Background story:

Current Earth, in the close future. Astronomers have found first signs of extraterrestrial life: huge sheet-like objects (with surfaces easily as large as the surface of the Earth) appear all around the sun. They do not approach Earth, but remain in stationary spots relative to each other. And it is nearly daily that new objects appear all of them at the same distance from the sun at half of Mercury's orbit.

After several weeks of observation, it is clear that the sheets cannot be natural events – starting with how they keep their exact position relative to each other and relative to the sun, to them being of some strange mix of trans-iron metals, and ending with sheets not being a natural rock/... formation. They MUST have been formed by aliens; even if we cannot make out exactly how they got transported to the sun. They definitely didn't get towed by spaceships or anything normal like that – one day a spot is empty, and the next there is one of those sheets. No clue how they appeared, no radiation anomalies, nothing.

We try communicating with what clearly has to be an alien civilization, but there is no answer to any of our attempts.

Gradually, more and more sheets appear in what is starting to look like a Dyson grid – if not the beginnings of a Dyson sphere.

And then, one day, a text message is displayed on all screens on the entire world: "Hello. We are the GGs, the Good Guys, an advanced civilization from the X system. Since you appear to have advanced to technological toddlerhood and seem to be prone to panicking, it has been decided it would be best to inform you of the current happenings. We are the GGs, after all, and if you kill yourselves due to your poor instinct control and backward fear response, we will have failed our mandate of Protecting All Lesser Races.

"Your star system has come to the attention of the BBs, the Bad Boys, who delight in eradicating entire civilizations. They will be in your neighborhood within the next decade, in two Earth years at the earliest. To prevent your planet from befalling harm, we are currently building a sun-powered defensive shield for your entire solar system. Whenever we turn on this shield, though, it will take the entire energy output of your sun – meaning that to a Lesser Race like you it will look like the sun got turned off.

"Don't worry, we know exactly how long we can keep the shield going without affecting your planet unduly at your current technological level. We will not be the cause of human deaths, and we implore you to keep a well enough order on your planet to not cause deaths amongst yourselves.


"The Good Guys"

After a short demonstration of the full functionality of the sun-off switch, humanity actually believes the GGs. They start to wonder about how long they could expect the sun to be turned off at the longest...


Quick Facts (those who do not want to read the background story):

  • Earth: current technological level
  • Aliens can and will turn off the sun temporarily. This may be a repeated event. Aliens say they won't turn off the sun long enough to damage the planet or humanity, but don't give any exact numbers.
  • Earth has forewarning: earliest sun-out is in two years


How long can the sun be turned off without planet Earth taking enough damage to endanger human survival? (Assume that the sun will be turned on again after this period of time, and humanity/Earth should be able to bounce back within a few months. The next sun-out will come only once humanity/Earth has recovered.)

There is enough forewarning to establish sufficient food stores and shelters etc., so at least a part of humanity should be able to weather the event in pretty comfortable conditions. Afterwards, sunlight will be just as available as it was before again. However, what about

  • Non-human life? How long can plants survive without sunlight? How long until the ecosphere will be damaged so badly that it cannot be... reforested again when the sun turns on again? What about animal life? Species dying out is acceptable, but not to the point of an extinction-level event (less than 10% of all land + maritime species)
  • Oxygen? Does this even become a factor due to the restrictions on killing plant-life and temperature?
  • Temperature? How long until the average temperatures have dropped more than, say, 10 degrees Celsius?
  • Natural catastrophes? Will the sudden absence of sunlight cause huge hurricanes / ice storms /...? What about a continued absence of sunlight?

A ball-park figure would be enough for me; is it a couple of days that Earth can weather only, a couple of weeks, or might Earth even be capable of enduring 2-3 months without sunlight?

EDIT: I am looking for the longest possible time the sun can be turned off in one go. 4 hours daily is not what I am looking for :)

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    $\begingroup$ They can't just use the half away from Earth? And store excess power in some form of battery? (also, how long earth can go depends on frequency. If you 'turn it off' for 4 hours every day, a lot of plants will still die. I think you want "recovery time per hour turned off") $\endgroup$
    – Tezra
    Oct 19, 2016 at 21:15
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    $\begingroup$ The Earth is so tiny compared to its distance from the sun that only about 0.00000005% of the sun's emitted radiation hits the Earth. The GGS could use 99.99999995% of the sun's emitted energy and the Earth would not be affected at all. I wonder if they actually have Earth's best interests in mind... $\endgroup$ Oct 19, 2016 at 21:21
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    $\begingroup$ «one day a spot is empty, and the next there is one of those sheets.» saying they arrive at night is too much of a classic joke. Why can’t space-based solar observatories see them move into position? $\endgroup$
    – JDługosz
    Oct 20, 2016 at 7:20
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    $\begingroup$ @JDługosz: Or just people on the other half of the planet? $\endgroup$
    – MichaelS
    Oct 20, 2016 at 7:54
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    $\begingroup$ Even if they must take the entire current output of the Sun, there is no need to turn off all light to Earth. For the period when the Sun's output is being consumed by the aliens, light stored by routing it on a longer path (which could crisscross the solar system for an arbitrary length/time) could be switched to illuminating the Earth. While this might change the direction from which the light is coming, this technique could be used to have little or no impact on the total light hitting the Earth. $\endgroup$
    – Makyen
    Oct 20, 2016 at 16:31

11 Answers 11


While no one has ever turned off the sun, there are some historical reference points.

