Puzzling quotes from astronauts about earth size

Lambda 10/28/2018. 6 answers, 6.945 views
the-moon observational-astronomy moonlanding

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.

6 Answers

Rob Jeffries 10/29/2018.

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.

stackzebra 10/28/2018.

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?

James K 10/29/2018.

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


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".

antlersoft 10/28/2018.

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.

David Hammen 10/29/2018.

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.

Hartmut Braun 10/28/2018.

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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