Puzzling quotes from astronauts about earth size

  • 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.


    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.

    @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).

    @Kevin - Take the cube root of 50 to get the ratio of the Earth's diameter to that of the Moon.

    @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.

    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.**

    @Fattie Smaller is a comparative adjective and the moon compared to other planetary objects in our solar system is comparatively small.

    Hi @Lambda , there are 10 or so "big" moons in our system, and our Moon moon is one of those 10 "big" moons. The majority of moons in our system are tiny rocks. Based on your question, my guess is that you thought our Moon was "a small rock"; so, you assumed (not unreasonably) that Earth would be HUGE when seen in comparison. In fact this is not the case, they're "about" the same size. As fully explained in Rob's answer. Cheers!

    @DavidHammen Gloves! big space suit gloves! Relax, I know it's a matter of point o view, just jesting a little *wink* ;)

  • ProfRob

    ProfRob Correct answer

    3 years ago

    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.


    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.

    @Arthur that is true if size means solid angle. I will clarify.

    Even at ground level you can blot out the entire Earth with your thumb, if you put it close enough to your eye.

    @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".

    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.

    @DavidHammen "Big thumbs" to his friends (or "short arms" ?). My thumb does not quite subtend 2 degrees at arms length. It is 2 cm wide and my eye to thumb length with outstretched arm is 67cm. So 1.7 degrees. It's close - I'll reword.

    Or perhaps you have small thumbs or long arms. A good rule of thumb is that a man's thumb is about an inch wide.

    The astronauts weren't tall or fat, and you don't need an outstretched arm (esp. in the Apollo CM) to appreciate the effect. Time on the moon was too precious for much philosophizing.

    I guess it would depend on whether the Earth was near the moon's horizon, or at a high angle, which would make it appear smaller. Still I would imagine it would look big to them, relative to the moon, but relative to their earthbound perception of the Earth, it would appear very, very small. I'm guessing they were trying to make the latter point? Good question though--it's the first time I've thought about this!

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Content dated before 7/24/2021 11:53 AM