Convenient Eclipse

The heroes are in a sticky situation somewhere. The writer needs them to be somewhere else in the next act/scene, but no established plot device will allow this to happen. So the writer makes the story happen at a specific time which provides a (just about) plausible solution. On any other day of the year the heroes would be caught/killed, but sheer dumb luck on their part (or sheer bad luck on the antagonist's part) yields an unexpected outcome.

A convenient way to set up a God Guise. Compare/contrast with Deus Ex Machina, Contrived Coincidence. For literal eclipses, see Total Eclipse of the Plot.

Comic Books

 * Tintin used a Convenient Eclipse in "Prisoners of the Sun": when held prisoner by a surviving group of Incas and pending execution by sun-lit pyre, Tintin claims the date of the eclipse is Captain Haddock's birthday, causing the Inca priest to schedule their execution for that day. During the day itself, Tintin fakes being able to command the sun and the Incas let them go. It's a little more believable than many examples of this trope, as the Inca leader tells Tintin that he must die within a month, but can choose which time for the execution (being a full month, the chance that an eclipse actually would occur in that time period is a little higher) On the other hand, this ends up creating a factual error: the Incas were skilled astronomers and knew what eclipses were and how to predict them.
 * Also, Tintin was given a newspaper that had astronomical tables printed in it. Good thing they didn't give him the sports section.
 * Parodied in the Don Rosa comic story "The Once and Future Duck", where Donald Duck is about to be executed by (the historical) King Arthur, and he orders his nephews to wow Arthur by predicting an eclipse. After protesting "That only works in old movies and comic books, Unca Donald!", they reply "You have two options: get them to pack up camp and move to Madagascar, or stay here and convince them to delay the execution for 237 years!" Don is instead saved from execution when Gyro beeps the horn on his truck and scares the whole camp silly.

Film

 * Ladyhawke. "Night without a day, day without a night."
 * In The Fugitive, Harrison Ford leaves the federal lockup with Tommy Lee Jones in hot pursuit. However, a conveniently passing St. Patrick's Day parade provides the perfect cover for the hero to escape. On most other days of the year, the streets would be relatively empty and Harrison Ford would be captured within moments. (It should be noted that this sequence was a late addition to the script. As filmed, Jones' exit from the lockup is delayed briefly by a security door closed in an attempt to halt Ford; as originally scripted, the delay would have been long enough for Ford to get away.)
 * Apocalypto: Just as Jaguar Paw is about to be sacrificed via heart-ripping by a Maya priest, a solar eclipse makes the priest believe that their god is satisfied with the sacrifices already done, saving Jaguar Paw's life. This seems unlikely, since Maya astronomers and priests understood eclipses as well, and should have known it was going to happen anyway. Pre-Columbian South American civilizations just can't catch a break, it seems.
 * Well they aren't exactly  now are they?
 * The priest looks at the eclipse, gives a small smile to himself and ends the sacrifices. He knew it was coming.
 * Sort-of inverted in From Dusk till Dawn 2: the hero escapes the vampires into the sunlight... Then guess what happens. Note that the moon must have wanted the vampires to win; it literally comes racing across the sky then screeches to a halt when it reaches the proper position.
 * The musical remake of Little Shop of Horrors has the evil plant coming to Earth during an unexpected eclipse. Never mind the fact that real eclipses are predicted decades in advance, so there's never an "unexpected" one.
 * Which would make an alien ship passing between Earth and the Sun pretty unexpected, huh?
 * Die Hard 2 has the bad guys heavily relying on a bad storm on the exact day they need to hold the airport to ransom. On a day with good visibility, it would have been nearly impossible to trick pilots into crashing by messing with the landing systems.
 * In Robin Hood (the Patrick Bergin version), the heroes need to gain access to the Big Bad's castle. They use the fact that it is All Fool's Day, and no group observing the festival can be denied admission, to get in and execute their plan. On any other day of the year, presumably, the castle guards would simply reply by riddling the merry men with crossbow bolts.

