# QuestionTemperature on the Moon?

#### mossathewriter

What would be the digit recorded by a thermometer suspended in the shade in a hit-by-sunrays site on the Moon? bearing on mind that the thermometer would be in a vacuum, since there is no air on the Moon, and that the seething temperature of the surroundings would have no effect on the thermometer, since heat does not transfer through a vacuum.

#### billslugg

A thermometer would stabilize at a temperature of the average of what it sees. Take the percentage of outer space at -457°F seen by it, the temperature of the shaded Moon surface at -280°F seen by it and the temperature of the sunlit surface at +260°F seen by it, then calculate a weighted average.

#### mossathewriter

A thermometer would stabilize at a temperature of the average of what it sees. Take the percentage of outer space at -457°F seen by it, the temperature of the shaded Moon surface at -280°F seen by it and the temperature of the sunlit surface at +260°F seen by it, then calculate a weighted average.
One centimeter above the ground and upward anywhere on the Moon, whether the site is sunlit or shaded, it is -457° F, as there is no medium through which the heat of the Moon's surface can transfer upwardly.

#### billslugg

You are correct that there is no medium to transfer heat by conduction. However, heat transfer by radiation heats any lunar soil in sunlight to +260°F. Any soil in the shade loses its heat to the open sky which is at -457°F. Any bit of soil in the shade which looks at both unshaded soil and the open sky reaches a temperature in between the two in proportion to how much of each it sees.

Ken Fabian

#### mossathewriter

You are correct that there is no medium to transfer heat by conduction. However, heat transfer by radiation heats any lunar soil in sunlight to +260°F. Any soil in the shade loses its heat to the open sky which is at -457°F. Any bit of soil in the shade which looks at both unshaded soil and the open sky reaches a temperature in between the two in proportion to how much of each it sees.
If heat transfer by radiation, why, then, the temperatures of the outer layers of all the planets of the Solar System are colder than those of the inner layers: they should be hotter as they are nearer to the sours of radiation?

#### billslugg

The outer layer of the Earth's atmosphere, the thermosphere, is very hot 900°-3,600°F . This is due to the exposure to the Sun's radiation and the fact there are so few molecules to transfer the heat to. In fact, a thermometer up there would read freezing temperatures. There would be so much radiative heat loss and so little transfer of heat from the few molecules that are there.

#### mossathewriter

How to accept the contradiction between these two facts: the outer space is at –270º C, and the heliosphere, which encloses the entire Solar System, is at 100,000 ºC ?

#### billslugg

This is a bubble of hot gas inside a large room with freezing cold walls. How did such hot gas get that far out into the cosmos without cooling down? Temperature is a measure of how fast a particle is moving. In order to reduce the temperature of a ballistic particle leaving the Solar System, it must have energy removed from it. There is no mechanism to do this for an isolated particle other than bumping into another one. Unfortunately, in outer space there might only be a few million atoms per cubic meter. Encounters would be very rare.

mossathewriter

#### Ken Fabian

Radiant heat, conduction and convection are the ways energy transfers and there is no conduction or convection between the ground and the thermometer in the example.

Temperature is dependent on how fast the molecules are moving, which is not the same as how much heat (energy) is present. Not many molecules (thin gas) means not much energy even at high, even extreme temperatures. The thermometer won't read temperature of the surrounding near vacuum so much as how much radiant energy it absorbs (which depends on it's materials and surface) minus how much of that heat the thermometer radiates away. And if the shading material reflects rather than absorbs then radiant heat from the ground can reflect back at the thermometer. If it absorbs then it will radiate back at the thermometer. You can posit some ways that avoid those I suppose..

There is radiation. However there is conduction between internal heat within the moon and the surface as well as sideways between ground in sun and ground in shade. A thermometer above ground but with a line of sight to unshaded ground would affect the reading too. The thermometer won't be measuring the temperature of the near vacuum, but registering the radiant heat it absorbs.

#### mossathewriter

You are correct that there is no medium to transfer heat by conduction. However, heat transfer by radiation heats any lunar soil in sunlight to +260°F. Any soil in the shade loses its heat to the open sky which is at -457°F. Any bit of soil in the shade which looks at both unshaded soil and the open sky reaches a temperature in between the two in proportion to how much of each it sees.
Where does the confusion, on my side, lurk? I based my question, concerning temperature in a shaded place on the Moon, on the principle on which vacuum flasks are manufactured: that is, the heat of the hot beverage contained does not transmit by radiation from the inner bottle of the flask to the outer mantle. Note that "my thermometer" does not touch the soil, it is suspended in a shaded place on the Moon.

#### billslugg

It is true that conductive heat transfer will not heat up your thermometer. There are not enough molecules of air in this near perfect vacuum. Radiative heat transfer, however, does not care about vacuums. It only cares about the difference in temperature between the two objects exchanging radiation, and their emissivities. This is explained by Stefan's Law.

mossathewriter

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