<p>The &quot;dark&quot; side of the Moon is only truly dark during full Moon. Everywhere on the Moon there is day and night as well. The dark side of the Moon is called like that because we do not see it from Earth, since the Moon shows us always the same side due to <a href="https://en.wikipedia.org/wiki/Tidal_locking" rel="nofollow noreferrer">tidal locking</a> - not because it is always dark there.</p>
<p>In other words: During (lunar)  day, the Sun heats up the surface of the Moon and it gets pretty warm, even on the (not so) &quot;dark&quot; side of Moon. This explains the <span class="math-container">$100 {\rm K}$</span> you are citing.</p>

CC BY-SA 4.0
2021-03-31T10:11:50.093Z
Why 100K and not 3K (of space)
Ganesh Kulkarni
CC BY-SA 4.0
2021-03-31T10:32:24.613Z
During the Lunar day (of 300 hours), the Sun has plenty of time to heat up the surface, and the latter is radiating heat back, e.g. towards astronauts. Anyhow, we mostly talk about *surface* temperature.
B--rian
CC BY-SA 4.0
2021-03-31T19:43:14.920Z
Even when not illuminated, the far side may well pick up a few degrees from turbulent solar wind. Black body temp. in its orbit is around -23C.
Wayfaring Stranger
CC BY-SA 4.0
2021-04-02T23:55:32.530Z
This doesn't explain why the surface of the Moon at the end of the lunar night hasn't cooled down to the ambient temperature of the nearby space. Is the surface being kept warm by residual heat in the Moon's interior? Does the nature of the surface have higher absorption than emission? Or is 100 Kelvin the ambient temperature of space at that distance from the Sun?
Chappo Hasn't Forgotten
CC BY-SA 4.0
2021-04-03T14:48:30.267Z
This is implied but not stated explicitly in @B--rian’s answer: things take time to cool down. If you take a pan of boiling water and put it in the refrigerator, it doesn’t instantly reach the temperature of the refrigerator.   So when the sun sets for any given part of the Moon’s surface, the surface starts cooling down, and will continue to cool for about 14 days, until the Sun rises again for that part of the Moon.  That’s not long enough for it to cool down to the ambient temperature of space around it.
Eric Jensen
CC BY-SA 4.0
2021-03-31T09:56:31.107Z
2021-03-31T10:34:40.747Z
2021-03-31T10:34:40.747Z
B--rian
<p>The regolith absorbs solar radiation and conducts some of it to the subsurface during the day. As the lunar night begins, heat is lost rapidly near the surface. As the surface becomes colder than the subsurface, heat stored in the subsurface flows to the surface, and the surface cooling slows. In other words, the rapid cooling seen in the first part of the night in the Diviner graphs is the near-surface cooling, the subsurface cools slowly because it has the near surface as additional insulation.</p>

CC BY-SA 4.0
2021-12-11T12:04:32.083Z
further reading: answers to [Do we know the fluctuations and constant temperature depth of the moon?*](https://astronomy.stackexchange.com/q/30525/7982) and [What would be the temperature range measured 2 meters above the Moon's surface? @10m? @50m?](https://space.stackexchange.com/q/39577/12102) and [Where to get data of the received/measured moon temperature of the last 50 years?](https://space.stackexchange.com/q/52275/12102)
uhoh
CC BY-SA 4.0
2021-12-09T19:32:27.080Z
2021-12-09T19:32:27.080Z
Ray
<p>The temperature of the space between Earth and the Moon is around 3 Kelvin (<span class="math-container">$\approx -270^\circ {\rm C}$</span>). The moon has no atmosphere, therefore the heat convection is not possible from the bright side to the dark side on Moon.</p>
<p>Now, if the temperature of the space around the moon is <span class="math-container">$3 {\rm K}$</span>, then how the dark side of the moon is having a temperature of <span class="math-container">$100 {\rm K}$</span>?</p>
<h2>References</h2>
<ul>
<li><a href="https://sciencing.com/temperatures-outer-space-around-earth-20254.html" rel="nofollow noreferrer">https://sciencing.com/temperatures-outer-space-around-earth-20254.html</a></li>
<li><a href="https://www.space.com/14725-moon-temperature-lunar-days-night.html" rel="nofollow noreferrer">https://www.space.com/14725-moon-temperature-lunar-days-night.html</a></li>
</ul>

