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If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?

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    $\begingroup$ What would you consider the actual "start" of this process? $\endgroup$ – Carl Witthoft Mar 2 at 20:35
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    $\begingroup$ FWIW, in about 1.1 billion years it will be too hot for most lifeforms currently on Earth, long before the Sun starts moving off the main sequence and becoming a red giant. See Timeline of the far future. $\endgroup$ – PM 2Ring Mar 2 at 22:12
  • $\begingroup$ @PM2Ring I quoted you below $\endgroup$ – uhoh Mar 2 at 23:18
  • $\begingroup$ The Sun (or any other star) increases its radiative output almost constantly when at main sequence stage. See "faint young sun paradox". The process is rather slow (say, 20 or 30% for all its past history of, say, 4.5 bn years). If we interpolate over the humanity history (say, 100ky) we get some 0.0001% increase. Not a big deal, since 11-year sun cycle gives about 1% difference. $\endgroup$ – fraxinus Mar 3 at 11:19
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    $\begingroup$ While the expansion of the Sun is enormously slow and not visible on a human time-frame, Earth's eccentricity varies a bit faster, and about 200,000 years ago Earth had an eccentricity of about .05, similar to the Moon's eccentricity. That means, at Perihelion, it was like having a "super sun", similar to the super moons we get today. .05% eccentricity mean a 10% variation in degrees of arc and a 21% variation in brightness that the Sun would swing back and forth every 6 months. I think the idea of super-suns and 21% variation in the visible size of the Sun is kinda cool. $\endgroup$ – userLTK Mar 6 at 21:05
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If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?

I'd say no for a couple of reasons.

  1. @PM2Ring's comment:

    FWIW, in about 1.1 billion years it will be too hot for most lifeforms currently on Earth, long before the Sun starts moving off the main sequence and becoming a red giant. See Timeline of the far future.

  2. Our eyes and vision processes accommodate (adjust for) changes in brightness, we walk indoors from a sunny day to a well lit room and the brightness of things has dropped by 99% or nearly a factor of 100 within a few seconds and yet though we are somewhat aware of the change of tens of percent per second we hardly notice it.
  3. Our vision system is constantly adjusting its white point to accommodate changes in the color of available light. A white sheet of paper looks white to us even as the color of ambient light changes because our vision is always "color balancing" (not exactly the right term but it's something like that).

From @Luaan's comment

It's called "white balancing" - calibrating vision for ambient light to make white surfaces (hopefully) appear white. Sometimes it doesn't quite work properly, and you get fun things like The Dress (since different people's vision chooses different arbitrary ambient conditions, and end up disagreeing wildly about the colors involved)

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    $\begingroup$ It's called "white balancing" - calibrating vision for ambient light to make white surfaces (hopefully) appear white. Sometimes it doesn't quite work properly, and you get fun things like The Dress (since different people's vision chooses different arbitrary ambient conditions, and end up disagreeing wildly about the colors involved). $\endgroup$ – Luaan Mar 3 at 7:17
  • $\begingroup$ @Luaan aaaaaahhhh!!! I've stumbled into meme galaxy! :-) Thanks, I'll make an a̶c̶c̶o̶m̶m̶o̶d̶a̶t̶i̶o̶n̶ edit. $\endgroup$ – uhoh Mar 3 at 9:01

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