Now another, deceptively similar question: if the Sun disappears this instant, how long before the Earth will stop orbiting the point where it used to be? The common answer: gravity travels with the speed of light, so also 8.5 min. This answer is obvious, simple and wrong. Yes, dead wrong. Why? because static gravity is not like light, it’s more like electric field, only worse.
I think you’re calling this more wrong than you should be. You follow it up by arguing that if the sun instantly disappears, there’ll never be a gravity change, so the 8.5min is wrong. But really, what you’re doing is showing that the sun can’t instantly disappear in the sense that people mean the question. The natural follow-up is to modify the question to something like the “what if the sun split into two halves that rapidly moved apart” version, and in that case, unless I’m mistaken, it would be 8.5 minutes before the earth’s orbit changed.
I think you’re calling this more wrong than you should be. You follow it up by arguing that if the sun instantly disappears, there’ll never be a gravity change, so the 8.5min is wrong. But really, what you’re doing is showing that the sun can’t instantly disappear in the sense that people mean the question. The natural follow-up is to modify the question to something like the “what if the sun split into two halves that rapidly moved apart” version, and in that case, unless I’m mistaken, it would be 8.5 minutes before the earth’s orbit changed.