Sorry for late response: the optical depth definition for the photosphere is used because that's the point where the plasma becomes effectively transparent. IOW, a photon emitted radially outward at that level will on average travel to infinity without being scattered by the plasma. A photon emitted at a depth of 3/4, OTOH, will scatter off intervening material once or more on average, meaning that the observed properties of the radiation don't reflect the conditions where it was emitted. Put another way, the photosphere is the deepest level where the spectrum provides direct info about conditions.
ThinksMarkedly wrote:tlb wrote:If all the planets are in a line out from the Sub (a planetary parade), then the CM is as far outside of the Sun as it can get and NO planet is orbiting about a point contained within the Sun. So I do not understand why you limit this to Jupiter. Not that my understanding is of great import.
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And, of course, for a big ball of plasma, where does the Sun end anyway? Why must it be at the photosphere? And why is that defined as optical depth of 2/3 instead of 3/4 or something else? For a gravitational phenomenon, I'd be much more interested in what fraction of the mass of the star / system is inside of a given radius. Even then we'd have an arbitrary number, like 99% or 99.8% - the Sun is 99.86% of the mass of the entire system, but how much of that is within its nominal radius of 695,700 km?
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