An omen, no doubt about that. This surely would have driven ancient soothsayers into a state of consternation. Whether it was an auspicious omen or an inauspicious one, that wasn't so certain. I thought about all sorts of dog-related imagery and expressions to see if any would provide a clue to interpreting it, such as 'The dogs of war', 'Let sleeping dogs lie', 'Hour between dog and wolf' -- even "Release the hounds!" -- but couldn't make up my mind.
But then, close examination showed that the dog had its tongue out, which I took to be a positive sign since it probably indicates a peaceable and engaging mood. If the heavens were sending a message, it seems to be at the very least one of reassurance. It might even be a personal one, since the Dog is my Zodiac animal (by the Chinese 'Zodiac', not the western one).