The earth is 149.600.000 km away from the sun and the speed of light is 299 792.458 kilometres / second, therefore, it would take sunlight, on average, 8 minutes and 20 seconds to reach earth.

Now, imagine the following scenario, which I think we can all relate to: you are at the beach and the sun is shining, what a wonderful day! But no, suddenly clouds appear and bye-bye good time, because the sun gets in the clouds. Some time passes, and you can see the sun coming out of the clouds, and the second this happens, you feel the warm embrace of our extraordinary favourite star.

My question is, from what I said in the beginning, sunlight travels to earth in 8 minutes and 20 seconds, however, we immediately feel it when the sun gets out of the clouds, why is that? Because from what I can see, it should take 8 minutes and 20 seconds

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    $\begingroup$ I am suspecting this is a joke/troll question. $\endgroup$ – MichaelK Jan 17 '18 at 11:32
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    $\begingroup$ @MichaelK Why? Sounds like a reasonable question to me. $\endgroup$ – user1583209 Jan 17 '18 at 12:01
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    $\begingroup$ @user1583209 Unless the person believes that the clouds are far out in space, this question is entirely unreasonable. $\endgroup$ – MichaelK Jan 17 '18 at 12:05
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    $\begingroup$ @MichaelK, in fact, that last paragraph would even be logically unsound if the clouds were in fact on the sun instead of on earth... think about it. "Seeing no clouds anymore" is equivalent to "intense light reaches our body", no matter how far the clouds are. Nice little nugget her. I guess we can assume that the OP simply meant why we feel warm immediately when the first unclouded influx of sun appears, and the last paragraph is just a little overlooked misformulation. $\endgroup$ – AnoE Jan 17 '18 at 12:10

Sunlight is hitting earth at all times. If clouds block the sunlight that happens at a height $h$ above earth's surface. If clouds let the sunlight pass, there is already light just beyond the cloud (which started at the sun some 8 min earlier). It is not that the light has to travel all the way from the sun when that happens.

You will see the sun coming out of the clouds, only a time $h/c$ after the clouds have given way to the sun, because in order for you to see, light has to travel from the clouds to you, which it does at the speed of light. Similarly the warmth (being carried by photons/light) has to travel the same distance and is travelling at the same speed.

Or said differently, the warm embrace/heat/energy that you feel travels at the same speed as the image you see with your eyes. That's why it does not matter whether the clouds are at $h=2~km$ or whether the blocking occurs close to the sun.

If the blocking occurred close to the sun, you would see the opening of the blocking ca. 8 minutes after it happened, but you would also feel it 8 min after it happened. Thus you would experience both at the same time.

An analogy:

Imagine a car standing at a tunnel entrance that is blocked. You are standing at the opposite end of the tunnel. Now the road is opened and the car drives through the tunnel towards you. The car reaches you ("warmth") at the same moment that you receive the information that the tunnel is not blocked anymore. It does not matter how long the tunnel is or how fast the car is driving.

  • $\begingroup$ Wrong. If the blocking would occure close to the sun, then it would take roughly 8 minutes for you to see the change. But it is true that if you see it, you also feel it. $\endgroup$ – MaDrung Jan 17 '18 at 10:53
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    $\begingroup$ @MaDrung True, if the blocking would occur close to the sun it would take 8 min for you to see it. But it would also take 8min for you to feel it. The point is that you feel and see it at the same moment. $\endgroup$ – user1583209 Jan 17 '18 at 10:55
  • $\begingroup$ Maybe a water tap would be a better analogy? The moment you open it, the water starts to pour out, because the whole water line between the tap and the pump station is not empty but filled with water already. $\endgroup$ – Joker_vD Jan 17 '18 at 12:31
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    $\begingroup$ @Joker_vD : This would explain why there is light already, but how do you make the analogy to warmth/visible light? $\endgroup$ – user1583209 Jan 17 '18 at 12:34

The answer by user1583209 is correct . Simply radiation from the sun covers a large spectrum of frequencies, of which a small part is the infrared felt as heat on the skin

enter image description here

and all wavelengths travel at the same speed, c. So light and heat are felt/seen at the same time. The sane is true in a total eclipse, when the moon covers the sun. Heat and light happen in coincidence.

In addition to the infrared which directly heats the atoms and molecules of our body, visible when hitting skin is absorbed and the energy degrades to infrared wavelenghts, the same with ultra violet which turn on the melanin of the body, but the energy ends up at the infrared wavelengths.

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    $\begingroup$ +1 for the example of the eclipse and for explaining what that "warm embrace" is. $\endgroup$ – user1583209 Jan 17 '18 at 11:58
  • $\begingroup$ Infrared light does not posses any special power to confer warmth. Anything that absorbs light gets warmer as a result. Also, anything that is warm, radiates light (a.k.a., black body radiation). You don't need visible light to "degrade to infrared" in order to explain why it can warm things up. $\endgroup$ – Solomon Slow Mar 14 '18 at 19:53
  • $\begingroup$ @jameslarge sorry,but what we call "warm" is the infrared wavelengths that add to the lattice vibrations and rotations that our neurons interpret as "hot" . Visible light is atomic transitions that have to be degraded to the lattice level . Another tack: the black body radiation of our body is in the infrared, not the visible. $\endgroup$ – anna v Mar 14 '18 at 20:11
  • $\begingroup$ OK, I was not aware that the lattice vibrations in the interior of some solid body were understood as a form of light. But I guess, if that's true then it conveniently eliminates any need to explain how the energy can be "converted" to black body radiation at the surface. $\endgroup$ – Solomon Slow Mar 14 '18 at 20:50

While the sun is far enough away that there is a time delay between light leaving the sun and hitting the earth, clouds are only about 2-6 kilometres above the earth's surface (depending on the type of cloud). This means that the light is blocked only 2-6 kilometres away from the earth's surface, from which the light only takes a fraction of a second to reach. This means that when a cloud blocks the sun's light, it only takes a fraction of a second for us to notice.

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    $\begingroup$ I think this is wrong. See my answer above. $\endgroup$ – user1583209 Jan 17 '18 at 10:28
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    $\begingroup$ I'm confused at the difference @user1583209 $\endgroup$ – CooperCape Jan 17 '18 at 10:34
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    $\begingroup$ It does not matter whether the light (warmth) needs fraction of seconds because the image (you seeing the clouds recede) takes the same amount of time to reach you. So basically you experience the warmth and the view of receding clouds at the same time, but fractions of seconds after it happened. $\endgroup$ – user1583209 Jan 17 '18 at 10:40
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    $\begingroup$ This doesn't really seem relevant. The reason the heat and light hit you the same is because the heat and light are both travelling as the suns radiation at the same speed. It doesn't matter where the blockage occurs. You feel it the same as you see it in both cases. $\endgroup$ – JMac Jan 17 '18 at 14:11

protected by Qmechanic Jan 17 '18 at 13:56

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