Science Questions

How long does it take for heat to reach us from the sun?

Sun, 22nd Feb 2009

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Phil Kenyan asked:

How long does it take for heat to reach us from the sun? It’s about 8 minutes, the distance between the sun and the Earth because light travels at about 1 billion km/h and the sun is about 150 million km away from the Earth.


Chris - The answer is that heat arrives in the form of light because it’s in the form of infra-red radiation, largely, that’s reaching us from the sun.  It’s reaching us in the form of radiation so that too takes 8 minutes to get here.


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Phil Kenyon asked the Naked Scientists: Hi Chris,   I enjoy your section in the Redi Direko show on Cape Talk and wondered if you could discuss the following? I was thinking about what would happen if the sun suddenly 'went-out'!   Would we have 8.5 minutes of sunlight and 14 years of heat? 299792 km/s - Speed of light 152000000 km to sun 507.02 seconds for sunlight to arrive 8.45 minutes for sunlight to arrive 0.34 km/s speed of sound 447058824 seconds for sound to arrive Therefore 14 years for sound to arrive Speed of sound = speed of heat Therefore it takes 14 years for heat to arrive? Regards, Phil Kenyon (Somerset West) What do you think? Phil Kenyon , Mon, 12th Jan 2009

Why?  Chemistry4me, Mon, 12th Jan 2009

Sound doesn't travel through a vacuum so 'heat' wouldn't get to us that way.
The 'heat' transfer you refer to is what can happen in gases liquids and solids - molecules vibrating and bumping into each other. The speed of sound in the medium affects the speed of heat conduction but:
The heat which arrives from the Sun is in the form of Infra red radiation (another wavelentgth of electomagnetic wave). (Same speed as light). lyner, Mon, 12th Jan 2009

As the sun flickers out there will be no more photons coming our way.
When those photons stop our 'light' goes out.
That will create a rapid cooling.

The only heat will be from our Earth's core.
That will not be enough to warm the surface and the air.
All the water falls out from the atmosphere.
And I suppose the air will break down into a supercooled gas?

very few organisms will survive this.
Perhaps inside the earth?
How fast Earth will cool depends on how much warmth it has before.
And how fast that heat will dissipate.
I would guess the planet would be dead in a couple of days.

As for heat in air?
You lost me there. yor_on, Mon, 12th Jan 2009

Sorry for being nitpicking, heat is in the form of all the electromagnetic radiation. lightarrow, Mon, 12th Jan 2009

Feel free to pick all you like, my friend. I was being deliberately sloppy in an attempt to seem more approachable. I shall avoid it in future. :-)

But I could nitpick back and say that em radiation is not heat (i.e. internal energy) at all but electric and magnetic energy (or even the dreaded photons). Heat is a term which is used very sloppily. lyner, Mon, 12th Jan 2009

Hmmm...yes, but, wouldn't you define heat as those energy exchanged between two bodies by virtue of a temperature difference of them? Just thinking. lightarrow, Mon, 12th Jan 2009

How about this definition of what would happen to our Earth if it cooled down by an absence of Sun :)

"The atmosphere on earth is about 10 km thick. Ten kilometer is how far atoms at room temperature can move against the gravitational force. If the temperature of the air were ten times smaller (which is about 30 K or -240 degrees Celsius), the atmosphere would be only 1 km thick. At 30 microkelvin, the atmosphere would shrink to a mere millimeter, and at 30 nanokelvin, the height of the atmosphere would be 1 micrometer, hundred times less than the thickness of the human hair. (Of course, air is not an ideal gas and would have liquified by then).  "

"Freshly liquefied air consists of 78.1% nitrogen, 21.0% oxygen, 0.9% argon, and very small amounts of rare gases and hydrogen in solution. Its boiling point is approximately −195°C. Because of fractional evaporation, its oxygen concentration and its boiling point increase with time. ..As the temperature of liquid air rises, the nitrogen evaporates first at −195.8°C, the argon next at −185.7°C, and the oxygen last at −183°C."

Seems chilly.
the solarsystem seems to have a temperature around 40-50 K (-233 - -223 C.)
But I still don't know how long time it would take?
Black body radiation?
yor_on, Tue, 13th Jan 2009

you suck my coch or die mudder fukker, Mon, 22nd Feb 2010

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