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On an alternative Earth live the people of Tcelonia.

One of their founding myths goes that, in the dawn of times, when they were under the menace of being attacked by an enemy population, for an entire month the night sky of the entire planet remained as bright as under the full moon, frustrating all the attempts of a surprise attack of the enemy who, at the end, gave up an left.

Since then, continues the myth, the favor of the gods has never left Tcelonia, which thrived and prospered.

What natural event can reasonably explain the characteristics of the phenomenon?

  • Night sky as bright as when the full moon shines
  • Duration of one month
  • Spanning the entire dark side of the planet
  • No severe and adverse effects on life

A single event is preferred, however I am also open to combination of more events leading to the same result. The less the events, the better.

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    $\begingroup$ Supernova? $\endgroup$ – nzaman Dec 21 '18 at 11:31
  • $\begingroup$ @nzaman, can it brighten up the sky like the full moon for a month without wiping out life? $\endgroup$ – L.Dutch Dec 21 '18 at 11:32
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    $\begingroup$ From the link above "The supernova SN 1006 ... was the brightest recorded star ever to appear in the night sky, and its presence was noted in China, Egypt, Iraq, Italy, Japan and Switzerland. It may also have been noted in France, Syria, and North America...one-quarter the brightness of the Moon." " SN 1054 may have been four times as bright as Venus, and it remained visible in daylight for 23 days and was visible in the night sky for 653 days." $\endgroup$ – nzaman Dec 21 '18 at 11:38
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    $\begingroup$ @nzaman Four times as bright as Venus is still far dimmer (about seven magnitudes, or a factor of around 500) than the full Moon. But it’s true that a supernova that was as bright as the full Moon still wouldn’t damage the Earth. $\endgroup$ – Mike Scott Dec 21 '18 at 11:47
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    $\begingroup$ How about having a megawatt laser on every square meter of the surface of Asia and point them at the moon? $\endgroup$ – VLAZ Dec 21 '18 at 11:53
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Supernova. A type II-P supernova maintains a fairly constant brightness for several months, and one at an appropriate distance (something like 500 light years) will be as bright as the full Moon for that time period. It will need to be on the opposite side of your planet from its own star, as seen from where the planet is in its orbit. Betelgeuse will do the same for the Earth, some time in the next million years. At that distance, it will not have any adverse effects on the planet or its biosphere — the light will be all that’s noticeable.

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    $\begingroup$ Apparently I overestimated the damaging potential of a supernova... $\endgroup$ – L.Dutch Dec 21 '18 at 11:48
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    $\begingroup$ @L.Dutch Supernovas are very destructive, and you wouldn’t want to be within 100 light years of one, which is a vast volume of space. But they’re even brighter than they are destructive, so they are bright over an even larger volume of space. $\endgroup$ – Mike Scott Dec 21 '18 at 11:55
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    $\begingroup$ @BladeWraith It would be easily noticeable to our modern technology, but not to a mythical pre-technological civilisation at the dawn of time, which is what we’re talking about here. The light would be all that they could detect. $\endgroup$ – Mike Scott Dec 21 '18 at 11:58
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    $\begingroup$ Those who enjoy this answer should check out Clarke’s short story “The Star.” $\endgroup$ – KRyan Dec 21 '18 at 16:40
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    $\begingroup$ >Likely the brightest observed stellar event in recorded history, the intensity of SN 1006's light was a little more than ["]a quarter that of Moon light" $\endgroup$ – Mazura Dec 21 '18 at 19:24
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A volcano may have erupted that sent a lot of fine ash onto the upper atmosphere.

However, unlike the volcanoes on Earth and due to a different chemical makeup, this volcano's ash did not keep sunlight from coming in. It also refracted light reasonably well, and by night the air over the terminator and towards the night side refracted and reflected a significant amount of sunlight back to the planet. A bystander at the equator at midnight would see a fully dark sky atop their head, but the horizon would have an eerily beautiful Belt of Venus that would shine as bright as the full moon.

After a month the ash dissipated.

Cue to end of times cults arising whenever that volcano goes off again.

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    $\begingroup$ I like this answer because if the story continues in historic times, it can have a lot of story significance even if the population themselves aren't versed in astronomy. $\endgroup$ – nostalgk Dec 21 '18 at 20:43
  • $\begingroup$ High altitude ash will cause a glowing sky after sunset and before sunrise but not through the night. $\endgroup$ – Loren Pechtel Dec 23 '18 at 4:54
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Aurora.

aurora https://photographynewsblog.wordpress.com/2012/05/01/aurora-borealis-bright-ccanada/

The aurora borealis (northern lights) form when charged particles emitted from the sun during a solar flare penetrate the earth's magnetic shield and collide with atoms and molecules in our atmosphere. These collisions result in countless little bursts of light, called photons, which make up the aurora...

