromagnetic waves heat an object with their energy.

Since there are no(or in reality very few) particles to heat up in the vaccum of space, it cant heat up.

Space isn’t cold.

Space isn’t hot, either.

Space is a vacuum, and since temperature is a property of matter, a vacuum doesn’t have a measurable temperature.

Well, it sort of does, since it’s not a perfect vacuum–there’s some matter floating around, but it’s so little that its temperature doesn’t matter.

If you were floating in space between the earth and the sun, you would have to worry about overheating, not freezing. The thing you see in sci fi sometimes where people insta-freeze in space is pure fiction. Even when not in direct sunlight, you’d cool down fairly slowly, since there’s no air to carry the heat away from you.

Space doesn’t have a temperature in any meaningful sense

The space is not cold, there is nothing to get warm or cold.

Until you get to Earth, which is something to get warm, from energy from the sun.

So, the reason you might think the space between the Earth and Sun is cold is because it’s so very empty. For you to feel hot or cold, usually, it’s all the little stuff around you, mostly air molecules, bumping into you and either giving you energy (warmth) or taking some energy away (cold.) So, when there’s absolutely nothing around, there’s nothing bumping into you to help insulate you (creating a buffer so that you don’t heat up really fast or lose too much energy too fast.) So you’re only losing heat through the same process by which the heat of the Sun travels, via radiation. If you were to be facing the sun in space, the front of you would start to get very, very warm very quickly, as all of that radiation is now being blasted directly onto you (you’ll need some really high SPF for this haha.) At the same time, your back would being to feel very, very cold, as you’ll be radiating your own energy with nothing returning it. So, in actuality, the space between the Sun and Earth isn’t hot nor cold, it’s just full of radiation. Until you introduce something that can absorb that radiation, then you can see the temperature gradient that forms on the matter that you’ve introduced due to the amount of radiation received (the front receiving the most, the back receiving almost none, and the sides receiving a little. This is the reason we have our seasons btw, more direct and less direct radiation due to the tilt in our axis. As you may have heard before, we’re actually farthest away from the Sun in our orbit during spring in the Norther Hemisphere.)

I learned this from Robert Heinlein in “Have Spacesuit, Will Travel” in the 1980’s: Vacuum is NOT cold. Vacuum is NOT hot. Vacuum has no temperature.

If vacuum was cold then a thermos couldn’t keep soup hot.

Vacuum is simply the absence of molecules. Because there are no molecules, heat cannot travel by many of the means you are used to.



conduction, which is something touching something else to allow heat to transfer,


convection, which is really a special case of conduction because it’s just heat transfer to a fluid which then moves and carries molecules around so they can do more conduction.

The heat you almost always experience is actually one of these two types. Heat is really just vibration of molecules.

Radiant heat, that comes in the form of light, is just part of the electromagnetic spectrum (it’s light, whether you can see it or not) which interacts with matter and causes it to vibrate. That vibration is heat, and it is then transferred by the methods described above.

Because there is nothing in between to heat up. Radiation is not “hot” it has not a temperature. Radiation hits matter, absorbs it, matter heats up, radiates heat away again.

There’s a lot to unpack there.

Space isn’t really either cold or hot. It’s … nothing much. It’s mostly vacuum, so there’s not really a single temperature there. (That’s why thermoses have vacuum between the inside part and the outside part — so heat won’t conduct out to cool off Daddy’s coffee, or in to warm up your milk).

Space can feel cold, because you radiate heat all the time. When you stick toast in the toaster and push down the knob, the wires in the toaster get hot. They radiate reddish light, which is why they look red. But they also radiate infrared — a kind of light that’s too red to see. The light and the infrared carry heat energy. They carry enough heat energy to toast your toast! You are warm, so you always radiate a lot of energy outward as infrared light, too. Right now, you’re surrounded by a warm room that is radiating infrared light back in to you, which balances out the light you’re radiating out. That keeps you comfortable. In deep space, nothing radiates back at you very strongly, so you can cool off quite quickly unless you have special clothing on to prevent that. (Silvery things, like a suit made out of tinfoil, work great for that, because they don’t glow very well in infrared.)

Space can also feel hot if you’re near the Earth. That’s because the Sun radiates a lot of sunlight onto you, and the sunlight can warm you up. That’s part of why the Earth is warm so we can live here. The outermost part of the Earth (the upper layers of the atmosphere) settles down to about 0 degrees Centigrade. We feel warmer than that because of the “greenhouse effect”. Our air lets in sunlight, which heats up the Earth, but air also blocks in infrared light, which keeps the ground from cooling off very well. So the outer part of the Earth’s atmosphere settles down to about 0 degrees Centigrade, but the ground is quite a bit warmer than that, on average.

Space near the Earth is also full of very, very thin gas that is very, very, very hot. In interplanetary space, there are about 50 atoms in each teaspoon. Near you, right now, there about 500,000,000,000,000,000,000 molecules of air in each teaspoon. So space is pretty close to empty. But the atoms that *are* there act like a gas, and that gas is at about 100,000 degrees. Astronauts don’t get burned by it, because there’s just not very much of it — so it doesn’t hold very much heat. Sort of like how you can stick your hand inside a 350 degree oven for a few seconds and feel fine, but if you put your hand in 120 degree water it will feel scalding hot instantly. The hot water dumps a lot more heat into your hand than the much hotter air in the oven does.

So there are at least three different temperatures in space, all at once: -270 degrees centigrade, which is the temperature of the “room” around you if you block out the Sun and Earth; about 0 degrees centigrade, which is the temperature a basketball would reach if it were floating around in space near the Earth, from sunlight landing on it; and about 100,000 degrees centigrade, which is the temperature of the material in outer space.