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An eclipsing binary is a binary star system that periodically eclipses one another. There is a drop in total brightness when the primary eclipses the secondary and a smaller drop when the secondary partially eclipses the primary.

If an earth like planet were to orbit the center of mass of the system and be in a position that periodically produces these eclipses, would the inhabitants notice these changes in brightness.

I am working under the assumption that the period of the binaries is on a human time scale on the order of days, months, or years. I am also working under the assumption that the primary is much brighter than the secondary.

Edit: I probably should of rephrased the title. What I want to know is whether the change in brightness would be noticeable. Think of a total solar eclipse darkening the sky. Would anything to that effect happen at either drop in brightness.

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  • $\begingroup$ Welcome to worldbuilding. What do you mean with the planet be in a position that periodically produces these eclipses? $\endgroup$
    – L.Dutch ♦
    Commented Nov 8 at 3:20
  • $\begingroup$ Assuming you mean "periodically experiences these exlipses," I find it difficult to believe that any planet in the goldilocks zone of a binary star system would be capable of missing the fact that there's two really bright spots out there and that those spots periodically cross one in front of the other. Unless the binary is the combination of a super massive star combined with something like a white dwarf such that the planet's goldilocks zone is really far away then maybe, maybe, it could be missed. Maybe the better Q is to ask the conditions where it could be missed. $\endgroup$
    – JBH
    Commented Nov 8 at 3:26
  • $\begingroup$ I meant that it's orbit is somewhat parallel to the orbits of the stars around each other so that the eclipses are a more regular occurrence. The 3 bodies are close to all moving on a 2D plane in space. $\endgroup$
    – Quinali Solaji
    Commented Nov 8 at 3:28
  • $\begingroup$ @JBH I'm more interested in whether the sky would get noticeably darker during the eclipses. Although you raise an interesting question. $\endgroup$
    – Quinali Solaji
    Commented Nov 8 at 3:35
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    $\begingroup$ @QuinaliSolaji It's a question of ratio. If the luminosity and size of the two stars are identical, you see the greatest darkening effect as only 50% of the light gets through (I'm assuming the distances between starts and the planet allows for a perfect occlusion of the distant star). That's a 1:1 ratio. As the ratio increases, the darkening effect decreases. Obviously this is complicated by the types/colors/luminosities of the stars, but I believe that's the basic rule. $\endgroup$
    – JBH
    Commented Nov 8 at 3:46

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Based on my personal experience as a human, the answer would be no.

This is based on my experience of a partial solar eclipse. Because our sense experiences the brightness logarithmic, even a 30% drop is not something that you notice unless you are looking for it.

This is based on an assumption that you are asking about a day to day experience of an uncaring person. Different in brightness or colour could be possible spotted by a trained person.

Detecting and tracking of such binaries would be possible with stone age astronomical instruments such as a stick that casts a shadow or a camera obscure. Or watching the sunset.

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    $\begingroup$ I mostly agree with this. You can tell it's darker if you're paying attention, shadows would potentially look different (in this case sharper), and your solar panels would definitely register the drop, but people going about their day wouldn't necessarily notice $\endgroup$
    – Zags
    Commented Nov 8 at 19:36
  • $\begingroup$ Shadows would definitely be sharper when one star was behind the other. $\endgroup$
    – Loren Pechtel
    Commented Nov 11 at 21:38
  • $\begingroup$ Even 99% is something that can be hard to tell--a cloudy day is 1% as bright as a sunny day but the 'cloudy day' feeling is mostly caused by the lack of sharp shadows--you don't squint to see things or anything. 99% totality looks weird because of the shadows, but if you aren't looking for that, it feels like "strangely cold sunny daytime" until totality fully hits $\endgroup$
    – Kaia
    Commented Nov 15 at 20:20
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Measuring the variation in the light curve caused by the mutual eclipsing is one of the methods used to detect binary stars.

Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity when one star passes in front of the other. The brightness may drop twice during the orbit, once when the secondary passes in front of the primary and once when the primary passes in front of the secondary.

If we can detect the luminosity change while being several light years away, for sure anyone orbiting the binary will notice it.

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  • $\begingroup$ I doubt anyone would notice the dimming without instruments. We have evolved to ignore unimportant things like the sun gradually dimming because of a cloud going by. People don't look up unless the dimming is rabid, such as a large bird flying between you and the sun casting a sharp-edged shadow, as such birds might have attacked our children in the past. Dimming over a period of days is not going to be noticable - would you notice if it's 10% more overcast than yesterday? $\endgroup$
    – Pete Kirkham
    Commented Nov 9 at 15:39
  • $\begingroup$ @PeteKirkham, people have noticed the sun getting weaker from June to January since well before measuring instruments more advanced than human eyes were invented. $\endgroup$
    – L.Dutch ♦
    Commented Nov 9 at 16:42
  • $\begingroup$ The sun does not get noticable weaker because the Earth's orbit is almost circular. They do notice that the days are shorter, and the sun is lower in the sky at noon, neither of which is going to happen with a binary. I've got sunburnt up a mountain on a bright winter day, and varying one day to the next is unlikely to be enough to create a seasonal temperature change that is more than weather. $\endgroup$
    – Pete Kirkham
    Commented Nov 9 at 17:34

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