Volcanic Eruption Data

Following the May 20, 1883 eruption of the Krakatoa volcano which spewed ash into the air that slightly reduced the amount of sunlight reaching the Earth:

Average global temperatures fell by as much as 1.2 degrees Celsius in the year following the eruption. Weather patterns continued to be chaotic for years and temperatures did not return to normal until 1888.

Mount Tambora's eruption on April 10, 1815 (also in Indonesia) was the most powerful in recorded history:

The eruption caused global climate anomalies that included the phenomenon known as "volcanic winter": 1816 became known as the "Year Without a Summer" because of the effect on North American and European weather. Crops failed and livestock died in much of the Northern Hemisphere, resulting in the worst famine of the 19th century.

In particular:

The 1815 eruption released sulfur dioxide (SO2) into the stratosphere, causing a global climate anomaly. Different methods have estimated the ejected sulphur mass during the eruption: the petrological method; an optical depth measurement based on anatomical observations; and the polar ice core sulfate concentration method, using cores from Greenland and Antarctica. The figures vary depending on the method, ranging from 10 to 120 million tonnes.

In the spring and summer of 1815, a persistent "dry fog" was observed in the northeastern United States. The fog reddened and dimmed the sunlight, such that sunspots were visible to the naked eye. Neither wind nor rainfall dispersed the "fog". It was identified as a stratospheric sulfate aerosol veil.[10] In summer 1816, countries in the Northern Hemisphere suffered extreme weather conditions, dubbed the Year Without a Summer. Average global temperatures decreased about 0.4–0.7 °C (0.7–1.3 °F),4 enough to cause significant agricultural problems around the globe. On 4 June 1816, frosts were reported in the upper elevations of New Hampshire, Maine, Vermont and northern New York. On 6 June 1816, snow fell in Albany, New York, and Dennysville, Maine.[10] Such conditions occurred for at least three months and ruined most agricultural crops in North America. Canada experienced extreme cold during that summer. Snow 30 cm (12 in) deep accumulated near Quebec City from 6 to 10 June 1816.

The second-coldest year in the Northern Hemisphere since c.1400 was 1816, and the 1810s are the coldest decade on record, a result of Tambora's 1815 eruption and another possible VEI 7 eruption that took place in late 1808 (see sulfate concentration figure from ice core data). The surface temperature anomalies during the summer of 1816, 1817, and 1818 were −0.51 °C (−0.92 °F), −0.44 °C (−0.79 °F) and −0.29 °C (−0.52 °F), respectively.7 As well as a cooler summer, parts of Europe experienced a stormier winter.

This climate anomaly has been blamed for the severity of typhus epidemics in southeast Europe and the eastern Mediterranean between 1816 and 1819. The climate changes disrupted the Indian monsoons, caused three failed harvests and famine contributing to the spread of a new strain of cholera originating in Bengal in 1816. Many livestock died in New England during the winter of 1816–1817. Cool temperatures and heavy rains resulted in failed harvests in Britain and Ireland. Families in Wales travelled long distances as refugees, begging for food. Famine was prevalent in north and southwest Ireland, following the failure of wheat, oat, and potato harvests. The crisis was severe in Germany, where food prices rose sharply and demonstrations in front of grain markets and bakeries, followed by riots, arson, and looting, took place in many European cities. It was the worst famine of the 19th century.

A volcanic eruption in 1257 CE in Lombok, Indonesia probably caused the Little Age Age. What happened half the world away (where all of the effects were from loss of sunlight rathe than direct impacts of the volcano):

Reports in 1258 recount the presence of a dry fog, giving the impression of a persistent cloud cover to contemporary observers. Medieval chronicles say that in 1258 the summer was cold and rainy, resulting in floods and bad harvests, with cold lasting from February to June. In both Europe and the Middle East, changes in atmospheric colours, storms, cold and severe weather were reported in 1258-1259. In Europe, too much rain damaged crops and caused famines followed by epidemics. Reports of the effects of the eruption, including failure of crops and famine as well as weather changes, exist for northwest Europe. Crop failures, and a famine in London have been linked to this event. Witnesses reported a death toll of 15,000 to 20,000 in London. Matthew Paris of St Alban retells how until mid-August in 1258, the weather alternated between cold and strong rain, causing high mortality.

Swollen and rotting in groups of five or six, the dead lay abandoned in pigsties, on dunghills, and in the muddy streets.

— Matthew Paris, chronicler of St. Albans,

The resulting famine was severe enough that grain was imported from Germany and Holland. The price for corn increased in Britain, France and Italy. Outbreaks of disease occurred during this time in the Middle East and England. Problems were also recorded in China, Japan and Korea. Other effects of the volcanic eruption include a lunar eclipse in May 1258, where the Moon was completely darkened. With and after winter 1258-1259, exceptional weathers were reported less commonly, but the winter 1260-1261 was very severe in Iceland, Italy and elsewhere.

