# When did humans realize that there is no air on the Moon?

What is the easiest evidence that there is no air on the Moon?

Frankly speaking, I myself have no idea how to prove it.

• We had a similar question on here recently, but some tiresome person decided to ban it as "off topic". Perhaps we will have better luck this time. hsm.stackexchange.com/questions/571/… – fdb Mar 3 '15 at 16:21
• @fdb It was closed only to reject a migration. There was no cause to migrate it; therefore, it was closed. Don't use the phrase "some tiresome person" - at least half a dozen people were against the migration and supported closure, whereas only those who migrated it were at first for it - and if I remember correctly, they changed their minds. – HDE 226868 Mar 3 '15 at 19:37
• @fdb Note that the question was not closed by anyone from this site, but rather by Pops, who is a member of the Stack Exchange community team. – Danu Mar 3 '15 at 20:29
• @wdlang. What I was trying to say, perhaps not successfully, is that you are asking the wrong question. You should not be asking when people realised that there is no air on the moon. You should be asking whether anyone EVER believed that there is air on the moon. Do have a look at my answer below. – fdb Mar 3 '15 at 21:58
• @fdb (and the rest who commented in this chain) Could you guys save this type of discussion for chat please? It provides nothing useful in regards to the question at hand. – b1nary.atr0phy Jan 8 '17 at 0:51

There is no sharp difference between "no air" and "very little air". That it is "very little" is seen when we observe the Moon with a telescope. An atmosphere creates a visible haze, especially on the edge of the object (because of refraction). Especially well visible it would be during Solar eclipses. So when people started to observe the Moon with telescopes it was clear that there is very little air. The more powerful the telescopes that were used, the smaller this "very little" became.

For practical purposes it was clear that there is no air by the end of the 18th century.

For comparison, comets usually have atmospheres (visible with a naked eye) but they are so rarified that "there is no air" on the comets, for all practical purposes.

EDIT. Of course when I say that "it was known in 18-th century" this means "it was known to the experts, scientists". It is easy to show that this was NOT known to the "general public". Many writers (and some movies in the beginning of 20-th centiry) described travels to the Moon, and they did not know or did not care that a human cannot breath there. Jules Verne's travelers, for example has no provision for breathing on the Moon. Jules Verne wrote in the second half of 19-s century. (That air is necessary for survival was shown in 17-s century).

• Is there evidence that people in the 18th century actually used the reasoning you outline in your first paragraph? – Jack M Mar 4 '15 at 0:00
• Yes, it is. Actually Jules Verne (From Earth to the Moon, Chapter 20) gives a survey of opinions of scientists on the question whether Moon has an atmosphere. – Alexandre Eremenko Mar 4 '15 at 1:19
• @AlexandreEremenko Actually, Jules Verne was the 19th Century. 18th Century = 1700's, 19th Century = 1800's. – DrZ214 Apr 21 '16 at 1:26
• "So when people started to observe the Moon with telescopes it was clear that there is very little air." What exactly did they use as a frame of reference? It's not like they could look at the earth from a distance to compare the difference. – b1nary.atr0phy Jan 8 '17 at 0:48
• @b1nary.atr0phy: Besides the direct naked eye observation, there is mathematics. In 18th century there was a sufficiently developed theory of refraction to predict how an atmosphere at a distance should look. – Alexandre Eremenko May 7 at 17:10

Here are three evidences that do not require a telescope.

1) No clouds are ever seen on the moon's surface. This is the weakest evidence, since the clouds could be too small to see with the naked eye, or there could be too little water in the air to form clouds.

2) When the moon moves over a star, occulting it, the star winks out quickly. If the moon had air, the air would take a little time to cause the star to loose its brightness.

3) The terminator, the dividing line between light and darkness on the face of the moon, is jagged due to craters and mountains, but it is very sharp. An atmosphere would make it fuzzy.

Therefore, there was good reason to conclude the lack of air on the moon long before the telescope was invented.

• @Alexandre Eremenko There are clouds on Mars. mars.jpl.nasa.gov/MPF/science/clouds.html – Conifold Mar 3 '15 at 22:39
• @Conifold: Nice to know this. Are they visible by the naked eye from 380,000 km distance (the average dist to the Moon)? – Alexandre Eremenko Mar 3 '15 at 23:08
• @AlexandreEremenko Some are: blogs.discovermagazine.com/d-brief/2015/02/17/… – Conifold Mar 4 '15 at 1:12
• @Rory Daulton, OK, I removed my comment. – Alexandre Eremenko Mar 4 '15 at 1:15
• I don't think (this is an opinion) any of this is hard evidence. $1)$ and $2)$ are weak and I'm not sure about $3)$, either. On the other side, your answer made me think about how a total solar eclipse would look if the moon had an atmosphere; I believe the refraction would be noticeable. Edit: I just saw @AlexandreEremenko mentions this idea in his answer. – hjhjhj57 Mar 6 '15 at 8:06

You could also ask when people decided that there was air on Earth. The concept of “air”, as opposed to “wind”, is a relatively advanced (and relatively abstract) notion, which probably came up at about the time of Empedocles (or whoever it was that first posited four elements: earth, fire, water, air). Aristotle taught that air fills the whole space between the surface of the Earth and the sphere of the moon; above that sphere everything is made of the fifth element, aether. This implies that there is no air on the moon. Somehow I don’t think anyone ever thought there was air on the moon.

• It is wrong to credit Aristotle (or any of the ancients) with "no air on the Moon:-) According to their theories, other planets were also "above" and thus had no air. But we know that all of them except Mercury have atmospheres. – Alexandre Eremenko Mar 3 '15 at 22:17
• I am just asking: who DID think there is air on the moon? – fdb Mar 3 '15 at 22:27
• Jules Verne, From Earth to the Moon, Chapter 20, gives a survey of opinions on this question. – Alexandre Eremenko Mar 3 '15 at 23:02
• I will have a look. – fdb Mar 3 '15 at 23:30
• Galileo claimed in the Sidereus Nuncius that it was "obvious" that "not only the Earth but the Moon also is surrounded by a vaporous sphere." – Viktor Blasjo Jul 31 '17 at 16:07