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I am trying to understand why we can assume that all light rays from the Sun are in parallel.

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    $\begingroup$ If sun rays were all just moving radially then the sun would be seen as a tiny yet very bright speck. $\endgroup$
    – Zac67
    Commented Jun 10 at 20:33
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    $\begingroup$ They are not parallel, and are only assumed to be at large scales. The shadow Eratosthenes used to measure the circumference of the Earth would have had a fuzzy edge due to the slight angle of the photons emmitted at the edges of the Sun. His experiment only worked because he measured the different angles over a very large distance. Someone doing more modern celestial navigation with a sextant will only use the very edge of the Sun to get precision to within a mile. $\endgroup$ Commented Jun 10 at 21:43

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No. They are given out in all directions.

The rays are parallel just because the sun is far away. Any photons that reach you from the sun must be going in the same direction because they have both travelled from the sun to you.

But not exactly in the same direction. This is why the whole disc of the sun appears bright; photons will travel towards you from both the top and the bottom of the sun's disc as you see it.

But if I have two holes in a piece of card, the sun's rays as they pass through the holes will both have come from the sun, so will have had to travel parallel.

Another way to think of this is just to note that the sun appears to be in the same direction in the sky no matter where you view it from, because it is very distant. The rays of the sun are all parallel to that direction.

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  • $\begingroup$ a small caveat to "...the whole disc of the sun appears bright" might be limb-darkening $\endgroup$
    – uhoh
    Commented Jun 13 at 0:15
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The Sun's photons get emitted uniformly in all directions, but we only see the photons that get emitted in our direction.

At the Earth's surface, the Sun's angular diameter is about half a degree. So any illuminated surface point is receiving rays within that narrow cone.


The Sun's angular diameter varies over the year, as the Earth-Sun distance changes. Here's a graph from a previous answer of mine:

Sun Angular diameter

(There are 3600 arc-seconds in 1 degree).

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  • $\begingroup$ a small caveat to "...photons get emitted uniformly in all directions" might be limb-darkening $\endgroup$
    – uhoh
    Commented Jun 13 at 0:16

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