The K-Pg extinction event

The most pertinent precedent is the K-Pg extinction event (formerly known as the K-T extinction event) 66 million years ago that killed off the dinosaurs clearing the way for mammals:

[A] 10-to-15-kilometre (6.2 to 9.3 mi) space rock hurtled into Earth at Chicxulub on Mexico's Yucatán Peninsula. The collision would have released the same energy as 100 teratonnes of TNT (420 ZJ), over a billion times the energy of the atomic bombings of Hiroshima and Nagasaki.

The consequences of the Chicxulub impact were of global extent. Some of these phenomena were brief occurrences that immediately followed the impact, but there were also long-term geochemical and climatic disruptions that were catastrophic to the ecology. . . . the impact would have inhibited photosynthesis by creating a dust cloud that blocked sunlight for up to a year. Further, the asteroid struck a region of sulfur-rich carbonate rock, much of which was vaporized, thereby injecting sulfuric acid aerosols into the stratosphere, which might have reduced sunlight reaching the Earth's surface by more than 50%, and would have caused rain and ocean water to become acidic. The acidification of the oceans would kill many organisms that build shells from calcium carbonate. At Brazos section, the paleo-sea surface temperature dropped as much as 7℃ for decades after the impact. It would take at least ten years for such aerosols to dissipate, and would account for the extinction of plants and phytoplankton, and of organisms dependent on them (including predatory animals as well as herbivores). Some creatures whose food chains were based on detritus would have a reasonable chance of survival.

New studies show that this caused mass extinctions at levels previously unrealized:

Extinction rates are markedly higher than previously estimated: of 59 species, four survived (93% species extinction, 86% of genera).

The "good news" is that even a year with an at least 50% reduction in solar energy penetration wasn't enough to cause all life on Earth to go extinct, although the vast majority of life on Earth including virtually all "megafauna" did.

The "bad news" is that a far shorter deprivation of sunlight, particularly if it was more complete than the K-T event, would still be a big problem.

The 7 degrees of temperature drop from a 50% reduction in sunlight for on year might be particularly useful in estimating the impact on the climate from a 100% reduction in sunlight for some period of time shorter than that. After all, we know from the lower impact volcanic events that even a 1-2 degree temperature drop for a year or so is a big thing.

The Heat Budget Of Earth

We know from basic thermodynamics that heat loss is a function of temperature difference. Heat radiated from the parts of the Earth where people live is mediated by the atmosphere which is between us and empty space. Empty space is about 3 degrees K.

But, the top of the atmosphere, even after the K-Pg extinction event, would still have been heated by the sun without interruption - less of that heat would have made it to the surface of the Earth, but the heat on the top of the clouds would still have slowed radiation of heat to outer space. To evaluate that you need a grip on the Earth's heat budget in normal times:

To quantify Earth's heat budget or heat balance, let the insolation received at the top of the atmosphere be 100 units, as shown in the accompanying illustration. Called the albedo of Earth, around 35 units are reflected back to space: 27 from the top of clouds, 2 from snow and ice-covered areas, and 6 by other parts of the atmosphere. The 65 remaining units are absorbed: 14 within the atmosphere and 51 by the Earth’s surface. These 51 units are radiated to space in the form of terrestrial radiation: 17 directly radiated to space and 34 absorbed by the atmosphere (19 through latent heat of condensation, 9 via convection and turbulence, and 6 directly absorbed). The 48 units absorbed by the atmosphere (34 units from terrestrial radiation and 14 from insolation) are finally radiated back to space. These 65 units (17 from the ground and 48 from the atmosphere) balance the 65 units absorbed from the sun; thereby demonstrating no net gain of energy by the Earth.

So, about a third of the solar energy normally received on Earth immediately goes into space through radiation, and two-thirds of the solar energy normally received on Earth is absorbed by the atmosphere and ultimately ends up in space, but more slowly, preventing the surface from losing energy all that much faster.

The Precedent Of Nighttime

It would also help to know the typical differences between night and day temperatures:

As solar energy strikes the earth’s surface each morning, a shallow 1–3-centimetre (0.39–1.18 in) layer of air directly above the ground is heated by conduction. Heat exchange between this shallow layer of warm air and the cooler air above is very inefficient. On a warm summer’s day, for example, air temperatures may vary by 16.5 °C (30 °F) from just above the ground to waist height. Incoming solar radiation exceeds outgoing heat energy for many hours after noon and equilibrium is usually reached from 3–5 p.m. but this may be affected by a variety of different things such as large bodies of water, soil type and cover, wind, cloud cover/water vapor, and moisture on the ground.

Diurnal temperature variations are greatest very near the earth’s surface.

High desert areas typically have the greatest diurnal temperature variations. Low lying, humid areas typically have the least. This explains why an area like the Snake River Plain can have high temperatures of 38 °C (100 °F) during a summer day, and then have lows of 5–10 °C (41–50 °F). At the same time, Washington D.C., which is much more humid, has temperature variations of only 8 °C (14 °F); urban Hong Kong has a diurnal temperature range of little more than 4 °C (7.2 °F).

Averaging the high and the low provides about 18 °C of average temperature difference between day and night.

The temperature loss from a single night without the Sun would probably triple in 36 hours without sunlight, so even the best buffered areas might lose 24 °C and the least buffered areas, like deserts, would lose much more, perhaps 54 °C or more, which would be enough to turn hot deserts into frozen deserts. (Cooler temperatures would also shut down evaporation leading to extreme aridity in every place that depends on rain once the exiting moisture in the air was exhausted.)

