'Observed celestial events' that you can base stories on are few and far between. For instance, "the journey of the Sun of God through the Zodiac" is not something that a normal person observes; you don't see the stars when the sun is near them. So the concept of this journey means you first have to accurately map the stars along the ecliptic, and record their rising and setting accurately enough to be able to say where the Sun is among them; then someone assigns shapes to the patterns of stars, which, if you are starting from scratch (as opposed to thinking of the myths you already have) is extremely arbitrary, and then you make up a story which has some meaning, you think, about, say, the Sun moving into Aquarius. Then you have to sell that story to the general population who haven't the faintest idea why you're talking about "the Sun moving into Aquarius".
The occasional very noticeable event could easily generate a myth or two - eclipses, comets. The phases of the moon are very obvious, and regular; I could believe stories based on that, I suppose. But they compete with happenings on earth - seasons, weather, animal behaviour, etc. - that are more noticeable. But, most of all, they compete with human stories.
Myths are mainly fables, based on human behaviour - sibling rivalry, over-reaching ambition, warnings of disaster that are ignored, wasting away the wealth you have, and so on. I think it's far more likely that most are just made up to illustrate our behaviour - like novels are now.
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