CC BY-SA 4.0
2021-03-31T10:41:16.693Z
The temperature of (gas in) space between the Earth and the Moon is *not* 3 K; even the first of your links (which does misleadingly claim that "empty interstellar space" has a temperature of 3 K) doesn't say that.
Peter Erwin
<the-moon><temperature><celestial-mechanics>
Why temperature of dark side of moon is not 3 Kelvin
Ganesh Kulkarni
# Why temperature of dark side of moon is not 3 Kelvin

(Asked by: username_0 on 2021-03-31)

The temperature of the space between Earth and the Moon is around 3 Kelvin ($\approx -270^\circ {\rm C}$). The moon has no atmosphere, therefore the heat convection is not possible from the bright side to the dark side on Moon.

Now, if the temperature of the space around the moon is $3 {\rm K}$, then how the dark side of the moon is having a temperature of $100 {\rm K}$?

## References

* <https://sciencing.com/temperatures-outer-space-around-earth-20254.html>
* <https://www.space.com/14725-moon-temperature-lunar-days-night.html>

**Tags:** the-moon, temperature, celestial-mechanics

> **Comments:**
> **username_1**: The temperature of (gas in) space between the Earth and the Moon is *not* 3 K; even the first of your links (which does misleadingly claim that "empty interstellar space" has a temperature of 3 K) doesn't say that.  

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## 2 answers

### Answer by username_2 on 2021-03-31. Score: 5

The "dark" side of the Moon is only truly dark during full Moon. Everywhere on the Moon there is day and night as well. The dark side of the Moon is called like that because we do not see it from Earth, since the Moon shows us always the same side due to [tidal locking](https://en.wikipedia.org/wiki/Tidal_locking) - not because it is always dark there.

In other words: During (lunar) day, the Sun heats up the surface of the Moon and it gets pretty warm, even on the (not so) "dark" side of Moon. This explains the $100 {\rm K}$ you are citing.

> **Comments:**
> **username_0**: Why 100K and not 3K (of space)  
> **username_2**: During the Lunar day (of 300 hours), the Sun has plenty of time to heat up the surface, and the latter is radiating heat back, e.g. towards astronauts. Anyhow, we mostly talk about *surface* temperature.  
> **username_3**: Even when not illuminated, the far side may well pick up a few degrees from turbulent solar wind. Black body temp. in its orbit is around -23C.  
> **username_4**: This doesn't explain why the surface of the Moon at the end of the lunar night hasn't cooled down to the ambient temperature of the nearby space. Is the surface being kept warm by residual heat in the Moon's interior? Does the nature of the surface have higher absorption than emission? Or is 100 Kelvin the ambient temperature of space at that distance from the Sun?  
> **username_5**: This is implied but not stated explicitly in @username_2’s answer: things take time to cool down. If you take a pan of boiling water and put it in the refrigerator, it doesn’t instantly reach the temperature of the refrigerator.   So when the sun sets for any given part of the Moon’s surface, the surface starts cooling down, and will continue to cool for about 14 days, until the Sun rises again for that part of the Moon.  That’s not long enough for it to cool down to the ambient temperature of space around it.  

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### Answer by username_6 on 2021-12-09. Score: 3

The regolith absorbs solar radiation and conducts some of it to the subsurface during the day. As the lunar night begins, heat is lost rapidly near the surface. As the surface becomes colder than the subsurface, heat stored in the subsurface flows to the surface, and the surface cooling slows. In other words, the rapid cooling seen in the first part of the night in the Diviner graphs is the near-surface cooling, the subsurface cools slowly because it has the near surface as additional insulation.

> **Comments:**
> **username_7**: further reading: answers to [Do we know the fluctuations and constant temperature depth of the moon?*](https://astronomy.stackexchange.com/q/30525/7982) and [What would be the temperature range measured 2 meters above the Moon's surface? @10m? @50m?](https://space.stackexchange.com/q/39577/12102) and [Where to get data of the received/measured moon temperature of the last 50 years?](https://space.stackexchange.com/q/52275/12102)  

---