This suggests that ambient temperature drops will wreck havoc in a matter of hours or days by exposing people and livestock and plants to temperatures that are cold enough to kill them from exposure, while loss of photosynthesis leading to famines and the like (given food stores on hand) might take months; after all, lots of places have months of winter when nothing grows already.


Many people, animals and plants would die from exposure to cold in even a single day entirely without any Sun with a night before and after the period without the Sun (as opposed to merely being obscured by particulates in the atmosphere).

A "safe" period during which the Sun could be turned off would be significantly less than 12 hours.

An entire 12 hours without Sun deprives every place on Earth from Sun for 24 to 36 hours, while 4-6 hours would lead to Sun deprivation for no more than 18 hours continuously in the worst hit areas, and would have less continuous deprivation in many places. Breaking up a continuous period completely without the Sun for even a few hours makes a huge difference relative to having the Sun turn off just before sunrise and stay off until what would ordinarily be sunset in terms of planetary and local temperature drops.

On the other hand, Paige Ksnak's estimate of one degree per minute is probably too high, mostly because the atmosphere is still a blanket that would slow heat loss and buffer the temperature of the Earth until it lost all of its heat, which we know takes more than twelve hours without exposure to the Sun to happen because parts of the planet experience that much time without Sun every day. Indeed, even just six or seven hours of Sun followed by twenty-one hours of dark, while making it much cooler, doesn't cause catastrophe if you're ready for it.

So, I suspect that the practical limit would be driven by temperature losses from sustained continuous periods of time without the Sun and would be on the order of 4-6 hours tops before severe consequences and mass death followed.

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    $\begingroup$ I suspect “temperature loss from a single night would triple, for 36hrs w/o sun” is an overestimate. The temperature ranges you quote are air temperature; ground and water temperature fluctuate much less. In a period without sunshine, ground and water would act as a heat bank, providing a significant brake on the drop in air temperature. So I would guess the answer is somewhere between your estimate and @IndigoFenix’s? $\endgroup$ Oct 20, 2016 at 9:46
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    $\begingroup$ You wrote: “An entire 12 hours without Sun deprives every place on Earth from Sun for 24 to 36 hours [...]”. Actually it's from 0 to 36 hours. Those places where the blackout starts at 18:00 local apparent solar time will only be deprived of twilight, or a little more if it's summer or spring. $\endgroup$ Oct 20, 2016 at 10:42
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    $\begingroup$ The notion is that temperature loss is much greater if two natural nights are added to the day between them without light, leading to a great many hours without solar heat, rather than a situation when several hours of light break up the period between nights. There are precedents of places above the Arctic Circle being entirely deprived of sun for sustained periods, and those places are very, very cold even with the heat of the atmosphere somewhat limiting heat loss. In warmer climate not ready to cope with that, such chills would be deadly to plant and animal life and people too. $\endgroup$
    – ohwilleke
    Oct 20, 2016 at 17:55
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    $\begingroup$ In the 1257 eruption story, you talk about corn prices in England, which is odd, because corn was brought to Europe by Columbus, over 240 years later. $\endgroup$
    – user9981
    Oct 21, 2016 at 17:17
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    $\begingroup$ This is a plant called corn in England which is not maize which is a New World crop. Maize was named after the former in English. $\endgroup$
    – ohwilleke
    Oct 21, 2016 at 20:26

Others are estimating the results of the temperature change, but I wanted to point out another answer from humanity to the GGs:

Do you good guys really need the entire output of the sun?

Earth only occupies a circle 0.005 degrees in diameter from the sun. They could capture effectively all of the sun's energy, but leave a tiny pinhole aimed at our planet and we'd be nearly unaffected.

Lastly, I wanted to address the part where you describe:

but remain in stationary spots relative to each other.
Gradually, more and more sheets appear in what is starting to look like a Dyson grid – if not the beginnings of a Dyson sphere.

Objects in a conventional Dyson sphere are not static relative to each other, and they're not static relative to the orbiting planets or the star they surround. They orbit just like everything else. They're composed of many separate layers of sheets each orbiting the sun.

Don't imagine a stationary soccer ball, with each giant sheet static relative to the others. Instead, imagine a rubber band ball, with each strand consisting of closely-spaced tiny sheets. There would be some tidal forces from other sheets passing above and below each orbit, and some slight course corrections required, but the idea is that it would be basically stable, rather than requiring non-science-based engines to hold the objects static against the unbelievable gravity of the sun.

Also, don't imagine the bladder in the soccer ball as occupying all of the space inside except a tiny foam gap between the bladder and the panels - at half the orbit of Mercury, it's more like a ping-pong-ball in a sphere that can contain a ping-pong table.

And the shell components don't have to appear by magic warp jumps - they could instead do a fantastic dance where they all come in from interstellar space at extreme speeds, gravity brake around Jupiter and aim at chaotic-appearing angles above, below, and next to the sun. We wouldn't see them coming in for a long time if they were very cold and came from interstellar space edge-on to minimize damage from micrometeoroids and interstellar dust. After sweeping around Jupiter, they'd eventually pass through the ever-increasing cloud around the sun by what would appear to be a ridiculous amount of luck but would actually be extremely careful, complicated, precise choreography. They'd aerobrake in the sun's atmosphere (!), pop back out to their target apoapsis, and circularize.

