Re: [Phys-l] Temperature in Space

[John Denker <jsd@av8n.com>]

David Abineri wrote:
> A follow-up question if I may on the Giancoli question about an animal freezing at high altitude
> even though the temperature is 700C.

Keep in mind that the animal would not necessarily freeze.
A sufficiently furry animal would do just fine, as would
a would a person wearing sufficiently thick clothes.
(Breathing would be an issue, but that's a separate issue.)

> Where the temperature is 700C at high altitude, what instrument does one use to measure this?
> How does one avoid picking up external radiation effects if one just wants the average kinetic
> energy of the air molecules?

You can use a chopper to measure the time-of-flight of the
particles ==> speed ==> kinetic energy ==> temperature.

> On the moon in daylight the temperature is about 250C 

That depends strongly on the time of day.

> but there are virtually no air molecules.
> Would the lunar soil be hot and would this register on a regular thermometer stuck into the
> ground?

Yes, the soil gets hot, and this registers on the most
prosaic of thermometers.

>  Did the astronauts wear air conditioners 
> because primarily of the solar radiation rather than any local 
> temperature?   

The local temperature is of considerable importance to the
overall radiation budget;  an astronaut (or any other object)
near the surface is looking at 2pi steradians of black-body
radiation at 250C (or whatever the local temperature is).

When you add up the heat load from this, it is of the same
magnitude as the heat coming directly from the sun ... which
is *not* a coincidence.  (Think about it.)

==========

In principle, you could get an object very cold on the moon,
even in daytime without an air conditioner, by use of the
following trick:

                         <--- cold sky --->

            S
           SSS  <--- sun
            S



             -------     <--- parasol



               bbbbb     <--- object
               wwwww

     mmmmmmmmmmmmmmmmmmmmmmmmmmm  <--- lunar surface

The point is that the object is white (or shiny) on the bottom
so it does not come into equilibrium with the hot lunar surface,
and is black on top so it comes in to equilibrium with the cold
(very cold!) sky, and it is protected from direct sunlight by a
parasol.


The astronauts did not use this trick because of the bother of
keeping the parasol and other details lined up just right.