The aurora borealis most commonly occur between 60°-75° latitude, but during great geomagnetic storms the auroral oval expands equatorially and can reach 30° latitude or further.

https://www.aurorahunter.com/what-causes-the-northern-lights.html

The aurora borealis can light the sky as bright as a moonlit night. I have seen it. In your world, as in ours, exceptionally bright aurorae happen during solar storms.

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  • $\begingroup$ But would aurora last a month? Coronal mass ejections are discrete events and while there are several a day during peaks of solar activity, they move in separate direction, so only small fraction of them hits the Earth. $\endgroup$ – Jan Hudec Dec 22 '18 at 21:51
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    $\begingroup$ @JanHudec - I do not know what governs the length of solar storms, or how long they can be. I found this link which describes one in historical times that lasted 9 days. wired.co.uk/article/…. Other stars might experience longer ones than our sun. There are a lot of different kinds of stars! $\endgroup$ – Willk Dec 22 '18 at 22:04
  • $\begingroup$ But would a 1 month long solar storm strong enough to light up the sky until the equator be safe for life? $\endgroup$ – L.Dutch Dec 23 '18 at 5:02
  • $\begingroup$ I know aurora-causing solar storms can mess up electronics (via EMR) but I have never heard that they pose a danger to life. It is an interesting question. Earth's magnetosphere lets radiation through but must be good enough to block all the charged particles, causing them to expend their energy as visible light. If you are concerned for your world with its more energetic sun and brighter aurora, ramp up the internal dynamo of your planet to augment its magnetospheric shields. More particles from the star + stronger planetary shields = HUGE LIGHT SHOW! $\endgroup$ – Willk Dec 23 '18 at 16:33
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Another sun passed by the solar system and disappeared into the black void again.

You only need 'moon brightness' so it can be far enough away to not present any danger (it will be farther away than our sun so tidal effects are less).
You'd have to calculate the actual distance for the appropriate brightness, but you can vary the intrinsic luminosity, so that gives you enough variation. And if the distance is such that it passes through the outer edges of the solar system and rips something away, who cares.

The brightness will not rise abruptly, but given enough speed, you can make a period of a month with 'significant' light plausible.

The sun will have to pass on the side of earth diametrically opposed to the sun, but that also is possible.

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  • $\begingroup$ Was about to post this, so I looked for rogue white dwarf and found this: universetoday.com/117778/… $\endgroup$ – Asoub Dec 21 '18 at 12:12
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    $\begingroup$ Unfortunately thay sun would be closer to this planet than Jupiter is to Earth, which would cause the planet to change orbit. $\endgroup$ – Renan Dec 21 '18 at 13:30
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    $\begingroup$ and rips something away, who cares. - We do, because that's us, if not the entire solar system. $\endgroup$ – Mazura Dec 21 '18 at 19:17
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    $\begingroup$ I don't think this works. The Sun would have full-Moon brightness at a distance of 0.15 light-years, which is still close enough to wreck the Solar System. Since the mass-luminosity relation has an exponent of 3.5, brightness increases faster than gravity, and a sufficiently bright star can be far enough away to not cause problems. However, to get the desired one-month duration of brightness, the star may need to be moving faster than light. $\endgroup$ – Mark Dec 21 '18 at 22:59
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    $\begingroup$ It's going to slowly rise and slowly fall, I don't think it's going to meet his requirements. $\endgroup$ – Loren Pechtel Dec 23 '18 at 4:55
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If the solar system encountered a homogeneous debris field with just the right size of rocks, that were big enough to not be pushed away by the sun's bow shock, and small enough to not get sucked into Jupiter and to definitely burn up in our atmosphere... then yeah ;)

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  • $\begingroup$ I'm making all this up. Does Jupiter even ever intersect its own previous orbit, or is the sun going too fast? We kinda need Jupiter to do its thing. "homogeneous" is a handwave... $\endgroup$ – Mazura Dec 21 '18 at 19:08
  • $\begingroup$ The problem with that is the amount of debris collising with planets. It has happened before, and is orobably how we got oceans. But I don't think macroacopic life would handle that very well should it happen again. See Late Heavy Bombardment $\endgroup$ – Renan Dec 21 '18 at 21:00
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    $\begingroup$ I like it. Most "shooting stars" are about the size of a grain of rice. So imagine an enormous number of those all at once, and it could light up the sky without anything hitting the ground. Plus it would be pretty spectacular to see, adding to the memorability of the event. $\endgroup$ – nasch Dec 21 '18 at 21:11
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There's a nearby black hole that has no companion so it's normally pretty quiet. Something wandered by and got sucked in--not a direct hit but it came close enough it was shredded and now the black hole is very active while it eats the stuff that got trapped in the accretion disk.

As there is no ongoing source of mass the disk will in time be eaten and the light will fade away.

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