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Comment by compscistd | Hacker News Reader

Weird question, how large would an alien spaceship have to be to position itself between the sun and the earth and cause us major problems? Distance would also play a factor as would the degree of “major problem” (varying effects on plant life/temperature). The equivalent of finding a parked car in front of your driveway, on a cosmic scale.

Would be fun to learn the mathematics, physics, and macrobiology to solve this!

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An object capable to completely shade Earth must be larger than Earth itself. The Moon can only shade a small fraction of Earth at a time, see eclipse path maps, for example https://www.timeanddate.com/eclipse/map/2024-april-8.

This is plainly untrue.

Hold a finger up in front of a torch, and look at the size of the shadow cast. Move the finger closer to the torch, and watch the shadow area increase.

A shadow is always larger than the light-facing area of the object blocking the light. This is true even of your own shadow in sunlight, even though your distance from the light source means the change is so tiny you don't even notice it.

                      ------
                ------     S
          ------           S
    ------     A           S
    E          A           S
    ------     A           S
          ------           S
                ------     S
                      ------

    *drawing not to scale

For the shadow to be significantly larger than the shading object, the object must be significantly closer to the light source.

For Earth, in solar orbit, that means you'd have to get inside the orbit of Venus.

Then you have a huge energy cost in keeping your shading object in the correct position relative to Earth. Its natural orbit is not going to work at all.

A finger is comparable size to a torch. Not much light goes around a finger despite the non-zero size of the light source making the rays both non-parallel and originating from different places.

Sol has a diameter of ~100 times the Earth. A shade near Earth would have to be similar size to Earth to block most of the light, and likewise similar size to Sol if it was near Sol.

That's a good point. I made the mistake of treating the sun as a point light source, which of course it is not! https://www.timeanddate.com/eclipse/shadows.html explains the difference clearly.

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