Can't figure out where to put your astronomy related example? Leave it here. When we get enough like it, a new trope will begin to form. Kind of like a solar system, if you think about it.

Mangled Celestial Motion - Relative Planetary and stellar placement

• There is an energy snack commercial where a basketball player (Lamar Odom) shows off his ability to dunk to the Moon. On his way, he tells Saturn to get out of his way. If we're suspending our disbelief enough to buy that a guy jump to the Moon, Saturn might as well be between the Earth and the Moon. The next guy says he's going to dunk on Pluto. Okay. For all we know, that might be a shorter trip than one to the Moon, in this ficton's astronomy.
• In Lara Croft: Tomb Raider, all the planets including Pluto (which at the time, was a planet) are aligned. Okay. Pluto's orbit is highly erratic, tilted at an angle of 17 degrees to the ecliptic and highly eccentric (being the wrong kind of ellipse). It will probably never align closely with all the other planets in the lifespan of the solar system, but you can't have a really good planetary collection without having the whole set.
• Doctor Who: The 1996 TV movie places Gallifrey, the Doctor's home planet, some 250 million light years away from Earth, on "the other side" of the Milky Way. That's about 249.9 million light years past the other edge of it. The Milky Way is estimated to be only between 80 and 100 thousand light years across.
• Towards the end of J.K. Rowling's Harry Potter and the Order of the Phoenix, in mid or late June, Harry is taking his astronomy O.W.L. and charting Orion. Orion is not visible in the night sky in mid or late June at any latitude. The same scene also has him looking around for Venus (which is never more than 47 degrees away from the sun) around midnight. Needless to say, he didn't do particularly well on that O.W.L.. Just as well. Muggles have to be better at something.
• When the aliens attempt to reach Earth in Calvin and Hobbes: The Series, the narrative inexplicably swaps Jupiter and Saturn's positions in the solar system.

Mangled Celestial Motion - Made-To-Order Eclipses

• Apocalypto features a solar eclipse. The very next night, there's a full moon, which is odd considering that an eclipse can only happen at a new moon. The Moon is obviously like a great big lightbulb a writer can turn up or down, depending on the level of light needed at night.
• Buffy the Vampire Slayer: The Season 3 finale features a solar eclipse that goes from zero to total in about five seconds, then stays that way for the duration of the entire climactic battle. Apparently the mayor's magic is powerful enough to first speed up the earth's rotation, then stop it dead in its tracks for a while.
• Ladyhawke: A full moon is followed a few days later by a solar eclipse, followed a few days later by a quarter moon. It is almost like this story about people turning into hawks just wasn't about correct astronomy at all.
• Something alike happens in the Spanish TV series Aguila Roja. A few days before a full moon there's a solar eclipse.
• Heroes has issues with eclipses. The pilot has a reasonably brief eclipse, but it's visible in both New York City and Tokyo at the same time. The eclipse that robs everyone of their powers in Season 3 is even worse; not only is it visible across the globe, it lasts for hours. Why? Because is it says so, right here in the script, that's why.

Mangled Celestial Motion - Constellations Are Constant

• Stargate: The way that constellations are used as a Cartesian coordinate system begins with the idea that a constellation is a fixed point in space. That probably makes it easier to build things you can just walk through to cross interstellar distances.
• Subverted in Stargate SG-1. The SGC cannot dial any address other than Abydos until Daniel Jackson discovers information that shows how to shift gate addresses to compensate for passing time.
• Space Mutiny: Constellations are repeatedly referred to as meaningful divisions of space.
• In Doctor Who, the Doctor specifically mentions that his homeworld of Gallifrey was located in the Kasterborous constellation. How can one's own homeworld be in a constellation, when constellations are arbitrary shapes in the night sky as visible from your homeworld? We can't say that Earth is in a certain constellation because to us constellations are only visible from here. Additionally, stars that appear to be close together in a constellation may actually be hundreds of light-years apart; they might only look close together because of the scale involved.
  • The Trial of a Time Lord repeatedly refers to Earth's entire constellation being moved by the Time Lords, ravaging Earth in the process and turning it into Ravelox.
  • This error also crops up in The Five Star Stories, with many characters referring to the eponymous stars as a constellation despite living on planets orbiting them. They may just be talking poetically, though.
• Star Control II: All the stars in a constellation are close to each other, forming contiguous regions on the hyperspace map. The manual explains that these constellations are not the same constellations visible from any given planet, but were created after hyperspace was mapped from the patterns on the map.
• Shot down in Men In Black, where the cryptic clue "The galaxy is on Orion's belt," is quickly dismissed as blathering nonsense.
• In Prometheus, the titular ship's destination is derived from an image of five stars which shows up in ancient sites around the world. The archaeologist hero says that a certain region of a very distant galaxy is the only possible match for this stellar configuration.