Literature
"In fact, to state the Matter plain, The Sun will be eclips'd again ... To aid a troubled English Gent This Astronomical Event Is by some Holy Power sent! (And is, by Scepticks, thought to be Suspicious in its frequency.)"
 * In A Connecticut Yankee in King Arthur's Court (and any similar guy-from-now-ends-up-in-medieval-times plot), the hero just happens to be around on the same day as a total eclipse, which he can use to his benefit. On any other day of the year, he would simply be run through.
 * Did anyone else wonder how on Earth a factory foreman from 19th-century Connecticut could have predicted that? Honestly, how many people here who aren't astronomers can name the date of ANY eclipse, ever, let alone one over 1,000 years ago? They weren't even using the Gregorian calendar in King Arthur's time!
 * Rule of Cool?
 * Occurs in the book King Solomons Mines (though not the movie versions). The heroes use a convenient lunar eclipse to con themselves out of death at the hands of African natives. Is slightly subverted when one of the villains tries explaining that the eclipse is a natural occurrence that will pass soon, but no one bothers to listen to her.
 * It helps that one of the characters was carrying an Almanac at the time, which documents that sort of thing, and which is often logically carried on long expeditions like the one they were on.
 * Still a remarkable fortune that an eclipse happened right at the same night they were to be executed.
 * The real-life solar eclipse visible in Maine on July 20, 1963 is a plot point of the Stephen King novels, Gerald's Game and Dolores Claiborne. I believe it's mentioned in some of his other works as well.
 * Enid Blyton adventure story The Secret Mountain (1941). The main characters need to escape from the titular mountain. They find out that there's to be a solar eclipse the next day, so at the appropriate moment their father throws his hunting knife off the mountain. The lights go out and the tribe think he's killed the sun, at which point the "big white bird" turns up to carry the heroes to safety before the tribe realise they've been had.
 * Subverted in Shadow of Earth. Where a modern day American woman is trapped in a 'the Spanish took over all of the Americas and never developed beyond feudalism' parallel universe. The main character is annoyed about the lack of handy eclipses. Luckily being a natural blonde gets her a relatively good deal anyway.
 * In the Gene Wolfe novel Urth of the New Sun a convenient eclipse saves the protagonist Severian's life from attack by Aztecs, but this is a subversion because
 * Parodied in one of Jack Handey's books: "I bet a fun thing would be to go way back in time to where there was going to be an eclipse and tell the cave men, 'If I have come to destroy you, may the sun be blotted out from the sky.' Just then the eclipse would start, and they'd probably try to kill you or something, but then you could explain about the rotation of the moon and all, and everyone would get a good laugh."
 * Subverted in the short story by Augusto Monterroso El Eclipse. Fray Bartolomé Arrazola tries to do this when he is about to be sacrificed by Mayans, unfortunately for him their astronomers already predicted all Solar and Lunar eclipses
 * Parodied, along with several related tropes by David Langford in "A Tale of the Jungle":


 * Subverted in The Last Camel Died At Noon, an Affectionate Parody of King Solomons Mines. A family of adventure archaeologists are in a lost civilization and looking to impress the natives. The wife asks her husband if a Convenient Eclipse coming up by any chance, and his response is essentially, "How the Hell would I know? I'm an archaeologist, not an astronomer."

Live Action TV

 * 1960's Batman episode "The Cat and the Fiddle". Batman and Robin are tied under giant magnifying glasses so they'll be broiled to death by concentrated sunlight. An eclipse gives the time to move one of the glasses so it burns through their bonds and frees them.

Video Games

 * Subverted in the Interactive Fiction game Bureaucracy: At one point, the player is captured by natives. If he tries to run the eclipse predicting program, it will predict the eclipse as being "Yesterday", and the natives will mention having seen it.
 * In the video-game adaptation of The Darkness, when Jackie initiates his attack on his uncle's island hideaway, the sun has started to rise, and the player is left with the horrifying realization that the Darkness, which is reliant on, well, darkness to provide Jackie with his shield and offensive powers that make him capable of taking down the entire Mafia all alone, won't be any help. And then the Darkness conjures an eclipse to happen on the spot, restoring your abilities.
 * Not that The Darkness directly conjured the eclipse as much as it kept Jackie on the hook long enough that it would occur at just the right time for Jackie's raid, ensuring that Jackie would have the power required to pull off the kind of atrocities that The Darkness needed to fully take him over.
 * In Treasure of the Rudras this is how the story progresses

