The story was going along nicely, when I came to a sentence that stated that they were flying so high that the earth looked to be about the same size as the moon. Now, I know this is a fanciful tale and doesn't involve anything scientific, but a comparison like that completely suspends disbelief for me. If it said they flew 10 or 20 km, even 100 km or 1,000 km (remembering Mt Everest is 8.85 km, and already the air there is thin enough to make breathing difficult), I could accept this. Sure, there would be no breathable air (a space shuttle flight often has an average altitude of 300 km), but having those numbers I wouldn't necessarily imagine them past the space shuttle's stalking ground. I'd just go, yep, okay, and move on. But even if I paused to think about the numbers, I'd still imagine them attached to the planet. (Mind you, this might be a sticking point for other members of society, particularly those more familiar with the numbers than I am.)
The moon is about a quarter of the earth's size (the mass is a lot less, but we're not concerned about that). So, for the earth to look a similar size to the moon, they'd need to be four times further from the earth than the moon is. (Admittedly, the story says "hardly bigger", but that is "hardly" an ameliorating condition.) For the size of the earth to be even four times the size of the moon, our adventurous couple would have to be the distance of the moon away! (Remember, it took the Apollo astronauts about eight minutes to leave the atmosphere, and three days to reach the moon.) Considering, Pegasus is doing this as a joy flight, he's one very fast winged horse! But we're actually talking four times further away. I don't think so.
Unfortunately, with a comparison like that, I am going to stop and think about it. Rip. I'm out of the story.
Funnily enough, I also had problems with the next sentence, which has a much more subtle problem for me. The next sentence talks about how they [Bellerophon and Pegasus] amazed people in far off lands, who thought that the pair "must have come down out of the sky". I immediately stopped and thought, but they did!.
Although it doesn't say it, that sentence implies to me that they thought [mistakenly] that the pair had come from the sky. Maybe I'm reading more into this than other people would (and maybe you need to see the full context), but I had no doubt that that was what the writer was implying. If I'm wrong in the implication, then why would we need the sentence at all? Why state that the people thought they came from exactly where they did come from? What does that add to the story? In any case, it's another strike against the story, one as equally dangerous as suspension in disbelief, because like that one it shakes my faith in the writer.
As writers, we need to hold onto our reader's trust. We do this by setting up the rules of our story in the story opening (whether that's as simple as "this is the normal world, and the normal rules apply" -- done without our necessarily giving any conscious thought to what we are doing). We do this by then not violating those rules. So we can have a winged horse and strange creatures, but unless they have SRBs (sorry, Solid Rocket Boosters -- I love those NASA acronyms!) attached to their underbellies, chances are they're not going to go skipping from planet to planet -- unless we've set up this possibility.
We also do this by writing to a professional standard, employing a professional standard of spelling, punctuation and grammar because all of these things give the reader faith in the writing, faith that the writer knows what they are doing, faith that the writer is in control. And if there is one thing the writer must have, it's control over their own work! (And while some might argue that the editor can fix all of this, you must first convince an editor that you are a professional writer, professional in your approach and intent, and you can hardly do that if you don't approach your work professionally, if you don't embrace the rules of your own trade.)