Mangled Celestial Motion - Spacey Schedule Errors

• In She-Wolf of London, Randi tells Ian not to worry about her transforming into here wolf form because "there won't be another full moon for months".
• In Time Scout, reading stars like a clock is portrayed as much more difficult and complex than it actually is.

Mangled Astrophysics - Timescales of Stellar Evolution

• In the novellization of Revenge of the Sith, Anakin recalls a visit to a black dwarf star system: a frigid dwarf of hypercompacted trace metals, hovering a quantum fraction of a degree above absolute zero. Even now there are no such objects yet, since they require tens of billions of years to cool, and the Universe isn't old enough; and Anakin lived "a long time ago", that's in a universe even younger than ours.
  • As if anyone ever pays attention to the "long time ago" part anyway. Or if they do, it's easily resolved as being "a long time ago" from the perspective of the narrator.

Mangled Astrophysics - Supernovae: Causes and Effects

• In A Swiftly Tilting Planet, one of the sequels to A Wrinkle in Time, we learn yet another reason not to throw a nuclear war on Earth:
  Gaudior: You know some of the possibilities if your planet is blown up.
  Charles Wallace: It just might throw off the balance of things, so that the sun would burst into a supernova.
  Though, to be sure, it's a trope in the series that "all things are connected", and indeed, one character in A Wrinkle in Time is herself a former star who blew herself up to fight the cosmic enemy.
• In the 2009 Star Trek reboot/alternate continuity film, the Romulan system is destroyed by the shockwave from a supernova...trouble is, the star shown exploding is an average-looking yellow Main Sequence star (like our own), which are neither hot enough nor massive enough to generate a supernova. Supernovae form almost exclusively from extremely massive blue-white stars.

Mangled Astrophysics - Nova

In more than one work of fiction, a star like the sun is said to end its life by "going nova." The implication is that a nova is merely a supernova on a smaller scale.

In reality, a nova is an outburst caused by a white dwarf sucking material off of a companion star; when enough material is accreted on the white dwarf's surface, it gets hot enough and dense enough to undergo nuclear fusion. This produces an outburst that reaches peak brightness in a few hours, then cools back down again over a few days or weeks. Some white dwarfs are known to be "recurrent novae", undergoing a nova outburst at more-or-less regular intervals of a few years or decades.

When a sunlike star ends its main sequence lifetime, it doesn't "go nova", it swells into a red giant over the course of a few thousand years.

Even fictionally "blowing up the sun" should never, ever result in a "nova." Supernova, maybe, but not nova.

Telescopes 101

• In Lara Croft: Tomb Raider, Lara Croft observed the alignment of Pluto and Neptune through a telescope in a room full of what looked like 20 floodlights. If these two had tried to be too dim to see in that situation, Lara would have just kicked their butts. They must have been really trying to be bright little planetary bodies that night out of sheer terror.
• The first Ad Bumper (indeed very first segment) of this collection of 1984 commercials and ad bumpers shows a ground-based observatory frantically turning to get a view of a UFO. Not only is the UFO in question too close to resolve clearly by the telescope, but observatories can't turn that fast.

