<p>How did Erastothenes and/or earlier scientist-philosophers determine that the sun's rays arriving at the earth are parallel? Atmospheric and perspective effects would have suggested otherwise. He used this idea in part to calculate the circumference of the earth.</p>

CC BY-SA 4.0
2024-05-01T16:28:12.090Z
Perhaps because the difference in angles after rays traveled 93 million miles given the size difference of the two bodies made for rays close enough to parallel to calculate a close-enough circumference of the earth?
Greg Burghardt
CC BY-SA 4.0
2024-05-01T17:08:49.133Z
@GregBurghardt the question is how did Erastothenes know they were "parallel enough", because he didn't know the Sun was 93 million miles away.   Hipparcus computed the distance to the Moon, but that was about 200 years after Erastothenes, and that measurement was the basis for the first Sun distance measurements.
Greg Miller
CC BY-SA 4.0
2024-05-01T20:44:12.793Z
Cross out "93 million miles" and insert "a longgggggggggggg way" in the comment of @GregBurghardt.
Lee Mosher
CC BY-SA 4.0
2024-05-01T21:00:19.747Z
Physics has an exact duplicate: [How did Eratosthenes know the suns rays are parallel?](https://physics.stackexchange.com/q/81849/329416)
Greg Burghardt
<earth><sun-rays>
Determination that the suns rays are parallel
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# Determination that the suns rays are parallel

(Asked by: username_0 on 2024-05-01)

How did Erastothenes and/or earlier scientist-philosophers determine that the sun's rays arriving at the earth are parallel? Atmospheric and perspective effects would have suggested otherwise. He used this idea in part to calculate the circumference of the earth.

**Tags:** earth, sun-rays

> **Comments:**
> **username_1**: Perhaps because the difference in angles after rays traveled 93 million miles given the size difference of the two bodies made for rays close enough to parallel to calculate a close-enough circumference of the earth?  
> **username_2**: @username_1 the question is how did Erastothenes know they were "parallel enough", because he didn't know the Sun was 93 million miles away.   Hipparcus computed the distance to the Moon, but that was about 200 years after Erastothenes, and that measurement was the basis for the first Sun distance measurements.  
> **username_3**: Cross out "93 million miles" and insert "a longgggggggggggg way" in the comment of @username_1.  
> **username_1**: Physics has an exact duplicate: [How did Eratosthenes know the suns rays are parallel?](https://physics.stackexchange.com/q/81849/329416)  

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