There is a scale by which to rank these kinds of films with two logical extremes. Hard science fiction is the kind that goes out of its way to be as realistic as possible. Actual science is incorporated with few (if any) liberties taken. Spaceships are designed to resemble machines that could plausibly exist in our future, the Moon and any planets depicted are designed to resemble the real things (or at least as best an approximation of the real thing as can be made from the information available).
Destination Moon is one of the earliest examples, as well as the first big science fiction blockbuster. It used the best science available at the time (most of which still holds up alright at least in theory), complete with sci-fi writer Robert Heinlein as a consultant. Films like this and Pal's later Conquest of Space would help clear the way for Kubrick's 2001: A Space Odyssey which also employed a respected science fiction author, Arthur C. Clarke, to assist in ensuring the science is as realistic as possible. More recently we've also had Contact, Moon, Gravity, and Europa Report.
Personally I'm quite fond of this genre but it does have its challenges. As a writer of hard science fiction, one very easy trap I've fallen into is to spend too much time explaining the science which has gotten in the way of developing my characters and the story. Destination Moon is an enjoyable film but to someone looking for light entertainment they might get frustrated with the characters constantly explaining little details. The trick is to find a balance of sorts. After all while you use actual science you don't have to explain it all down to the last detail, just enough to make the reader understand what you're talking about.
The other extreme is known as soft science fiction. These are the works where logic and science are cast out the window in favor of a creative story and whatever the writers think looks cool. There's lots of things you can expect to see here. These are usually the kind of works where at best nothing will be explained, and when it is, it isn't so much a scientific explanation as it is scientific-sounding gibberish concocted by the writers in a desperate attempt to sound like they know what they're talking about.
There's certainly lots of images you'll see in soft science fiction. Flying saucers are reasonably common, spaceships with artificial gravity that is never explained, faster than light travel, "asteroid fields" consisting of large clusters of rocks that are unrealistically close together, laser guns, "black holes" that somehow function like wormholes, and great big noisy space battles with lots of explosions. Interestingly, a good early example would be Rocketship X-M which was hastily thrown together to compete with Destination Moon. Forbidden Planet would be another great example, as would Star Wars.
Those are the two biggest extremes. However, once in a while I see films (and by extension, other stories) that reach a new low. Soft science fiction writers at least have some idea what they're doing. They might not have done as much research as those of hard science fiction but they may do something. I'm talking the kind of writers that do absolutely no research whatsoever, desperately attempt to make it look like they did, and only end up showing how little they really know.
Naturally, there's a lot of mistakes I've often seen in science fiction writing. Star Wars has a long list of offenses for violating established scientific fact without any sort of logical justification, but I'm not going to get into that right now. One thing that can be really annoying in bad science fiction is when people try to make their work sound grander or more futuristic by throwing around astronomical terms. It creates misinterpretations and misunderstandings of various scientific concepts.
As it happens, in addition to being a film student I have also taken a few basic astronomy classes, so I think it is worthwhile to go through some of the biggest mistakes I've found and set the record straight. First up I'll be revisiting my old arch-nemesis: Jean-Luc Godard. Alphaville already has a pathetic enough vision of the future, but what really nails the head on the coffin is the extent of the efforts to make it in any way "futuristic" which amounts to the script throwing around various astronomy terms.
After I published the article one thing that was pointed out to me, though I don't specifically recall when this happened in the film itself (granted, I don't want to remember much of that travesty and I was probably too busy trying to make sense of the incoherent plot to notice), that Godard apparently thinks light years are a measure of time.
Admittedly it's a bit hard to wrap your mind around at first but that unit is actually a measure of distance. Light travels at a constant speed, which means that there is a finite distance it can travel in one year (9.46 trillion km). That distance gives astronomers a unit that is far easier to work with on a cosmic scale, the same reason why we have astronomical units (AU) for measuring distances within our Solar System.
Really, though, the offence we all remember the film committing was that instead of actually attempting to create any kind of interesting future, Godard just throws around the word "galaxy". It's not the only time I've seen this kind of thing happen. I've seen other works which try to sound cool by using the word "galaxy" but Godard takes it to the next level because he clearly has no idea what the word means. The main character of Lemmy Caution keeps saying lines like "things are much different in this galaxy" implying intergalactic travel somehow exists even though no real technological progress is visible.
The Star Wars movies famously open with a title card reading "A long Time Ago, In a Galaxy Far, Far Away..." but why a galaxy? Disregarding the expanded universe and the later films, A New Hope (or, if you really want to be a purist, Star Wars) really only featured three different worlds (Tatooine, Alderaan, and the moon Yavin IV, presumably orbiting a planet). If we focus specifically on the original film, and disregard anything that comes from hindsight, there really was no need for it to be a galaxy. The plot could have worked just as well if Tatooine, Alderaan, and Yavin were all located in one Solar System.
I suppose it would be worthwhile to provide you with an idea of the scale of galaxies. The Milky Way galaxy in which we live is of course just one of billions if not trillions in the universe where we live. When you hear the term "galaxy", I suspect the image that comes to mind is a pretty fascinating one: a large glowing spiral disk. It certainly looks cool, but what is it really?
