Why do you have such faith in astronomical predictions?  Because the models used to predict them have proven accurate in predicting a huge number of past events.
That is, they have been well tested by the method of predicting future data, retesting, and verifying that the prediction worked.
Now, even our astronomical models could be flawed.  Indeed, there was nothing that showed the Ptolemaic model to be incorrect until the phases of Venus were discovered, and it too made a huge number of correct predictions over the centuries.  We have refined the model since then so we trust it more in at least the areas where it has proved reliable, allowing for future refinements if necessary.
But note that you have chosen a model that is the culmination of literal centuries of revisions.  You are using this model to imply that we should put forth the same amount of trust in current climate models.  Models which are far newer than the astronomical models, and which have fared far worse in tests.  Indeed, every time the predictions of our climate models have been compared to actual data found after the predictions, they have consistently predicted higher temperatures than what has actually occurred.  If the models had been revised in response to these failures we might have some more trust in them, but largely they have not.  Instead effort has been focused on figuring out what went wrong with the world to make it disagree with the predictions (be that in the form of hidden temperatures, government regulations slowing climate change, or whatever else).  Of course, since the models were meant to predict the climate in the very world that we live in, such after the fact arguments do little to shore up confidence.
And that's without even getting into the fact that it's much easier to confirm astronomical predictions in the first place.  What I mean is this: if you are told there will be an eclipse, you can simply watch for it, and see if it happens.  It either will or it won't.  But what can you personally do to verify the current global average temperature?  Even if you had access to literally every thermometer in the world, you'd still have a huge part of the Earth's surface unaccounted for, necessitating a model to guess the current temperature.  That means that your model's predictions aren't checked against a reality, but rather a second model.  But how can we be sure that the second model is accurate?