A civilization capable of this choreography should be able to design the swarm such that it always leaves a tiny gap where the light from the Sun can shine on the Earth.

  • $\begingroup$ Presumably one reason they might not want/ be able to apply the pinhole method is if they need the shell to rotate at a different angular frequency than one revolution per earth year. Then the hole becomes a slit across the entire device. $\endgroup$
    – origimbo
    Oct 20, 2016 at 16:30
  • $\begingroup$ That's exactly what you'd do, leave pinholes so all the planets get normal illumination. Otherwise Really Weird Stuff could happen, including the potential breakup of a planet, which could cause Ceti Alpha V. youtube.com/watch?v=Uhu5V8VRxFU $\endgroup$ Oct 21, 2016 at 0:07
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    $\begingroup$ This'd work if the sun was a laser, beaming light directly towards the Earth; but it isn't. The light from the Earth come from all over the hemisphere closest to us. If the aliens provided us with a collimated beam of light, we'd be fine, and wouldn't take much of a sun. what-if.xkcd.com/141 shows what'd happen if you sent us all of it. $\endgroup$
    – Dragon
    Oct 21, 2016 at 2:11
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    $\begingroup$ The swarm elements could be roughly stationary if they float on solar radiation pressure; the math says they'd have to weigh on the order of 1 gram per square meter. If they were, say, aluminum, they'd be around 0.3 microns thick (around 2-3 thousand atoms). The total mass would be on the order of $10^{15}~\textrm{t}$. $\endgroup$ Oct 21, 2016 at 7:56
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    $\begingroup$ @Dragon - You're correct, thanks for catching that. The pinhole would have to be almost as large as the diameter of the sun to leave the earth unaffected. The sun has a radius of about 0.7 million km, and the swarm has a radius of about 30 million km, so they would weakly illuminate an area around earth and lose a lot of energy in the process. But it would still capture more than 99% of the sun's output - I would at least ask. $\endgroup$
    – johannes
    Oct 21, 2016 at 13:20

We could manage for a few days at least without too many bad effects.

What we're dealing with here, essentially, is a planetwide long night; we don't get any direct energy from the sun at night and the amount of heat that bleeds over from the day side is basically insignificant.

It would take about a week for the global average temperature to drop below freezing. Typical ice ages are colder than that; we could expect to see some glaciation and the more sensitive life forms would have trouble adapting but life on the whole would be all right. Humans would be fine.

The biggest problem for these short-term periods is not the cold, but plants dying due to lack of energy. However, even this wouldn't have much of an impact for a while. Small plants would have the biggest problems, but could probably last a week or so; trees and shrubs can store enough energy to last months or even years. Algae tends to go dormant without light, so it could last a while, though oceanic food chains might be disrupted after a few weeks if it wasn't reproducing; not to the point of total extinction though. Temperate regions that were already in the middle of winter might fare best, since most plants would be dormant already (plants that have lost their leaves aren't photosynthesizing anyway). Most seeds would be fine even if their parent plant died.

I don't expect we'd be dealing with too many natural hurricane-type catastrophes; in fact since wind is powered by the sun we could probably expect there to be less storms. However, there would be problems in the opposite direction; no sun means no evaporation, which means no rain. Temperate regions can do without rain for a few weeks or even months, but rainforests where the plants are adapted for constant rainfall might be in for a rough time.

Over longer periods, of course, the problems would be a lot more significant. We'd have snowball Earth conditions after a few months, which would be the end of many terrestrial plants and all marine mammals. Oceans could stay liquid for a long time, maybe even years, beneath the ice, but with all the algae dead most marine life would die after a while anyway. Geothermal power and stored food might allow humans to endure for a while.

So let's say the following estimates:

3 days: No serious problems, just a long night

1 week: Rainforests have problems due to lack of rain, more sensitive life forms die

1 month: Most plants are dead, seeds and maybe a few dormant trees still alive, all but the hardiest of animals die from cold and/or lack of food, humans huddled around volcanoes and eating stored food (at least no problems with refrigeration, right?)

1 year: Average global surface temperature is -100 degrees, everything on land and most sea life is dead, microorganisms living near geothermal sea vents and the animals that eat them are fine and will be for many years to come. Maybe some people in deep-sea habitats and eating stored food can still survive for a while.