Technically there's two major types of galaxies, but for the sake of simplicity I'm going to focus on the type you're more likely familiar with: the spiral galaxy. You've probably seen images of this kind before and examples would include the Milky Way and the Andromeda galaxy. What you're really looking at is something vast. Our own galaxy is 100,000 light years across, that means it takes light precisely that long to get from one end to the other, just imagine trying to send a spacecraft across. What you're looking at when you see an image of a galaxy is billions of stars, many of which also have planets, but that's not all.
There's also plenty of dust, gas, and other things with which to make more stars. That's not even getting into the super massive black hole right in the middle of the central bulge. In fact what you see might not actually be the whole thing. I'm not going to get into the specifics, but there's another invisible part of the galaxy called the "halo" which is full of globular clusters (large groups of stars that orbit on irregular paths around the galaxy). In fact it is possible that what you can see is really only a very small portion, and that most of the galaxy is actually made up of an invisible substance known as "dark matter", though I won't be going into the specifics of that.
So as you can see, even if we did manage to develop interstellar travel, exploring the galaxy would be a serious challenge. There is no known way we could operate a spacecraft that could travel at the speed of light and even if we could there'd be the issues of time dilation, so good luck finding a way to travel 100,000 light years. I have no idea how the Star Wars universe is able to maintain a galactic community, get around the time dilation from going to light speed, and develop methods of communicating in real time from opposite ends of their galaxy.
Incidentally, if you want to remember the full scale of our galaxy, there is a little song you can listen to that is extremely helpful, by Monty Python of all people. This little musical number from The Meaning of Life is surprisingly accurate to the point where remembering the lyrics helped me with an assignment and my astronomy professor actually endorsed it as a study aid. If you have any difficulty remembering the facts, just think of the lyrics to the song and you should be fine.
So we've established just how big our own galaxy is, the next part to consider is just how far other galaxies are. I can't give you a precise figure, but what I can tell you is that we don't have much of a chance of reaching any of them. Contrary to Godard's vision, I'm pretty sure we won't be visiting any other galaxies in the near future. Whether we'll ever have any chance of reaching them is something I'm not sure anyone can definitively say, but based on our current understanding of the universe there is no way it's possible. Also, even if we did develop intergalactic travel, matters might be complicated by the constant expansion of the universe.
Another offender from the Star Wars franchise: Han Solo famously boasted that he'd made "The Kessel Run" in less than twelve parsecs. That sounds quite impressive and certainly gives you an idea of how cool a guy he is... until you realize that parsecs are a measure of distance, not time. In short, it's yet another means of measuring distances between objects outside of our solar system. According to Wikipedia a parsec is roughly 3.26 light years, so three times the distance light can travel in one year. It's like saying you could fly to the moon in less than 42 astronomical units. It really doesn't make any sense. Suddenly Han's claim doesn't sound so impressive, does it?
The upcoming movie Guardians of the Galaxy has a supporting character named Nebula, played by Doctor Who star Karen Gillan. The thing is she is an alien, and I'm suspecting one who's never been to Earth before, so it seems strange that her name would be a term created by English-speaking humans, even if it's a phenomenon she's seen before. This is another word I've heard misused in various works from time to time, and really it's a bit simpler than you might think.
Finally, this is one really problematic error I've seen a lot and it can get a bit annoying when you know the reality. The weird 1979 Disney film The Black Hole claims that a black hole can be used as a means of travelling from one point in space to another. This is not the case. In fact that film clearly had no idea what it was talking about given it's image of a "black hole" was a swirling blue vortex which makes you wonder why they even bothered keeping the name.
The problem here is that black holes are confused with wormholes. A black hole is so called because it is impossible to see. The reason why is simple: black holes are so massive and dense that past a certain point the escape speed (the minimum required speed to break free from an object's gravitational pull) becomes greater than the speed of light. This means that past the point of no return, known as the event horizon, nothing can escape, and that includes light.
That's not even getting into the specifics of what happens when you go into a black hole. The simplest version is that once you go past the event horizon you would be stretched out in a process literally called "specification" and torn apart, along with anything else that followed you into the black hole, until nothing but atoms remain. Here is a video that explains it in a bit more depth:
Wormholes on the other hand are something quite a bit different. While they are theoretically possible we still have yet to find any definitive proof of their existence. A wormhole is like a fold in space and time, that could provide a shortcut between two different points in the universe that could be thousands of parsecs apart. This is more in line with the ideas being explored in The Black Hole.
Hopefully this will help clear up any misunderstandings. Perhaps it will help some future science fiction writers who want to use some of these terms to use them properly. There are plenty of other mistakes and misunderstandings that emerge from time to time. For one thing you should never confuse moons and planets; the key distinction being that planets orbit stars and moons orbit around planets. Another common error is that asteroid fields are never as cluttered as you see in the movies and they're actually pretty easy to navigate. Those are topics for another discussion, however.