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I have no doubt that we have been to the moon. This question has nothing to do with a moon landing hoax. But, there are two quotes from two different astronauts regarding the size of the earth as viewed from the moon that are puzzling to me. Both quotes talk about how small the Earth looked. Shouldn’t the Earth look very large when viewed from the moon

It suddenly struck me that that tiny pea, pretty and blue was the Earth. I put up my thumb, shut one eye and my thumb blotted out the planet Earth. I didn't feel like a giant. I felt very, very small. — Neil Armstong

As we got further and further away it [the Earth] diminished in size. Finally it shrank to the size of a marble, the most beautiful you can imagine. That beautiful, warm living object looked so fragile, so delicate that if you touched it with a finger it would crumble and fall apart. Seeing this has to change a man. — James Irwin

I know the term large is subjective, but still, the comments seem off. Please let me know what I’m missing.

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  • $\begingroup$ OP, it could be that you think the moon is tiny? Not so - Earth and Moon are roughly the same size. Kind of a twin planet system. $\endgroup$
    – Fattie
    Commented Oct 29, 2018 at 17:43
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    $\begingroup$ @Fattie That doesn't sound right to me. Earth has almost 50 times the volume of the moon (1 trillion km^3 vs. 21 billion km^3). $\endgroup$
    – Kevin
    Commented Oct 29, 2018 at 20:08
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    $\begingroup$ @Kevin - Take the cube root of 50 to get the ratio of the Earth's diameter to that of the Moon. $\endgroup$
    – David Hammen
    Commented Oct 29, 2018 at 23:37
  • $\begingroup$ @jean - Gloves aren't needed. The Earth as seen from the Moon and a thumb at arm's length are more or less the same size. $\endgroup$
    – David Hammen
    Commented Oct 29, 2018 at 23:40
  • $\begingroup$ Hi Kevin. They are about the same size objects. Earth and Jupiter, or the Sun versus the Galaxy, or Saturn versus its moons, are radically different in size. My guess is the OP things the moon is a "small" thing. $\endgroup$
    – Fattie
    Commented Oct 30, 2018 at 3:00

6 Answers 6

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The Earth is 4 times the diameter of the Moon. The Earth viewed from the Moon will therefore appear to have 4 times the angular diameter of the Moon viewed from the Earth.

The Moon is easily obscured by a thumb at arm's length (by a factor of 3-4).

Now bring your thumb closer (because you can't fully extend your arm in a bulky space suit) and put on the biggest pair of ski gloves you can find.

It is not a great stretch of the imagination to think your thumb, in a spacesuit, would easily obscure something 4 times the size of the Moon.

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  • $\begingroup$ For objects seen from afar, apparent relative sizes are actually more about how large an area of our field of view they cover. A circular object with four times the diameter of another circular object will appear sixteen times larger. $\endgroup$
    – Arthur
    Commented Oct 29, 2018 at 9:37
  • $\begingroup$ @Arthur that is true if size means solid angle. I will clarify. $\endgroup$
    – ProfRob
    Commented Oct 29, 2018 at 9:44
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    $\begingroup$ Even at ground level you can blot out the entire Earth with your thumb, if you put it close enough to your eye. $\endgroup$
    – aroth
    Commented Oct 29, 2018 at 12:24
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    $\begingroup$ @Arthur there is a crucial illusion relating to the horizon involved in size perception: when the moon is setting low over the earth's horizon (from Earth), it appears subjectively (according to some studies) 30-40% larger even though in photos it is objectively the same size. In space it stands to reason that, thus, objects would appear "at their smallest". $\endgroup$
    – Darren Ringer
    Commented Oct 29, 2018 at 12:49
  • $\begingroup$ The Moon is easily obscured by a thumb at arm's length because the Moon's angular size as seen from Earth is about half a degree while a thumb at arm's length is about two degrees -- four times the size of the Moon. That is about the same size as the Earth as seen from the Moon. A glove isn't needed. $\endgroup$
    – David Hammen
    Commented Oct 29, 2018 at 18:16
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You can get a rough idea of how large the Earth appears from these two photos: enter image description here

enter image description here

It's not quite that large, is it?