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    $\begingroup$ This problem with your post is that this is NOT extended night. During the night we still have the sun, we just don't get it directly. We get both conduction and convection of heat. Hot air comes to us from by wind. That's why it's hotter on a summer night than a winter night. If night was simply no solar energy, than it would be equally cold at night winter and summer. Speaking of which, without the sun, we would be cold... Really cold... Like multiple times colder than the coldest winter in history, because winter is just the sun fractionally farther away, and we are talking about an off sun $\endgroup$ Oct 20, 2016 at 15:56
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    $\begingroup$ @EvSunWoodard Not true. Even at the speed of sound, it still takes 32 hours to go around the equator, so in the middle of the night, there is no way heat from the sun is reaching you via air. Distance from the sun is 100% negligible to the temperature, its Axial Tilt. The earth is closet to the sun in January, but in the Northern hemisphere, its middle of winter in January, and January/February are the coldest months of the year for us. $\endgroup$
    – Ryan
    Oct 20, 2016 at 17:31
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    $\begingroup$ "The biggest problem for these short-term periods is not the cold, but plants dying due to lack of energy." I disagree. It takes extraordinary measures to preserve subtropical and tropical plants from freezes and temperate plants from unseasonal freezes, as massive losses in Florida and Georgia fruit crops every time there is an unusual freeze even a bit before zero degrees C hits. The damage done by the freeze to plants would be permanent and far worse than lost photosynthesis. $\endgroup$
    – ohwilleke
    Oct 20, 2016 at 17:56
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    $\begingroup$ "Even at the speed of sound, it still takes 32 hours to go around the equator, so in the middle of the night, there is no way heat from the sun is reaching you via air." The heat from the sun is not reaching you in that way, but the cloud/atmosphere temperature does not instantly fall to the ambient 3 degrees K at night either, while in a sustained period with the Sun turned off that would start to happen, and the temperature of the atmosphere drives the speed at which Earth's surface cools because the bigger the discrepancy the faster the cooling per thermodynamics. $\endgroup$
    – ohwilleke
    Oct 20, 2016 at 18:02
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    $\begingroup$ @EvSunWoodard: again, this has nothing to do with the distance. The distance differences caused by the elliptic orbit around Sun are way larger than the distance difference cause by axis tilt, still, the influence of the elliptic orbit is almost insignificant. The axis tilt determines the energy/surface ratio and how long an area is exposed to the Sun (aka daytime). $\endgroup$
    – Holger
    Oct 21, 2016 at 14:17

To give a ballpark idea of how important energy received from the Sun is for sustaining energy (and thus life) on Earth:

Earth receives about 1000W/m^2 in energy from the sun. I'm using the number on the bottom adjusted for the reflectivity of clouds. Earth's surface as seen from the sun can be approximated as a circle with a radius of 6.371e6 meters. Good old A=pi*R^2 gives a disc of 1.275e14 m^2

1000J/(sec*m^2) * 1.275e14 m^2 = 1.275e17 J/s received by the Earth from the Sun

The mass of the oceans is 1.37e24 g, average depth is 379400 cm.

1.37e24g/379400 = 3.61e18g in the top 1 cm of ocean; half this number since only half of Earth is facing the Sun; 1.80e18g of water exposed to the Sun

Specific heat of water gives us 4.184 J/g to warm water by 1 degree C. How long does it take to warm the exposed water by 1 degree?

1.80e18g * 4.184J/g * 1s/1.275e17 = 59 seconds

So if we're approximating Earth to be made of water (rough estimate, of course; dirt takes less time to heat AND less time to cool), in just one minute of sunlight, you get enough heat on Earth to warm the top centimeter of the oceans by a degree. Doesn't sound like a lot at first? Don't forget that by missing out on sunlight, you're creating a big heat disparity; ocean currents will speed up as they try to equalize the cool spots. Fish in the upper layers of oceans will die quickly without needed heat from the sun. Photosynthetic organisms will be starved of energy; maybe they'll be okay missing out on a few minutes of sunlight a day (maybe), but much more than that and the populations of a multitude of organisms will take a nosedive as the effects add up.

  • 1
    $\begingroup$ Thanks for those awesome calculations! What I don't get: why would missing out on sunlight create a heat disparity? I would have expected the heat disparity to be created by sunlight (since it's heating the water instead of letting it cool down as it likes on the night side), $\endgroup$
    – subrunner
    Oct 20, 2016 at 5:56
  • $\begingroup$ Bit over the top, that calculation. 4.1J/g/K / (1000 J/s/m^2 / 10000g/m^2) = 41 s/K. Earths albedo is 0.3, so add another factor 3.3. $\endgroup$
    – Karl
    Oct 21, 2016 at 3:40

The Arctic winter illustrates how quickly the planet responds to a change in input (what climate science calls 'forcing'), although if you accept the CO2 drives the climate meme, then the response time must be orders of magnitude slower than seasonal transitions.

Of course, the premise is wrong since if you enclose the Sun with a sphere, that sphere will heat up and radiate energy towards the Earth and in the steady state, will be emitting as much energy as its absorbing. If the sphere was 'insulating', the space between the Sun and the sphere would heat without bounds and become hot enough to ionize the atoms comprising the sphere and it would become a plasma like the surface of the Sun itself.

Funny, how these basic constraints of thermodynamics are so often ignored, even by many scientists ...

Mankind would persist until we ran out of food or things to burn, whichever comes first, but that could be centuries, although the first few billion people and most of the rest of the planets biomass would die within weeks. Frozen, dead biomass will last decades to centuries, so food and fuel will remain available to those that remain for a long time.

  • 3
    $\begingroup$ "we are currently building a sun-powered defensive shield for your entire solar system" <-- They are utilizing the sun's power for some mystical shield, so I assume a lot of that heat energy will be put to use. $\endgroup$
    – LukeN
    Oct 20, 2016 at 20:38

To give you guys some perspective:

During winter, the effective output of the sun is 50% in the upper latitudes. If you've ever been to northern countries during winter, you know that it can get to -40C during cold periods.

What would happen if the sun were to be turned "off"? For continental locations, it would be worse than winter, temperatures will go below -100C in less than a week.

