# What is the daytime ambient temperature on the moon in shade?

I know that the daytime surface temperature can exceed 270°F (130°C) but if you measured the temperature 5 feet (1.5 metre) off the surface in shade what would the reading be?

To further specify the question, consider the following setup:

lets say you bolted a pointy ended plexiglass tripod (very low thermal conductivity) to the surface of the moon (assumed to be about 270°F (130°C) and at the top of that table 5' (1.5 metre) off the surface is a plexiglass sphere resting pressurized with air to 1 atm (101 kPa) a and in the center of the sphere is a glass of water is suspended by 3 threads of fishing line . 10' (3 metre) away from the tripod, a mirror is suspended reflecting light away from the tripod and such that it casts a shadow on the whole aparatus. Would the glass of water freeze? What would the temperature of the air likely be, roughly?

• I suppose the temperature will be different depending on amount of sunlit areas nearby, because the sunlit surface will be re-emitting diffuse light (visible and infrared) to the shadow area. I don't know did Apollo missions or automatic crafts ever measured temperature in the Moon's shadowed areas. – Heopps Aug 19 '18 at 7:05
• There is no air temperature to be measured 5 feet off the surface. There is a nearly perfect vacuum above the surface of the moon. – Uwe Aug 19 '18 at 17:09
• Yes I get that. Nevertheless the thermometer will measure something. What would it say? – GGizmos Aug 19 '18 at 17:10
• See this comment. – uhoh Aug 19 '18 at 17:14
• A temperature sensor will measure its own temperature within the sensor itself. If carefully applicated, the sensor may measure the temperature of a solid part of the LM. If the sensor is put into the regolith, it will indicate its temperature after some time for alignment. But some care is necessary to thermallly isolate the sensor from the rod outside the regolith heated by sunlight. – Uwe Aug 19 '18 at 17:28

There isn't really a "temperature five feet off the surface" because there isn't much of anything there to have a temperature. The Moon does have an "atmosphere" but typical pressure is around $0.3 nPa$ (wikipedia) and it will not tranfer enough heat to or from any reasonable thermometer to produce anything you could call an ambient temperature. Indeed the mean free path of molecules in such a gas is measured in kilometers, so they will interact with the surface more than with each other, making the whole question of temperature a bit moot.

What your thermometer will measure is the balance of radiant energy that it is absorbing with what it is emitting. That is, the temperature it reports will be the one at which there are in balance. The depends on exactly what is in line-of-sight of the thermometer, and also on how well it absorbs and emits different frequencies. If it has no line-of-sight on sunlit (or recently sunlit) rocks, or on the Earth it could get very cold indeed, since it is absorbing very little and can radiate into space. On the other hand, if it almost surrounded by warm rocks that either are, or recently have been heated by the sun, it will end up almost as hot as they are.

Based on the more exact specification added to the question: Assuming you are on a flattish bit of Moon, then basically you have to ask where the lines of sight from the glass of water go. Nearly half of them (going down) will hit the lunar surface. Most of the rest will go into empty space. A few may hit the Earth, and a number will hit the curtain. It's a little simpler if we replace the curtain by a mirror, reflecting the Sun's heat away rather than absorbing it. To a VERY VERY rough approximation then, half of what we hit is surface at 400K and half is space at 2.7K. Energy transfer due to radiation scales as the difference of the fourth power of temperature, so we get:

$$400^4-T^4 = T^4-2.7^4$$ (wikipedia)

with a solution around 336. So the water would not freeze, although it wouldn't boil either.

Among many other details, this solutions assumes that the water and the rocks are black bodies, absorbing and emitting evenly and effectively at all wavelengths and that the glass is transparent at all relevant wavelengths.

Edited to use correct equation, and consequently change the outcome. Thanks to commenters who pointed out my mistake.

• OK, that's sort of what I thought but lets say you bolted a pointy ended plexiglass tripod (very low thermal conductivity) to the surface of the moon (assumed to be about 270F and at the top of that table 5' off the surface is a plexiglass sphere resting pressurized with air to 1 atm a and in the center of the sphere is a glass of water is suspended by 3 threads of fishing line . 10' away from the tripod, a thick black curtain is suspended such that it casts a shadow on the whole aparatus. Would the glass of water freeze? What would the temperatur of the air likely be? roughly. – GGizmos Aug 19 '18 at 16:32
• @user1023110 It may take a while, but you will need to come to terms with the fact that the concept of "ambient temperature" does not apply. Without an atmosphere, everything is absorbing and radiating thermal infrared from everything else. The water is opaque (black) in infrared, but so is the glass and so is the plexiglass. They will receive radiation from the lunar surface from the lower hemisphere, and the thick black curtain will absorb sunlight and radiate as well. Your apparatus will receive this radiation, and radiate as well. It's a complicated mess and there's no single answer. – uhoh Aug 19 '18 at 17:12
• I understand that ambient temperature is a a dubious concept is near vacuum. Nevertheless the question does have a single answer if the parameters are specified. I'm looking for educated guesses about what the answer is. Either the water will freeze or it won't. What do you think? Are there other improtant parameters I have not specified? – GGizmos Aug 19 '18 at 17:16
• @uhoh I took that from the 270F given in the question. – Steve Linton Aug 20 '18 at 8:44
• Glass is generally opaque and a pretty good blackbody at "thermal" IR wavelengths. See for example answers to What are these very large, square panels on Inmarsat 5? and also What is the function of this array of what looks like mirrors on TESS? – uhoh Aug 20 '18 at 15:20

It just so happens I'm reading First Man - The Life of Neil Armstrong, by James R Hansen, Pocket Books, 2006.

On page 533, after Neil Armstrong & Buzz Aldrin returned to the lunar module, after their Lunar EVA, it states:

'Even though it was over 200 degrees Fahrenheit outside the LM, it was quite cold inside - something in the range 61 to 62 degrees Fahrenheit. "When we put the window covers on so that it would be relatively dark inside" Armstrong explains, "the temperature got quite brisk in the cockpit".'

• Temperature outside the LM could be measured only within the lunar regolith (moon dust) but not in the vacuum. The "over 200 degrees Fahrenheit" may be from a temperature sensor at the landing legs of the LM or somewhere at its surface. – Uwe Aug 19 '18 at 17:15

What is the ambient temperature on the moon? The temperature of the lunar regolith some inches below surface, of a moon rock, of an outside part of the LM, the gas inside the LM, a landing leg of the LM in contact with the regolith?

You could not measure an air temperature because there is no air. There is only a nearly perfect vacuum above the surface of the Moon, but there are too few molecules to measure their temperature with a conventional temperature sensor. On Earth a precise meterological measurement of air temperature is not that easy. You need a Stevenson Screen to minimize the influence of soil temperature, solar radiation, buildings around the point of measurement and plants and trees.

But on the moon something similar to a Stevenson Screen would not help because there is no air temperature to be measured. The temperature of a solid, a liquid or a gas with a pressure similar to that on Earth may be measured, but not that of a vacuum. On the Moon there are only solids to be measured. You need a definition of ambient temperature applicable to the Moon.