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    $\begingroup$ You cannot get any idea of how large/small something looks from photos like these. Obvious counterexample here sites.google.com/a/g.clemson.edu/the-mysteriously-massive-moon $\endgroup$
    – ProfRob
    Commented Oct 28, 2018 at 15:52
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    $\begingroup$ And indeed exactly these photos have been used by conspiracy theorists to claim that the Earth looks too small for the pictures to have been taken on the moon! e g. angelfire.com/moon2/xpascal/MoonHoax/ApolloEarth/… $\endgroup$
    – ProfRob
    Commented Oct 28, 2018 at 16:24
  • $\begingroup$ @RobJeffries Good points. It was their comments that I found interesting. Both mentioned how small it appeared. My guess is they knew their descriptions would be told and retold so they may have used more poetically descriptive terms rather than direct objective descriptions like, it looked four times bigger than the moon. So I guess my question really doesn’t have an answer because it asks why someone chose to describe something the way they did. They must have been taken by strong emotions by witnessing such a sight. The metaphorically term of small in a cosmic sense most likely applies too. $\endgroup$
    – Lambda
    Commented Oct 28, 2018 at 17:00
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    $\begingroup$ photo.stackexchange.com/questions/102426/… $\endgroup$
    – Muze
    Commented Oct 28, 2018 at 22:26
  • $\begingroup$ Link to more photos of the Apollo 17 EVA: hq.nasa.gov/alsj/a17/images17.html#MagB $\endgroup$
    – stackzebra
    Commented Oct 29, 2018 at 9:49
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If you reduce the scale of the Earth to make it the size of a tennis ball, then the moon would be about the size of a nerf ball (20mm or under an inch), and the moon would be about 7 feet from the Earth.

So you can get a good idea of what the Earth would look like from the Moon: Get a tennis ball and nerf ball set them up 7 ft (2 m) apart put your head next to the nerf ball, and the tennis ball will look about the same size as the Earth would look on the moon

https://www.youtube.com/watch?v=GCTuirkcRwo

Now hold up your thumb, cover the tennis ball and imagine "on it [is] everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives".

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    $\begingroup$ This isn't really that accurate. As stated in the accepted answer the Earth s diameter is ~3.5 times that of the moon. A tennis ball sized Earth leads to a Moon ball that would have a diameter of ~20 mm, almost half the size of a standard table tennis ball. $\endgroup$
    – Dave
    Commented Oct 29, 2018 at 7:53
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    $\begingroup$ I've changed to "nerf ball" as per the source. 20mm is about the right size. I think the scale is 19000000:1 $\endgroup$
    – James K
    Commented Oct 29, 2018 at 16:48
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    $\begingroup$ Nerf balls come in many shapes and sizes: nerf.hasbro.com/en-us/sports $\endgroup$
    – GreenMatt
    Commented Oct 29, 2018 at 18:19
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From the moon, the earth would appear to span across about 2 degree of sky. If you hold your thumb a few inches in front of your face (or your facemask if you are on the moon), your thumb will block about two degrees of your field of view (Armstrong mentions closing one eye because his thumb is close enough that the different perspective between his two eyes is significant). So there doesn't seem to be anything too surprising about these statements.

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  • $\begingroup$ A very handy number to remember: A thumb at arm's length spans about 2 degrees of arc. A thumb a few inches in front of an eye would span about 20 degrees of arc. $\endgroup$
    – David Hammen
    Commented Oct 29, 2018 at 18:24
  • $\begingroup$ Just to add to my previous comment, if you held your thumb a few inches from your face it would span dozens degrees, not two. This answer is very wrong. Why so many upvotes? $\endgroup$
    – David Hammen
    Commented Nov 2, 2018 at 1:07
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The average distance $D$ between the Earth and the Moon is about 385000 km. The Earth's mean radius $r_E$ is 6371 km while the Moon's mean radius $r_M$ is 1737 km. This means that when the Earth is seen directly overhead from the Moon, it subtends an angle of $2\arcsin\left(\frac{r_E}{D-r_M}\right)$, or 1.9°.

A good rule of thumb is that the "visual angle of the width of the thumb held at arm's length is about 2 degrees." (Specifically, it's 2.12°±0.26° at the joint per the referenced paper.) A typical person could just barely blot out the Earth as seen from the Moon with a naked (not gloved) thumb held at arms length. The angular size of a thumb at arm's length varies from person to person; from the referenced paper the visual size of a thumb held at arm's length varied between 1.23° and 3.20° for the 112 tested subjects.

My thumb at arm's length is 2.45° wide at the middle of the thumbnail, so I could easily blot out the Earth from the surface of the Moon with my thumb at arm's length.

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Watching the moon from earth, it seems to be larger at the horizon than at the sky. This means that optical illusions may play a role here, too. The lack of reference objects or maybe the different shape of the horizon on moon may also contribute to the apparent size of earth.

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    $\begingroup$ +1. While optical illusions shouldn't affect Armstrong's thumb, it could explain Irwin's comparison to a marble. I wouldn't compare 4x moon to a marble when I look at it with surroundings visible, but it sure might look like one when surrounded only by the darkness of space and no reference for distance. $\endgroup$
    – jpa
    Commented Oct 28, 2018 at 17:05

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