Bad Nomenclature

• In the manga version of Yu-Gi-Oh! GX, the Big Bad summons his most powerful monster—The Supremacy Sun, strongest monster in the Planet Series. Somewhat justified in that said Big Bad grew up in Ancient Egypt—Yu-Gi-Oh!'s Ancient Egypt... and made the Planet Series, himself.
  • This may be a translation issue, actually, given that a lot of things from Japan classify all celestial bodies as "planets" (like Super Mario Galaxy).
    • Strictly speaking, Japanese things actually call everything stars—"wakusei" (惑星) literally translates to something like "confusing star" ("planet", by the way, comes from a word meaning "wanderer", probably for similar reasons).
    • German has the same problem. Stern is the German word for both star and planet.
• In the World of Darkness supplement Infinite Macabre, the term "galaxy" is explicitly stated to mean "systems of stars orbiting one another" or "systems of planets orbiting one or more stars." Also, they're separated by hundreds of thousands of parsecs at least, millions of parsecs at most. By way of reference, the nearest "galaxy," using the above definition, to our "galaxy, again using the above definition, is the Centauri system, roughly 1.5 parsecs away. Talk about scale problems...

Fake Looking Celestial Bodies

• In the Star Wars Expanded Universe the planet Bespin is stated by canon to be a gas giant, but is milky-white and stripeless, and generally looks like Venus. A Venus-like planet is an even better place to put a Cloud City, but it's not a gas giant, even if canon states it to be such.
  • Fridge Logic: Not all gas giants look alike. A gas giant in the habitable zone would have white water clouds. An orange color could be caused by the presence of methane in the atmosphere (airborne methanogenic bacteria?). The planet could lack stripes if it had a slow rotation.
• This does NOT excuse the dull red color of the gas giant Yavin. It looks like a brown dwarf, not a gas giant.
  • Except that brown dwarfs are basically gas giants that are so big, they almost began nuclear fusion. Given that many star systems are binary, having one with a main sequence star and a brown dwarf is not inconceivable.
  • Also, brown dwarves actually are magenta colored, while a sufficently hot gas giant (as in, close to it's parent star) might slightly glow red. Whether it could harbor a habitable moon is up to debate, but it could also be just a matter of the planet's chemistry.

Nebulas misrepresented

• The Crab Nebula and the Horsehead Nebula (because of their interesting appearance) were once depicted as being the home address of various aliens. If not, then they were in the neighborhood. These are not very hospitable areas to have a planet. They are actually areas where new stars are born. Also, as with constellations, these nebulae will not retain their appearance to a person travelling in space as opposed to on Earth. So an alien wouldn't think of himself as being from, say, the "Horsehead Nebula."*
  Though of course he might describe his home that way, when talking to earth humans.
• There is a political cartoon where a father tells his son that every star has a system of planets. Not every star system, every star. Where do I even begin?
  • In fact, not every star has planets (very young blue stars don't, for example, since they don't have time to form), but it's in fact a matter of ongoing scientific debate on exactly how common are planets.
• In more than one work of science fiction — Star Trek II: The Wrath of Khan comes to mind — a nebula is treated like an opaque cloud where a space ship can easily hide from sight. In reality, nebulas are more rarefied than the best laboratory vacuum. The only reason a nebula is visible as a cloudlike structure is because it's light-years across, and astronomers have to peer through a ginormous expanse of this extremely tenuous material.

Comet Impacts Don't Work That Way

• Tank Girl. The opening narration says that a comet hit the Earth and somehow got rid of most of the liquid water on Earth (as shown by the dry sea beds), presumably by evaporating it. It also resulted in no rain falling for 11 years. Unless the comet was made of some crazy compounds designed to utterly destroy water molecules, the result of all that water evaporating should have been 1) the entire planet being flash-broiled, and 2) the mother of all greenhouse effects and downpours when all of that water vapor condensed and returned to the ground. (Unless the comet hit the Earth so hard that it sent all the water out into deep space; but any comet impact of that magnitude would also shatter the Earth's crust and turn it into a lava world.) The dialog of the Water And Power personnel implies that vast amounts of water are hidden under the desert, but it's even less likely that the comet impact could have caused that.

Completely inaccurate terminology

• In Highlander The Source, the galaxy suffers a major case of When the Planets Align. One character offhandedly dismisses the phenomenon, suggesting "that could just be orbital wobble." Actual "orbital wobble" is very, very different from what the film thinks it means.