Western Animation

 * Parodied in Darkwing Duck. Darkwing was to be executed by a group of medieval peasants for witchcraft, but noticed that the time of the execution coincided exactly with a solar eclipse and decided to threaten the peasants that he would block the sun if they didn't release him. Unfortunately, the newspaper dates were wrong and he ended up standing on the gallows for 24 hours, pretending to put out the sun, before it actually went dark. Peasants = amazed.
 * And it was probably parodied there because it had already been played straight on that show's predecessor, DuckTales (1987). Scrooge McDuck went into a South American country on the deadline day to resign a lease on the company he owned there, but, like Darkwing, he found himself scheduled to be executed by the local dictator instead. His nephews, who had inadvertently caused this predicament by tricking Scrooge—and through him, the entire world! -- into thinking it was the next day, consulted their super-exhaustive Junior Woodchucks Guidebook to try to fix things. And what do you know, a Convenient Eclipse was supposed to happen today! The boys showed the dictator the eclipse, thus correcting the date, and "Unca Scrooge" was not only free to go, but free to resign that lease.
 * The exact same plot was later recycled for the Tale Spin episode "The Time Bandit".
 * Avatar: The Last Airbender plays a lesser version where the eclipse comes in a few months.
 * And is also portrayed FAR more accurately than in most media, the full Eclipse itself lasting only a few minutes.
 * There's also a villainous inversion with the imminent arrival of Sozin's Comet creating a deadline of less than a year for the entire plot (otherwise Aang would have the option to take his time mastering all the elements and facing the Fire Lord only when he was completely ready)
 * Although the comet itself is a case of You Fail Science Forever, since comets are composed of rock and ice, not fire. Could be justified though in the sense that Avatar takes place in another world quite different from ours.
 * Parodied on The Simpsons in "Marge and the Monorail" when an eclipse causes the out-of-control solar powered train to stop temporarily and everyone breathes a sigh of relief... only for the sun to come back out and cause the train to start again.
 * "A solar eclipse. The cosmic ballet... goes on."
 * "Does anybody wanna switch seats?"
 * Parodied again in Fanboy and Chum Chum: When Fanboy is told he is going to die after naptime, he tries everything he can to keep everyone awake while he devises a plan. One of these efforts is him opening the curtains and saying they should go outside, but as soon as he does an eclipse comes, causing Fanboy to scream, "Curse you semi-elliptical orbit!!"
 * Downplayed in one of the old Superfriends cartoons, where Lex Luthor's Doomsday Device -- which magnifies the sun's rays and uses them as a weapon -- is foiled by an unscheduled eclipse. In truth, this eclipse is done on purpose after his plan succeeds, as a group of Sufficiently Advanced Aliens take pity on the Earth and use their Reality Warping powers to reverse time, adding the eclipse to Set Right What Once Went Wrong.

Real Life

 * Real Life example; British explorer James Cook's 1779 arrival in Hawaii coincided with a festival dedicated to the Polynesian god Lono. According to some accounts, the Hawaiian natives, who had never seen a European before, took him to be an incarnation of the god himself and deified him upon his arrival. (When he returned later in the year, after Lono's festival had ended, he was received less warmly - the natives stabbed him to death and reputedly ate parts of his body before returning it to the British for burial at sea.)
 * Another real life example that deserves mention: When the infamous Spanish Conquistador Hernando Cortés reached the Aztec Empire, they took him to be a god, thanks to many circumstances and prophecies. The Aztec believed that the god, Quetzalcoatl, was a pale, bearded god that had left by the Atlantic coast and would return the same year that Cortes arrived. This coincidence proved rather fatal for the Aztec, who didn't try to kill Cortes until he was too powerful.
 * Conversations recorded between Cortés and Moctezuma make it quite clear that Moctezuma recognized Cortés as another mortal human. It seems that him or one of his successors fabricated the story as a way of taking full credit for the conquest of the Aztec Empire; conveniently ignoring the fact that the Spaniards fought with over five times their number of Tlaxcaltec natives, fierce enemies of the Aztec.
 * Yet another real life example: Christopher Columbus was stranded in Jamaica in 1504 and declared that if the natives wouldn't help him and his crew, he would destroy the moon! Of course, he had the equivalent of the Farmer's Almanac tucked away in his cloak, much like the Connecticut Yankee.
 * Very conveniently for science, there was a solar eclipse in 1919, just a few years after Einstein had written his general theory of relativity. British scientist Eddington took photographs of the eclipse and used them to prove that Einstein's theory was correct, catapulting him to fame. Recently made into a rather good film "Einstein and Eddington" by the BBC, starring Andy Serkis and David Tennant respectively.
 * Total solar eclipses occur somewhere every 18 months - the 1919 eclipse was notable for happening at a convenient time, at a convenient place. Most solar eclipses are visible only at the ocean, anyway.
 * When the Spanish Armada was coming to invade England, the wind happened to blow just right to screw up their plans. This was referred to as the 'Protestant Wind'.
 * Worst of all, it was used twice.
 * Similarly, on two separate occasions, the Mongols launched massive invasion fleets against Japan and were thwarted when typhoons destroyed the fleets. This was the origin of the term Kamikaze ("divine wind"), due to the belief that Japan was being protected by the wind god Fuujin and/or the storm god Raijin.