Hourly Temperature Graph Maritime Region
In a maritime region, after 21 hours, temperatures dropped by 4.4C, or 5C per day. So after a week, it would be a drop of temperature of about 35.2C

Hourly Temperature Graph Inland Region
Inland, the temperature drop after losing sunlight is about 6C in 9 hours, or 16C per day. After a week, it would be about a loss of 112C.

Maritime regions will be luckier, the sea stores a lot of heat. Normally, the seas' temperature is a gradient from hottest at the top to coldest at the bottom (except rifts and vents). But if the sun were to go dark, the gradient will reverse. Hot water will still rise, but it will still cool fast enough so that it becomes freezing near the surface. Unlike what most people suggest, the oceans will not be able to give out all its heat effectively, since once the water surface temperature drops below -5c a thick crust of ice will form at the surface, blocking heat transfer and acting as an insulating shield. Even if most oceans will stay liquid under the surface for quite a while, Maritime life will quickly die out though due to lack of sunlight and food.

In short, tropical regions have at best one day before suffering irreversible damage. Artic regions might have 3-4 weeks as a conservative estimate, as most lifeforms there have abilities to survive long periods of glaciation. But expect polar bears and penguins to die out.

  • $\begingroup$ Seawater freezes at -2°C, but it has no density maximum at 3°C as fresh water, so the surface will not freeze before the ocean has cooled down completely. How should the water cool before it reaches the surface? $\endgroup$
    – Karl
    Oct 20, 2016 at 20:09
  • 1
    $\begingroup$ Do you have a source (or a plausible calculation) for your "will go below -100 °C in less than a week"? $\endgroup$ Oct 20, 2016 at 21:28

Umm ... since scientists have never performed experiments about turning off the sun in a laboratory, we don't know the exact implications. But I can think of some detrimental changes.

1- The sun is our primary source of heat and light. Turning off the sun for 30 minutes straight will drop the temperatures on the night sides enough to freeze water. We are talking about equatorial regions here.

2- Also expect some extremely vicious, huge vortexes forming due to the sudden difference of air pressure as the sun goes out. We are talking about storms like on Neptune. You don't want to get these. One such vortex (if the wind speeds reach Neptunian storm speeds) would easily wipe out any and all traces of human settlement in its region. Also => forget all wildlife and forests in the region.

3- Ocean life will suffer irrepairably. Turning off the sun for an hour daily will have immense implications for phytoplankton which would in turn ripple out into the entire marine food webs.

4- Also, the circadian rhythm of all complex creatures would be badly disturbed.

Conclusion: it is very very bad to turn off the sun. We don't know precisely what it would result in, but even turning off the sun for one hour daily for extended periods of time (for a couple months) would have long term and irrepairable consequences for climate patterns and ecology.

  • 16
    $\begingroup$ Point 1 is unbelievable. The night side of Earth gets no direct heat from the sun, and night lasts for hours. Point us to your source for this? Point 2 is also implausible: we get no such vortices at nightfall. Point 3 is implausible: a few days of really thick cloud have no noticeable effect on phytoplankton. Point 4 seems quite unlikely: total eclipses of the sun have no noticeable effect on circadian rhythms. $\endgroup$ Oct 19, 2016 at 22:46
  • 2
    $\begingroup$ @JohnDallman: I don’t understand your counter-argument to point 4.  Even if “total eclipses of the sun have no noticeable effect on circadian rhythms” were true, so what?  Eclipses of the sun last a few minutes; subrunner is asking about days, weeks, or months of darkness.  But your statement isn’t true; animals respond to an eclipse as they do to sunset.  Diurnal animals might not know when to wake up; nocturnal animals might stay active until they drop from exhaustion, etc. $\endgroup$ Oct 20, 2016 at 0:55
  • 4
    $\begingroup$ @DavidCram: How is the heat hitting one side of the Earth reaching the other side so fast? The atmosphere doesn't circulate nearly that quickly. $\endgroup$ Oct 20, 2016 at 7:44
  • 4
    $\begingroup$ OK, the night side would cool down to the degree that it normally does, and carry on doing so when there would normally be dawn. Ice will start advancing from the polar seas quite soon, but the claim in the answer that tropical seas on the night side would freeze over in 30 minutes is nonsense. $\endgroup$ Oct 20, 2016 at 12:40
  • 2
    $\begingroup$ @JohnDallman: Where is the claim that the tropical seas would free over in 30 minutes? Read clearly. I wrote the temperature would be enough to freeze water, not large bodies of water. It is one thing to freeze a glass of water and a whole other to freeze an ocean even if both are subjected to the same temperature for equal amount of time. Don't assume things I haven't written. $\endgroup$ Oct 20, 2016 at 13:28

I think answers that equate a 'turned off' sun with an extended night time are wrong. The Earth is ALWAYS getting warmed by the sun. It's 24/7 365(366 on leap year) energy. Even if your side of the planet happens to be on the opposite side from the sun there is a tremendous amount of energy being brought to you from the day side by winds. With the sun 'turned off' you don't get that.

I'm guessing it's minutes at best before some negative effects are noticed and major extinctions in less than a day.

