In a universe of lights, the moon shines bright. Two hundred thousand miles away, the moon’s gravitational attraction determines the high and low tides of our seas and oceans, holding earth’s cyclical tidal patterns in balance.
Rotating around the earth in 29 Earth-day cycles, the near side of the moon remains fixated on the earth’s surface while reflecting the sun’s bright glow. Dominating the night’s sky, the moon’s bright glow features prominently against a sea of stars, constellations, and nebulae.
With such a bright illumination, one question fills every stargazer’s mind: Just how hot is the moon? We’ll answer that question in this article.
Is the Moon Hot or Cold?
At any one point in time, the Moon is both very hot and very cold. Just like the Earth, at any given time, there are parts of the Moon that receive direct sunlight (called the lunar day) and there are parts of the moon that receive little to no direct sunlight (called the lunar night).
Unlike the earth, the lunar days and nights last for a long time – usually around 13 Earth days. During the lunar day, the sustained solar radiation incident on the Moon’s surface can cause surface temperatures to rise up to over 100℃ (sometimes up to 200℃ at high surface elevations).
In contrast, the lunar night can drop surface temperatures down to as little as -150℃ (sometimes down to -247℃ in polar craters).
What Affects the Moon’s Temperature?
Temperature on the moon is controlled by radiation and conduction.
Radiation is the electromagnetic energy traveling from the sun that is incident on the moon. This directly warms the surface of the moon like a microwave heats food.
Conduction is the flow of heat from a hot area of a surface to a cold area. On earth, conduction causes heat from hot sand to increase water temperatures at night. On the moon, the lunar dusk is the hot-cold boundary between the lunar day and the lunar night.
This boundary maintains a moderate (typically -20℃) temperature due to conduction. Conduction causes the hot surface of the moon in the lunar day area to exchange heat with the cold surface of the moon in the lunar night area, producing a more moderate thermal zone.
If you were to walk on the moon, you would stick to this thermal boundary area for best results.
Temperature of the Moon’s Surface
Calculating the exact temperature of the moon’s surface is difficult for a number of reasons.
First, surface elevation, angle, and shade directly affect the intensity of sunlight absorbed by the lunar surface. Mountainous areas on the moon are hot, reaching temperatures of over 200℃, while lower-lying areas maintain a more moderate 100℃.
In addition, mountains and craters may cast a shadow on the lunar surface blocking solar radiation and causing temperatures to quickly fall.
Other parts of the moon never receive direct solar radiation. For example, the lunar poles are angled away from the sun and so receive little to no direct solar radiation. The temperature in these areas gets pretty extreme, reaching temperatures down to -247℃.
For reference, absolute zero, the temperature at which all particles stop moving is -273℃. So these craters experience temperatures just 26℃ above absolute zero – that’s cold!
Temperature of the Moon’s Core
Similar to the Earth, the moon’s core is a hot mass of partially molten metals and materials that propagate heat through the moon’s mantle layer to the surface. Unlike the earth, the moon’s core is smaller, cooler, and is under much less pressure.
Ranging from 1,327℃ to 1,427℃, the moon’s core isn’t hot enough to conduct heat through 205 miles of mantle layers to the lunar surface. In fact, subsurface temperatures on the moon have been measured at -40℃. That’s like the temperature in Alaska after a heavy snowstorm – pretty cold!
The moon’s core is predominated with iron in a partially molten state. Research studies suggest that only 5-30% of the moon’s core is actually molten. The rest may be solid or in vaporous form. Scientists are attempting to map out the thermal profile of the mantle layers of the moon. (Ibid)
How Cold Is the Moon When It’s Dark?
Although the same side of the moon always faces the earth, the moon rotates while orbiting the sun. Accordingly, the lunar night processes around the lunar surface. When the sun isn’t shining on the surface, it can get very cold because there is no convective heating.
On the moon, temperatures can fall below -150℃ as the moon’s surface is only warmed by:
- Surface conduction from areas of the moon experience a lunar day
- Subsurface conduction from the moon’s core, and
- Trivial amounts of space radiation from far away stars.
In other words, the moon’s surface gets really, really freezing cold at night. When we look up at the moon, we only see the part of the moon in the lunar day. On rare occasions, the outline of the moon may be visible, produced by radiating heat reflecting off of the far side of the moon.
In addition, some parts of the moon are always in a lunar night. There are craters in the lunar poles which were measured by NASA’s Lunar Reconnaissance Orbiter to reach -247℃.
How Cold (or Warm) Is It During a Lunar Day?
During a lunar day, temperatures can soar up to 200℃ or higher. Without a lunar atmosphere, solar radiation can directly bake the moon’s surface.
The sun radiating the moon is like you sitting under a tanning lamp. Initially, it feels warm and relaxing, but if you sit there for 14 hours, you’ll get a pretty bad sunburn. If you sit there for 14 days…Well, we won’t try that here on earth.
How Did Astronauts Survive the Temperature on the Moon?
Careful planning was essential for a successful moon landing and subsequent lunar walk.
First, the landing location was carefully chosen. The astronauts landed in a region experiencing a lunar dusk (the boundary between lunar day and lunar night) to ensure that the temperature was not too extreme.
Next, the lunar spacesuits were enclosed in a reflective material to protect the astronauts from solar radiation from the sun and reflected solar radiation from the lunar surface. The helmet visors were also coated with gold to protect the astronaut’s eyes from the solar radiation.
The boots of the spacesuits were also insulated to reduce thermal conduction from the lunar surface.
Finally, the spacesuits were lined with an insulating material, a temperature-regulating liquid cooling system, and heaters to help maintain normal body temperature during the two-hour lunar exploration.
What Was the Temperature on the Moon When Neil Armstrong Landed?
Neil Armstrong landed on the lunar surface during the lunar dusk – the transition area between lunar day and lunar night. This transition area is heated by surface conduction, while having limited exposure to solar radiation.
Accordingly, where Neil Armstrong landed was likely to have been close to the average temperature of the moon’s surface, which is -20℃. However, air temperatures could have varied significantly due to the lack of a lunar atmosphere and there being no thermal convection on the moon.
Conclusion
The moon is the nearest celestial body to the earth and illuminates the night’s sky. The moon reflects solar radiation, producing the visible spectrum of light that we see from earth while also producing black-body thermal radiation.
Due to a lack of an atmosphere, the moon has strong thermal gradients and harsh temperatures. Without our atmosphere, the temperature variations on earth would mimic the harsh thermal conditions on the moon.