SoulWager t1_iyd1nvi wrote
Reply to comment by avdolian in ELI5: How is that space is “flat” yet we are able to look around the universe (up, down, left, right, etc.,) as if it were not flat? by nhabz
>You can draw parallel lines on a sphere and they stay parallel.
From the perspective of two people walking on those lines, at least one of them will need to constantly be turning left or right. If you're both walking parallel and straight, your paths will intersect 1/4 the way around the sphere.
avdolian t1_iyd26av wrote
If my friend stays one metre to my north and we both walk around the globe one of us will have walked further but they wouldn't have to constantly turn and we would never cross.
stools_in_your_blood t1_iyd2ye5 wrote
The one who isn't walking on a great circle (i.e. an "equator" of the sphere) is constantly turning away from you, in the sense that if they imagine their path laid out in front of them, it appears to curve to one side.
Put another way, imagine doing this not with walking but with cars with the steering fixed dead ahead. Try it with toy cars and a basketball if you have those objects handy.
SoulWager t1_iyd370q wrote
The only line of latitude that's not bending left or right is the equator. For an extreme example, imagine walking the line of latitude one meter from one of the poles.
A line of latitude one meter from the equator is still bending, just not as much.
Lines of longitude do not bend left or right, and they all intersect at the poles, even though they're all parallel at the equator.
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