  • 2
    $\begingroup$ Hello, and welcome to the site! You haven't provided any detail to your answer, but are rather commenting on other answers. This answer would perhaps be better served as a comment on the question. Alternatively, you could expand your last sentence to explain what negative effects would occur and how, and why/how major extinctions would occur in less than a day. $\endgroup$
    – Azuaron
    Oct 20, 2016 at 14:24
  • $\begingroup$ Please have a look at usual wind speeds compared to the rotational speed of the earth: it takes much longer than a night even for the fastest winds. $\endgroup$ Oct 20, 2016 at 21:25

First comment which I've not seen mentioned is that casualties will begin to mount immediately. People relying on daylight will be plunged into darkness and some will die because of it. Second point is that the Sun's flux varies by ~0.1% over the solar cycle. It isn't reasonable that the 0.00002% falling on the Earth at any one instant would make or break the GG's response to the BGs. Third point (which you probably know) is that the physics behind your Dyson sphere is seriously aphysical, requiring technologies and materials which contradict what we (think we) know about Science.

  • $\begingroup$ "the physics behind your Dyson sphere is seriously aphysical" -- can you elaborate? $\endgroup$
    – LukeN
    Oct 20, 2016 at 20:36
  • 1
    $\begingroup$ Right about the small amount of light falling on earth, and the Dyson sphere, but what's "people relying on daylight"? That's quite a long life of death that would befall them every night. $\endgroup$
    – Karl
    Oct 21, 2016 at 3:24
  • $\begingroup$ losing all light suddenly while driving 80 on the freeway is going to cause some fatal accidents, just from the surprise. $\endgroup$
    – Leliel
    Oct 21, 2016 at 3:50


If we're mainly concerned with avoiding irreversible changes to climate, then the Sun can be turned off for about 200 days. This is a low estimate, and slightly longer times would probably be fine. However, if the Sun is turned off for a year or so, then the Earth is likely to enter a "snowball state", where the oceans are frozen over and the ground is covered with snow. In a snowball state, even when the Sun is turned back on, the surface is white and reflective, so sunlight is ineffective at warming it back up. This could result in permanent glaciation.


This is based on a rough order-of-magnitude estimate, so the answer could be off by a factor of two or so.

If the oceans freeze, then the Earth will enter a stable "snowball" state, which is almost impossible to recover from because ice reflects sunlight, and even when the Sun is turned back on, it will not be efficient at warming the Earth back up.

Since water has a very high heat capacity, the oceans store most of the Earth's surface heat. However, the deep ocean does not mix with the surface very well. Oceans have a wind-mixed layer, which is about 50 meters deep on average, which remains at nearly uniform temperature. Water below this layer is more or less thermally insulated from the surface, and does not affect the calculation.

For the oceans to begin to freeze, the wind-mixed layer would have to be cooled from its present average temperature (about 16 degrees C) to the freezing temperature of ocean water (about -2 degrees C).

The wind-mixed layer is about 50 meters deep, so each square meter of ocean corresponds to 50 cubic meters of water. The heat capacity of water is around 4200 J per kg per degree C, so cooling down 50 cubic meters of water by 18 degrees C requires losing (50 m^3) x (1000 kg / m^3) x (4200 J / kg deg C) x (18 deg C) = about 3.8 billion joules.

The rate of heat loss per unit area is proportional to temperature raised to the fourth power (Stefan-Boltzmann law). However, Earth has an atmosphere, which acts as a thermal blanket, so the correct temperature to use is not the surface temperature, but the "effective temperature" as seen from space, which for Earth is about -21 deg C, or (273 - 21) = 252 Kelvins. Using the Stefan-Boltzmann law, Earth will radiate at:

sigma T^4 = (5.7 x 10^-8 W / m^2 K^4) x (252 K)^4 = 230 W / m^2.

So, each square meter of surface will lose 230 Joules of energy per second. Normally, this is balanced out by heat received from the Sun, but when the Sun is turned off, this heat loss will cool and eventually freeze the surface. The oceans will begin to freeze when they've lost around 3.8 billion Joules per square meter, which will take (3.8 x 10^9) / 230 = 16.5 million seconds = 190 days.

Now, in reality, as the Earth cools down, it will lose heat slightly slower, so this time will be a little bit longer. In addition, the ice crust will at first be thin, and could easily be melted, and will take some time to develop to a stable thickness. So, 190 days is a safe lower limit. If the time is doubled, however, then the chance of a "snowball state" becomes very high.


So the aliens plan on turning off the sun, which is technically an exploding star?

If the sun turned off, I think we should be more worried about an event related to gravity, rather than enough energy to keep our little planet at an ideal temperature. The g-waves would most likely shatter our atmosphere and flip the planet inside out. This would be an interesting event to watch live!

Imagine our solar system as a plain circle with the sun in the middle. Then imagine a box surrounding the circle, which would be this "solar shield" the aliens speak of. Then imagine the same image, just no sun. This of course means taking away the color, the energy, and the gravity...and then take the same image, with the sun turned on again, and everything inside the box is all messed up due to the alteration of gravity.

And then the other issue is actually getting an exploding star to "turn on" again. Have you tried to re-ignite a burned match? Its not the same.

I would change the story.

  • 5
    $\begingroup$ "Turning off" the sun in this question is due to using all of its energy output as with a dyson sphere, not completely removing it from existence or actually extinguishing it. $\endgroup$
    – Kevin
    Oct 20, 2016 at 16:56

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